change SimValue to contain and deref to a value and not just contain bits

crates/fayalite-proc-macros-impl/src/hdl_bundle.rs

@@ -30,7 +30,9 @@ pub(crate) struct ParsedBundle {
     pub(crate) field_flips: Vec<Option<HdlAttr<kw::flip, kw::hdl>>>,
     pub(crate) mask_type_ident: Ident,
     pub(crate) mask_type_match_variant_ident: Ident,
+    pub(crate) mask_type_sim_value_ident: Ident,
     pub(crate) match_variant_ident: Ident,
+    pub(crate) sim_value_ident: Ident,
     pub(crate) builder_ident: Ident,
     pub(crate) mask_type_builder_ident: Ident,
 }
@@ -125,7 +127,9 @@ impl ParsedBundle {
             field_flips,
             mask_type_ident: format_ident!("__{}__MaskType", ident),
             mask_type_match_variant_ident: format_ident!("__{}__MaskType__MatchVariant", ident),
+            mask_type_sim_value_ident: format_ident!("__{}__MaskType__SimValue", ident),
             match_variant_ident: format_ident!("__{}__MatchVariant", ident),
+            sim_value_ident: format_ident!("__{}__SimValue", ident),
             mask_type_builder_ident: format_ident!("__{}__MaskType__Builder", ident),
             builder_ident: format_ident!("__{}__Builder", ident),
             ident,
@@ -427,7 +431,9 @@ impl ToTokens for ParsedBundle {
             field_flips,
             mask_type_ident,
             mask_type_match_variant_ident,
+            mask_type_sim_value_ident,
             match_variant_ident,
+            sim_value_ident,
             builder_ident,
             mask_type_builder_ident,
         } = self;
@@ -523,7 +529,7 @@ impl ToTokens for ParsedBundle {
             semi_token: None,
         }
         .to_tokens(tokens);
-        let mut mask_type_match_variant_fields = mask_type_fields;
+        let mut mask_type_match_variant_fields = mask_type_fields.clone();
         for Field { ty, .. } in &mut mask_type_match_variant_fields.named {
             *ty = parse_quote_spanned! {span=>
                 ::fayalite::expr::Expr<#ty>
@@ -565,6 +571,58 @@ impl ToTokens for ParsedBundle {
             semi_token: None,
         }
         .to_tokens(tokens);
+        let mut mask_type_sim_value_fields = mask_type_fields;
+        for Field { ty, .. } in &mut mask_type_sim_value_fields.named {
+            *ty = parse_quote_spanned! {span=>
+                ::fayalite::sim::value::SimValue<#ty>
+            };
+        }
+        ItemStruct {
+            attrs: vec![
+                parse_quote_spanned! {span=>
+                    #[::fayalite::__std::prelude::v1::derive(
+                        ::fayalite::__std::fmt::Debug,
+                        ::fayalite::__std::clone::Clone,
+                    )]
+                },
+                parse_quote_spanned! {span=>
+                    #[allow(non_camel_case_types, dead_code)]
+                },
+            ],
+            vis: vis.clone(),
+            struct_token: *struct_token,
+            ident: mask_type_sim_value_ident.clone(),
+            generics: generics.into(),
+            fields: Fields::Named(mask_type_sim_value_fields),
+            semi_token: None,
+        }
+        .to_tokens(tokens);
+        let mut sim_value_fields = FieldsNamed::from(fields.clone());
+        for Field { ty, .. } in &mut sim_value_fields.named {
+            *ty = parse_quote_spanned! {span=>
+                ::fayalite::sim::value::SimValue<#ty>
+            };
+        }
+        ItemStruct {
+            attrs: vec![
+                parse_quote_spanned! {span=>
+                    #[::fayalite::__std::prelude::v1::derive(
+                        ::fayalite::__std::fmt::Debug,
+                        ::fayalite::__std::clone::Clone,
+                    )]
+                },
+                parse_quote_spanned! {span=>
+                    #[allow(non_camel_case_types, dead_code)]
+                },
+            ],
+            vis: vis.clone(),
+            struct_token: *struct_token,
+            ident: sim_value_ident.clone(),
+            generics: generics.into(),
+            fields: Fields::Named(sim_value_fields),
+            semi_token: None,
+        }
+        .to_tokens(tokens);
         let this_token = Ident::new("__this", span);
         let fields_token = Ident::new("__fields", span);
         let self_token = Token![self](span);
@@ -615,6 +673,25 @@ impl ToTokens for ParsedBundle {
                 }
             },
         ));
+        let sim_value_from_bits_fields = Vec::from_iter(fields.named().into_iter().map(|field| {
+            let ident: &Ident = field.ident().as_ref().unwrap();
+            quote_spanned! {span=>
+                #ident: v.field_from_bits(),
+            }
+        }));
+        let sim_value_clone_from_bits_fields =
+            Vec::from_iter(fields.named().into_iter().map(|field| {
+                let ident: &Ident = field.ident().as_ref().unwrap();
+                quote_spanned! {span=>
+                    v.field_clone_from_bits(&mut value.#ident);
+                }
+            }));
+        let sim_value_to_bits_fields = Vec::from_iter(fields.named().into_iter().map(|field| {
+            let ident: &Ident = field.ident().as_ref().unwrap();
+            quote_spanned! {span=>
+                v.field_to_bits(&value.#ident);
+            }
+        }));
         let fields_len = fields.named().into_iter().len();
         quote_spanned! {span=>
             #[automatically_derived]
@@ -623,6 +700,7 @@ impl ToTokens for ParsedBundle {
             {
                 type BaseType = ::fayalite::bundle::Bundle;
                 type MaskType = #mask_type_ident #type_generics;
+                type SimValue = #mask_type_sim_value_ident #type_generics;
                 type MatchVariant = #mask_type_match_variant_ident #type_generics;
                 type MatchActiveScope = ();
                 type MatchVariantAndInactiveScope = ::fayalite::ty::MatchVariantWithoutScope<
@@ -660,6 +738,34 @@ impl ToTokens for ParsedBundle {
                 fn source_location() -> ::fayalite::source_location::SourceLocation {
                     ::fayalite::source_location::SourceLocation::caller()
                 }
+                fn sim_value_from_bits(
+                    &self,
+                    bits: &::fayalite::__bitvec::slice::BitSlice,
+                ) -> <Self as ::fayalite::ty::Type>::SimValue {
+                    #![allow(unused_mut, unused_variables)]
+                    let mut v = ::fayalite::bundle::BundleSimValueFromBits::new(*self, bits);
+                    #mask_type_sim_value_ident {
+                        #(#sim_value_from_bits_fields)*
+                    }
+                }
+                fn sim_value_clone_from_bits(
+                    &self,
+                    value: &mut <Self as ::fayalite::ty::Type>::SimValue,
+                    bits: &::fayalite::__bitvec::slice::BitSlice,
+                ) {
+                    #![allow(unused_mut, unused_variables)]
+                    let mut v = ::fayalite::bundle::BundleSimValueFromBits::new(*self, bits);
+                    #(#sim_value_clone_from_bits_fields)*
+                }
+                fn sim_value_to_bits(
+                    &self,
+                    value: &<Self as ::fayalite::ty::Type>::SimValue,
+                    bits: &mut ::fayalite::__bitvec::slice::BitSlice,
+                ) {
+                    #![allow(unused_mut, unused_variables)]
+                    let mut v = ::fayalite::bundle::BundleSimValueToBits::new(*self, bits);
+                    #(#sim_value_to_bits_fields)*
+                }
             }
             #[automatically_derived]
             impl #impl_generics ::fayalite::bundle::BundleType for #mask_type_ident #type_generics
@@ -696,6 +802,7 @@ impl ToTokens for ParsedBundle {
             {
                 type BaseType = ::fayalite::bundle::Bundle;
                 type MaskType = #mask_type_ident #type_generics;
+                type SimValue = #sim_value_ident #type_generics;
                 type MatchVariant = #match_variant_ident #type_generics;
                 type MatchActiveScope = ();
                 type MatchVariantAndInactiveScope = ::fayalite::ty::MatchVariantWithoutScope<
@@ -735,6 +842,34 @@ impl ToTokens for ParsedBundle {
                 fn source_location() -> ::fayalite::source_location::SourceLocation {
                     ::fayalite::source_location::SourceLocation::caller()
                 }
+                fn sim_value_from_bits(
+                    &self,
+                    bits: &::fayalite::__bitvec::slice::BitSlice,
+                ) -> <Self as ::fayalite::ty::Type>::SimValue {
+                    #![allow(unused_mut, unused_variables)]
+                    let mut v = ::fayalite::bundle::BundleSimValueFromBits::new(*self, bits);
+                    #sim_value_ident {
+                        #(#sim_value_from_bits_fields)*
+                    }
+                }
+                fn sim_value_clone_from_bits(
+                    &self,
+                    value: &mut <Self as ::fayalite::ty::Type>::SimValue,
+                    bits: &::fayalite::__bitvec::slice::BitSlice,
+                ) {
+                    #![allow(unused_mut, unused_variables)]
+                    let mut v = ::fayalite::bundle::BundleSimValueFromBits::new(*self, bits);
+                    #(#sim_value_clone_from_bits_fields)*
+                }
+                fn sim_value_to_bits(
+                    &self,
+                    value: &<Self as ::fayalite::ty::Type>::SimValue,
+                    bits: &mut ::fayalite::__bitvec::slice::BitSlice,
+                ) {
+                    #![allow(unused_mut, unused_variables)]
+                    let mut v = ::fayalite::bundle::BundleSimValueToBits::new(*self, bits);
+                    #(#sim_value_to_bits_fields)*
+                }
             }
             #[automatically_derived]
             impl #impl_generics ::fayalite::bundle::BundleType for #target #type_generics
@@ -768,23 +903,33 @@ impl ToTokens for ParsedBundle {
         }
         .to_tokens(tokens);
         if let Some((cmp_eq,)) = cmp_eq {
-            let mut where_clause =
+            let mut expr_where_clause =
                 Generics::from(generics)
                     .where_clause
                     .unwrap_or_else(|| syn::WhereClause {
                         where_token: Token![where](span),
                         predicates: Punctuated::new(),
                     });
+            let mut sim_value_where_clause = expr_where_clause.clone();
+            let mut fields_sim_value_eq = vec![];
             let mut fields_cmp_eq = vec![];
             let mut fields_cmp_ne = vec![];
             for field in fields.named() {
                 let field_ident = field.ident();
                 let field_ty = field.ty();
-                where_clause
+                expr_where_clause
                     .predicates
                     .push(parse_quote_spanned! {cmp_eq.span=>
                         #field_ty: ::fayalite::expr::ops::ExprPartialEq<#field_ty>
                     });
+                sim_value_where_clause
+                    .predicates
+                    .push(parse_quote_spanned! {cmp_eq.span=>
+                        #field_ty: ::fayalite::sim::value::SimValuePartialEq<#field_ty>
+                    });
+                fields_sim_value_eq.push(quote_spanned! {span=>
+                    ::fayalite::sim::value::SimValuePartialEq::sim_value_eq(&__lhs.#field_ident, &__rhs.#field_ident)
+                });
                 fields_cmp_eq.push(quote_spanned! {span=>
                     ::fayalite::expr::ops::ExprPartialEq::cmp_eq(__lhs.#field_ident, __rhs.#field_ident)
                 });
@@ -792,9 +937,13 @@ impl ToTokens for ParsedBundle {
                     ::fayalite::expr::ops::ExprPartialEq::cmp_ne(__lhs.#field_ident, __rhs.#field_ident)
                 });
             }
+            let sim_value_eq_body;
             let cmp_eq_body;
             let cmp_ne_body;
             if fields_len == 0 {
+                sim_value_eq_body = quote_spanned! {span=>
+                    true
+                };
                 cmp_eq_body = quote_spanned! {span=>
                     ::fayalite::expr::ToExpr::to_expr(&true)
                 };
@@ -802,6 +951,9 @@ impl ToTokens for ParsedBundle {
                     ::fayalite::expr::ToExpr::to_expr(&false)
                 };
             } else {
+                sim_value_eq_body = quote_spanned! {span=>
+                    #(#fields_sim_value_eq)&&*
+                };
                 cmp_eq_body = quote_spanned! {span=>
                     #(#fields_cmp_eq)&*
                 };
@@ -812,7 +964,7 @@ impl ToTokens for ParsedBundle {
             quote_spanned! {span=>
                 #[automatically_derived]
                 impl #impl_generics ::fayalite::expr::ops::ExprPartialEq<Self> for #target #type_generics
-                #where_clause
+                #expr_where_clause
                 {
                     fn cmp_eq(
                         __lhs: ::fayalite::expr::Expr<Self>,
@@ -827,6 +979,17 @@ impl ToTokens for ParsedBundle {
                         #cmp_ne_body
                     }
                 }
+                #[automatically_derived]
+                impl #impl_generics ::fayalite::sim::value::SimValuePartialEq<Self> for #target #type_generics
+                #sim_value_where_clause
+                {
+                    fn sim_value_eq(
+                        __lhs: &::fayalite::sim::value::SimValue<Self>,
+                        __rhs: &::fayalite::sim::value::SimValue<Self>,
+                    ) -> bool {
+                        #sim_value_eq_body
+                    }
+                }
             }
             .to_tokens(tokens);
         }

crates/fayalite-proc-macros-impl/src/hdl_enum.rs

@@ -129,6 +129,7 @@ pub(crate) struct ParsedEnum {
     pub(crate) brace_token: Brace,
     pub(crate) variants: Punctuated<ParsedVariant, Token![,]>,
     pub(crate) match_variant_ident: Ident,
+    pub(crate) sim_value_ident: Ident,
 }
 
 impl ParsedEnum {
@@ -190,6 +191,7 @@ impl ParsedEnum {
             brace_token,
             variants,
             match_variant_ident: format_ident!("__{}__MatchVariant", ident),
+            sim_value_ident: format_ident!("__{}__SimValue", ident),
             ident,
         })
     }
@@ -207,6 +209,7 @@ impl ToTokens for ParsedEnum {
             brace_token,
             variants,
             match_variant_ident,
+            sim_value_ident,
         } = self;
         let span = ident.span();
         let ItemOptions {
@@ -409,6 +412,106 @@ impl ToTokens for ParsedEnum {
             )),
         }
         .to_tokens(tokens);
+        let mut enum_attrs = attrs.clone();
+        enum_attrs.push(parse_quote_spanned! {span=>
+            #[::fayalite::__std::prelude::v1::derive(
+                ::fayalite::__std::fmt::Debug,
+                ::fayalite::__std::clone::Clone,
+            )]
+        });
+        enum_attrs.push(parse_quote_spanned! {span=>
+            #[allow(dead_code, non_camel_case_types)]
+        });
+        let sim_value_has_unknown_variant = !variants.len().is_power_of_two();
+        let sim_value_unknown_variant_name = sim_value_has_unknown_variant.then(|| {
+            let mut name = String::new();
+            let unknown = "Unknown";
+            loop {
+                let orig_len = name.len();
+                name.push_str(unknown);
+                if variants.iter().all(|v| v.ident != name) {
+                    break Ident::new(&name, span);
+                }
+                name.truncate(orig_len);
+                name.push('_');
+            }
+        });
+        let sim_value_unknown_variant =
+            sim_value_unknown_variant_name
+                .as_ref()
+                .map(|unknown_variant_name| {
+                    Pair::End(parse_quote_spanned! {span=>
+                        #unknown_variant_name(::fayalite::enum_::UnknownVariantSimValue)
+                    })
+                });
+        ItemEnum {
+            attrs: enum_attrs,
+            vis: vis.clone(),
+            enum_token: *enum_token,
+            ident: sim_value_ident.clone(),
+            generics: generics.into(),
+            brace_token: *brace_token,
+            variants: Punctuated::from_iter(
+                variants
+                    .pairs()
+                    .map_pair_value_ref(
+                        |ParsedVariant {
+                             attrs,
+                             options: _,
+                             ident,
+                             field,
+                         }| Variant {
+                            attrs: attrs.clone(),
+                            ident: ident.clone(),
+                            fields: match field {
+                                Some(ParsedVariantField {
+                                    paren_token,
+                                    attrs,
+                                    options: _,
+                                    ty,
+                                    comma_token,
+                                }) => Fields::Unnamed(FieldsUnnamed {
+                                    paren_token: *paren_token,
+                                    unnamed: Punctuated::from_iter([
+                                        Pair::new(
+                                            Field {
+                                                attrs: attrs.clone(),
+                                                vis: Visibility::Inherited,
+                                                mutability: FieldMutability::None,
+                                                ident: None,
+                                                colon_token: None,
+                                                ty: parse_quote_spanned! {span=>
+                                                    ::fayalite::sim::value::SimValue<#ty>
+                                                },
+                                            },
+                                            Some(comma_token.unwrap_or(Token![,](ident.span()))),
+                                        ),
+                                        Pair::new(
+                                            Field {
+                                                attrs: vec![],
+                                                vis: Visibility::Inherited,
+                                                mutability: FieldMutability::None,
+                                                ident: None,
+                                                colon_token: None,
+                                                ty: parse_quote_spanned! {span=>
+                                                    ::fayalite::enum_::EnumPaddingSimValue
+                                                },
+                                            },
+                                            None,
+                                        ),
+                                    ]),
+                                }),
+                                None => Fields::Unnamed(parse_quote_spanned! {span=>
+                                    (::fayalite::enum_::EnumPaddingSimValue)
+                                }),
+                            },
+                            discriminant: None,
+                        },
+                    )
+                    .chain(sim_value_unknown_variant),
+            ),
+        }
+        .to_tokens(tokens);
         let self_token = Token![self](span);
         for (index, ParsedVariant { ident, field, .. }) in variants.iter().enumerate() {
             if let Some(ParsedVariantField { ty, .. }) = field {
@@ -534,6 +637,142 @@ impl ToTokens for ParsedEnum {
                 }
             },
         ));
+        let sim_value_from_bits_unknown_match_arm = if let Some(sim_value_unknown_variant_name) =
+            &sim_value_unknown_variant_name
+        {
+            quote_spanned! {span=>
+                _ => #sim_value_ident::#sim_value_unknown_variant_name(v.unknown_variant_from_bits()),
+            }
+        } else {
+            quote_spanned! {span=>
+                _ => ::fayalite::__std::unreachable!(),
+            }
+        };
+        let sim_value_from_bits_match_arms = Vec::from_iter(
+            variants
+                .iter()
+                .enumerate()
+                .map(
+                    |(
+                        index,
+                        ParsedVariant {
+                            attrs: _,
+                            options: _,
+                            ident,
+                            field,
+                        },
+                    )| {
+                        if let Some(_) = field {
+                            quote_spanned! {span=>
+                                #index => {
+                                    let (field, padding) = v.variant_with_field_from_bits();
+                                    #sim_value_ident::#ident(field, padding)
+                                }
+                            }
+                        } else {
+                            quote_spanned! {span=>
+                                #index => #sim_value_ident::#ident(
+                                    v.variant_no_field_from_bits(),
+                                ),
+                            }
+                        }
+                    },
+                )
+                .chain([sim_value_from_bits_unknown_match_arm]),
+        );
+        let sim_value_clone_from_bits_unknown_match_arm =
+            if let Some(sim_value_unknown_variant_name) = &sim_value_unknown_variant_name {
+                quote_spanned! {span=>
+                    _ => if let #sim_value_ident::#sim_value_unknown_variant_name(value) = value {
+                        v.unknown_variant_clone_from_bits(value);
+                    } else {
+                        *value = #sim_value_ident::#sim_value_unknown_variant_name(
+                            v.unknown_variant_from_bits(),
+                        );
+                    },
+                }
+            } else {
+                quote_spanned! {span=>
+                    _ => ::fayalite::__std::unreachable!(),
+                }
+            };
+        let sim_value_clone_from_bits_match_arms = Vec::from_iter(
+            variants
+                .iter()
+                .enumerate()
+                .map(
+                    |(
+                        index,
+                        ParsedVariant {
+                            attrs: _,
+                            options: _,
+                            ident,
+                            field,
+                        },
+                    )| {
+                        if let Some(_) = field {
+                            quote_spanned! {span=>
+                                #index => if let #sim_value_ident::#ident(field, padding) = value {
+                                    v.variant_with_field_clone_from_bits(field, padding);
+                                } else {
+                                    let (field, padding) = v.variant_with_field_from_bits();
+                                    *value = #sim_value_ident::#ident(field, padding);
+                                },
+                            }
+                        } else {
+                            quote_spanned! {span=>
+                                #index => if let #sim_value_ident::#ident(padding) = value {
+                                    v.variant_no_field_clone_from_bits(padding);
+                                } else {
+                                    *value = #sim_value_ident::#ident(
+                                        v.variant_no_field_from_bits(),
+                                    );
+                                },
+                            }
+                        }
+                    },
+                )
+                .chain([sim_value_clone_from_bits_unknown_match_arm]),
+        );
+        let sim_value_to_bits_match_arms = Vec::from_iter(
+            variants
+                .iter()
+                .enumerate()
+                .map(
+                    |(
+                        index,
+                        ParsedVariant {
+                            attrs: _,
+                            options: _,
+                            ident,
+                            field,
+                        },
+                    )| {
+                        if let Some(_) = field {
+                            quote_spanned! {span=>
+                                #sim_value_ident::#ident(field, padding) => {
+                                    v.variant_with_field_to_bits(#index, field, padding);
+                                }
+                            }
+                        } else {
+                            quote_spanned! {span=>
+                                #sim_value_ident::#ident(padding) => {
+                                    v.variant_no_field_to_bits(#index, padding);
+                                }
+                            }
+                        }
+                    },
+                )
+                .chain(sim_value_unknown_variant_name.as_ref().map(
+                    |sim_value_unknown_variant_name| {
+                        quote_spanned! {span=>
+                            #sim_value_ident::#sim_value_unknown_variant_name(value) => {
+                                v.unknown_variant_to_bits(value);
+                            }
+                        }
+                    },
+                )),
+        );
         let variants_len = variants.len();
         quote_spanned! {span=>
             #[automatically_derived]
@@ -542,6 +781,7 @@ impl ToTokens for ParsedEnum {
             {
                 type BaseType = ::fayalite::enum_::Enum;
                 type MaskType = ::fayalite::int::Bool;
+                type SimValue = #sim_value_ident #type_generics;
                 type MatchVariant = #match_variant_ident #type_generics;
                 type MatchActiveScope = ::fayalite::module::Scope;
                 type MatchVariantAndInactiveScope = ::fayalite::enum_::EnumMatchVariantAndInactiveScope<Self>;
@@ -574,6 +814,35 @@ impl ToTokens for ParsedEnum {
                 fn source_location() -> ::fayalite::source_location::SourceLocation {
                     ::fayalite::source_location::SourceLocation::caller()
                 }
+                fn sim_value_from_bits(
+                    &self,
+                    bits: &::fayalite::__bitvec::slice::BitSlice,
+                ) -> <Self as ::fayalite::ty::Type>::SimValue {
+                    let v = ::fayalite::enum_::EnumSimValueFromBits::new(*self, bits);
+                    match v.discriminant() {
+                        #(#sim_value_from_bits_match_arms)*
+                    }
+                }
+                fn sim_value_clone_from_bits(
+                    &self,
+                    value: &mut <Self as ::fayalite::ty::Type>::SimValue,
+                    bits: &::fayalite::__bitvec::slice::BitSlice,
+                ) {
+                    let v = ::fayalite::enum_::EnumSimValueFromBits::new(*self, bits);
+                    match v.discriminant() {
+                        #(#sim_value_clone_from_bits_match_arms)*
+                    }
+                }
+                fn sim_value_to_bits(
+                    &self,
+                    value: &<Self as ::fayalite::ty::Type>::SimValue,
+                    bits: &mut ::fayalite::__bitvec::slice::BitSlice,
+                ) {
+                    let v = ::fayalite::enum_::EnumSimValueToBits::new(*self, bits);
+                    match value {
+                        #(#sim_value_to_bits_match_arms)*
+                    }
+                }
             }
             #[automatically_derived]
             impl #impl_generics ::fayalite::enum_::EnumType for #target #type_generics

crates/fayalite/src/array.rs

@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: LGPL-3.0-or-later
 // See Notices.txt for copyright information
 
+use bitvec::slice::BitSlice;
+
 use crate::{
     expr::{
         ops::{ArrayLiteral, ExprFromIterator, ExprIntoIterator, ExprPartialEq},
@@ -9,6 +11,7 @@ use crate::{
     int::{Bool, DynSize, KnownSize, Size, SizeType, DYN_SIZE},
     intern::{Intern, Interned, LazyInterned},
     module::transform::visit::{Fold, Folder, Visit, Visitor},
+    sim::value::{SimValue, SimValuePartialEq},
     source_location::SourceLocation,
     ty::{
         CanonicalType, MatchVariantWithoutScope, StaticType, Type, TypeProperties, TypeWithDeref,
@@ -142,6 +145,7 @@ impl<T: Type + Visit<State>, Len: Size, State: Visitor + ?Sized> Visit<State>
 impl<T: Type, Len: Size> Type for ArrayType<T, Len> {
     type BaseType = Array;
     type MaskType = ArrayType<T::MaskType, Len>;
+    type SimValue = Len::ArraySimValue<T>;
     type MatchVariant = Len::ArrayMatch<T>;
     type MatchActiveScope = ();
     type MatchVariantAndInactiveScope = MatchVariantWithoutScope<Len::ArrayMatch<T>>;
@@ -178,9 +182,48 @@ impl<T: Type, Len: Size> Type for ArrayType<T, Len> {
             Len::from_usize(array.len()),
         )
     }
+
     fn source_location() -> SourceLocation {
         SourceLocation::builtin()
     }
+
+    fn sim_value_from_bits(&self, bits: &BitSlice) -> Self::SimValue {
+        assert_eq!(bits.len(), self.type_properties.bit_width);
+        let element = self.element();
+        let element_bit_width = element.canonical().bit_width();
+        TryFrom::try_from(Vec::from_iter((0..self.len()).map(|i| {
+            SimValue::from_bitslice(element, &bits[i * element_bit_width..][..element_bit_width])
+        })))
+        .ok()
+        .expect("used correct length")
+    }
+
+    fn sim_value_clone_from_bits(&self, value: &mut Self::SimValue, bits: &BitSlice) {
+        assert_eq!(bits.len(), self.type_properties.bit_width);
+        let element_ty = self.element();
+        let element_bit_width = element_ty.canonical().bit_width();
+        let value: &mut [SimValue<T>] = value.as_mut();
+        assert_eq!(self.len(), value.len());
+        for (i, element_value) in value.iter_mut().enumerate() {
+            assert_eq!(SimValue::ty(element_value), element_ty);
+            SimValue::bits_mut(element_value)
+                .bits_mut()
+                .copy_from_bitslice(&bits[i * element_bit_width..][..element_bit_width]);
+        }
+    }
+
+    fn sim_value_to_bits(&self, value: &Self::SimValue, bits: &mut BitSlice) {
+        assert_eq!(bits.len(), self.type_properties.bit_width);
+        let element_ty = self.element();
+        let element_bit_width = element_ty.canonical().bit_width();
+        let value: &[SimValue<T>] = value.as_ref();
+        assert_eq!(self.len(), value.len());
+        for (i, element_value) in value.iter().enumerate() {
+            assert_eq!(SimValue::ty(element_value), element_ty);
+            bits[i * element_bit_width..][..element_bit_width]
+                .copy_from_bitslice(SimValue::bits(element_value).bits());
+        }
+    }
 }
 
 impl<T: Type, Len: Size> TypeWithDeref for ArrayType<T, Len> {
@@ -247,6 +290,18 @@ where
     }
 }
 
+impl<Lhs: Type, Rhs: Type, Len: Size> SimValuePartialEq<ArrayType<Rhs, Len>> for ArrayType<Lhs, Len>
+where
+    Lhs: SimValuePartialEq<Rhs>,
+{
+    fn sim_value_eq(this: &SimValue<Self>, other: &SimValue<ArrayType<Rhs, Len>>) -> bool {
+        AsRef::<[_]>::as_ref(&**this)
+            .iter()
+            .zip(AsRef::<[_]>::as_ref(&**other))
+            .all(|(l, r)| SimValuePartialEq::sim_value_eq(l, r))
+    }
+}
+
 impl<T: Type, Len: Size> ExprIntoIterator for ArrayType<T, Len> {
     type Item = T;
     type ExprIntoIter = ExprArrayIter<T, Len>;

crates/fayalite/src/bundle.rs

@@ -8,14 +8,14 @@ use crate::{
     },
     int::{Bool, DynSize},
     intern::{Intern, Interned},
-    sim::{SimValue, ToSimValue},
+    sim::value::{SimValue, SimValuePartialEq, ToSimValue},
     source_location::SourceLocation,
     ty::{
-        impl_match_variant_as_self, CanonicalType, MatchVariantWithoutScope, StaticType, Type,
+        impl_match_variant_as_self, CanonicalType, MatchVariantWithoutScope, OpaqueSimValue,
-        TypeProperties, TypeWithDeref,
+        StaticType, Type, TypeProperties, TypeWithDeref,
     },
 };
-use bitvec::vec::BitVec;
+use bitvec::{slice::BitSlice, vec::BitVec};
 use hashbrown::HashMap;
 use std::{fmt, marker::PhantomData};
 
@@ -216,6 +216,7 @@ impl Bundle {
 impl Type for Bundle {
     type BaseType = Bundle;
     type MaskType = Bundle;
+    type SimValue = OpaqueSimValue;
     impl_match_variant_as_self!();
     fn mask_type(&self) -> Self::MaskType {
         Self::new(Interned::from_iter(self.0.fields.into_iter().map(
@@ -239,6 +240,20 @@ impl Type for Bundle {
     fn source_location() -> SourceLocation {
         SourceLocation::builtin()
     }
+    fn sim_value_from_bits(&self, bits: &BitSlice) -> Self::SimValue {
+        assert_eq!(bits.len(), self.type_properties().bit_width);
+        OpaqueSimValue::from_bitslice(bits)
+    }
+    fn sim_value_clone_from_bits(&self, value: &mut Self::SimValue, bits: &BitSlice) {
+        assert_eq!(bits.len(), self.type_properties().bit_width);
+        assert_eq!(value.bit_width(), self.type_properties().bit_width);
+        value.bits_mut().bits_mut().copy_from_bitslice(bits);
+    }
+    fn sim_value_to_bits(&self, value: &Self::SimValue, bits: &mut BitSlice) {
+        assert_eq!(bits.len(), self.type_properties().bit_width);
+        assert_eq!(value.bit_width(), self.type_properties().bit_width);
+        bits.copy_from_bitslice(value.bits().bits());
+    }
 }
 
 pub trait BundleType: Type<BaseType = Bundle> {
@@ -247,6 +262,93 @@ pub trait BundleType: Type<BaseType = Bundle> {
     fn fields(&self) -> Interned<[BundleField]>;
 }
 
+pub struct BundleSimValueFromBits<'a> {
+    fields: std::slice::Iter<'static, BundleField>,
+    bits: &'a BitSlice,
+}
+
+impl<'a> BundleSimValueFromBits<'a> {
+    #[track_caller]
+    pub fn new<T: BundleType>(bundle_ty: T, bits: &'a BitSlice) -> Self {
+        let fields = bundle_ty.fields();
+        assert_eq!(
+            bits.len(),
+            fields
+                .iter()
+                .map(|BundleField { ty, .. }| ty.bit_width())
+                .sum::<usize>()
+        );
+        Self {
+            fields: Interned::into_inner(fields).iter(),
+            bits,
+        }
+    }
+    #[track_caller]
+    fn field_ty_and_bits<T: Type>(&mut self) -> (T, &'a BitSlice) {
+        let Some(&BundleField {
+            name: _,
+            flipped: _,
+            ty,
+        }) = self.fields.next()
+        else {
+            panic!("tried to read too many fields from BundleSimValueFromBits");
+        };
+        let (field_bits, rest) = self.bits.split_at(ty.bit_width());
+        self.bits = rest;
+        (T::from_canonical(ty), field_bits)
+    }
+    #[track_caller]
+    pub fn field_from_bits<T: Type>(&mut self) -> SimValue<T> {
+        let (field_ty, field_bits) = self.field_ty_and_bits::<T>();
+        SimValue::from_bitslice(field_ty, field_bits)
+    }
+    #[track_caller]
+    pub fn field_clone_from_bits<T: Type>(&mut self, field_value: &mut SimValue<T>) {
+        let (field_ty, field_bits) = self.field_ty_and_bits::<T>();
+        assert_eq!(field_ty, SimValue::ty(field_value));
+        SimValue::bits_mut(field_value)
+            .bits_mut()
+            .copy_from_bitslice(field_bits);
+    }
+}
+
+pub struct BundleSimValueToBits<'a> {
+    fields: std::slice::Iter<'static, BundleField>,
+    bits: &'a mut BitSlice,
+}
+
+impl<'a> BundleSimValueToBits<'a> {
+    #[track_caller]
+    pub fn new<T: BundleType>(bundle_ty: T, bits: &'a mut BitSlice) -> Self {
+        let fields = bundle_ty.fields();
+        assert_eq!(
+            bits.len(),
+            fields
+                .iter()
+                .map(|BundleField { ty, .. }| ty.bit_width())
+                .sum::<usize>()
+        );
+        Self {
+            fields: Interned::into_inner(fields).iter(),
+            bits,
+        }
+    }
+    #[track_caller]
+    pub fn field_to_bits<T: Type>(&mut self, field_value: &SimValue<T>) {
+        let Some(&BundleField {
+            name: _,
+            flipped: _,
+            ty,
+        }) = self.fields.next()
+        else {
+            panic!("tried to read too many fields from BundleSimValueFromBits");
+        };
+        assert_eq!(T::from_canonical(ty), SimValue::ty(field_value));
+        self.bits[..ty.bit_width()].copy_from_bitslice(SimValue::bits(field_value).bits());
+        self.bits = &mut std::mem::take(&mut self.bits)[ty.bit_width()..];
+    }
+}
+
 #[derive(Default)]
 pub struct NoBuilder;
 
@@ -353,6 +455,7 @@ macro_rules! impl_tuples {
         impl<$($T: Type,)*> Type for ($($T,)*) {
             type BaseType = Bundle;
             type MaskType = ($($T::MaskType,)*);
+            type SimValue = ($(SimValue<$T>,)*);
             type MatchVariant = ($(Expr<$T>,)*);
             type MatchActiveScope = ();
             type MatchVariantAndInactiveScope = MatchVariantWithoutScope<Self::MatchVariant>;
@@ -391,6 +494,24 @@ macro_rules! impl_tuples {
             fn source_location() -> SourceLocation {
                 SourceLocation::builtin()
             }
+            fn sim_value_from_bits(&self, bits: &BitSlice) -> Self::SimValue {
+                #![allow(unused_mut, unused_variables)]
+                let mut v = BundleSimValueFromBits::new(*self, bits);
+                $(let $var = v.field_from_bits();)*
+                ($($var,)*)
+            }
+            fn sim_value_clone_from_bits(&self, value: &mut Self::SimValue, bits: &BitSlice) {
+                #![allow(unused_mut, unused_variables)]
+                let mut v = BundleSimValueFromBits::new(*self, bits);
+                let ($($var,)*) = value;
+                $(v.field_clone_from_bits($var);)*
+            }
+            fn sim_value_to_bits(&self, value: &Self::SimValue, bits: &mut BitSlice) {
+                #![allow(unused_mut, unused_variables)]
+                let mut v = BundleSimValueToBits::new(*self, bits);
+                let ($($var,)*) = value;
+                $(v.field_to_bits($var);)*
+            }
         }
         impl<$($T: Type,)*> BundleType for ($($T,)*) {
             type Builder = TupleBuilder<($(Unfilled<$T>,)*)>;
@@ -444,16 +565,12 @@ macro_rules! impl_tuples {
         impl<$($T: ToSimValue<CanonicalType>,)*> ToSimValue<CanonicalType> for ($($T,)*) {
             #[track_caller]
             fn to_sim_value(&self, ty: CanonicalType) -> SimValue<CanonicalType> {
-                ToSimValue::<Bundle>::to_sim_value(self, Bundle::from_canonical(ty)).into_canonical()
+                SimValue::into_canonical(ToSimValue::<Bundle>::to_sim_value(self, Bundle::from_canonical(ty)))
             }
             #[track_caller]
             fn into_sim_value(self, ty: CanonicalType) -> SimValue<CanonicalType>
             {
-                ToSimValue::<Bundle>::into_sim_value(self, Bundle::from_canonical(ty)).into_canonical()
+                SimValue::into_canonical(ToSimValue::<Bundle>::into_sim_value(self, Bundle::from_canonical(ty)))
-            }
-            #[track_caller]
-            fn box_into_sim_value(self: Box<Self>, ty: CanonicalType) -> SimValue<CanonicalType> {
-                ToSimValue::<Bundle>::box_into_sim_value(self, Bundle::from_canonical(ty)).into_canonical()
             }
         }
         impl<$($T: ToSimValue<CanonicalType>,)*> ToSimValue<Bundle> for ($($T,)*) {
@@ -474,24 +591,12 @@ macro_rules! impl_tuples {
                 let [$($ty_var,)*] = *ty.fields() else {
                     panic!("bundle has wrong number of fields");
                 };
-                let mut bits: Option<BitVec> = None;
+                let mut bits = BitVec::new();
                 $(let $var = $var.into_sim_value($ty_var.ty);
-                assert_eq!($var.ty(), $ty_var.ty);
+                assert_eq!(SimValue::ty(&$var), $ty_var.ty);
-                if !$var.bits().is_empty() {
+                bits.extend_from_bitslice(SimValue::bits(&$var).bits());
-                    if let Some(bits) = &mut bits {
-                        bits.extend_from_bitslice($var.bits());
-                    } else {
-                        let mut $var = $var.into_bits();
-                        $var.reserve(ty.type_properties().bit_width - $var.len());
-                        bits = Some($var);
-                    }
-                }
                 )*
-                bits.unwrap_or_else(BitVec::new).into_sim_value(ty)
+                bits.into_sim_value(ty)
-            }
-            #[track_caller]
-            fn box_into_sim_value(self: Box<Self>, ty: Bundle) -> SimValue<Bundle> {
-                Self::into_sim_value(*self, ty)
             }
         }
         impl<$($T: ToSimValue<$Ty>, $Ty: Type,)*> ToSimValue<($($Ty,)*)> for ($($T,)*) {
@@ -499,19 +604,15 @@ macro_rules! impl_tuples {
             fn to_sim_value(&self, ty: ($($Ty,)*)) -> SimValue<($($Ty,)*)> {
                 let ($($var,)*) = self;
                 let ($($ty_var,)*) = ty;
-                $(let $var = $var.to_sim_value($ty_var).into_canonical();)*
+                $(let $var = $var.to_sim_value($ty_var);)*
-                SimValue::from_canonical(ToSimValue::into_sim_value(($($var,)*), ty.canonical()))
+                SimValue::from_value(ty, ($($var,)*))
             }
             #[track_caller]
             fn into_sim_value(self, ty: ($($Ty,)*)) -> SimValue<($($Ty,)*)> {
                 let ($($var,)*) = self;
                 let ($($ty_var,)*) = ty;
-                $(let $var = $var.into_sim_value($ty_var).into_canonical();)*
+                $(let $var = $var.into_sim_value($ty_var);)*
-                SimValue::from_canonical(ToSimValue::into_sim_value(($($var,)*), ty.canonical()))
+                SimValue::from_value(ty, ($($var,)*))
-            }
-            #[track_caller]
-            fn box_into_sim_value(self: Box<Self>, ty: ($($Ty,)*)) -> SimValue<($($Ty,)*)> {
-                Self::into_sim_value(*self, ty)
             }
         }
         impl<$($Lhs: Type + ExprPartialEq<$Rhs>, $Rhs: Type,)*> ExprPartialEq<($($Rhs,)*)> for ($($Lhs,)*) {
@@ -537,6 +638,15 @@ macro_rules! impl_tuples {
                 .any_one_bits()
             }
         }
+        impl<$($Lhs: SimValuePartialEq<$Rhs>, $Rhs: Type,)*> SimValuePartialEq<($($Rhs,)*)> for ($($Lhs,)*) {
+            fn sim_value_eq(lhs: &SimValue<Self>, rhs: &SimValue<($($Rhs,)*)>) -> bool {
+                let ($($lhs_var,)*) = &**lhs;
+                let ($($rhs_var,)*) = &**rhs;
+                let retval = true;
+                $(let retval = retval && $lhs_var == $rhs_var;)*
+                retval
+            }
+        }
     };
     ([$($lhs:tt)*] [$rhs_first:tt $($rhs:tt)*]) => {
         impl_tuples!([$($lhs)*] []);
@@ -564,6 +674,7 @@ impl_tuples! {
 impl<T: ?Sized + Send + Sync + 'static> Type for PhantomData<T> {
     type BaseType = Bundle;
     type MaskType = ();
+    type SimValue = ();
     type MatchVariant = PhantomData<T>;
     type MatchActiveScope = ();
     type MatchVariantAndInactiveScope = MatchVariantWithoutScope<Self::MatchVariant>;
@@ -596,6 +707,16 @@ impl<T: ?Sized + Send + Sync + 'static> Type for PhantomData<T> {
     fn source_location() -> SourceLocation {
         SourceLocation::builtin()
     }
+    fn sim_value_from_bits(&self, bits: &BitSlice) -> Self::SimValue {
+        assert!(bits.is_empty());
+        ()
+    }
+    fn sim_value_clone_from_bits(&self, _value: &mut Self::SimValue, bits: &BitSlice) {
+        assert!(bits.is_empty());
+    }
+    fn sim_value_to_bits(&self, _value: &Self::SimValue, bits: &mut BitSlice) {
+        assert!(bits.is_empty());
+    }
 }
 
 pub struct PhantomDataBuilder<T: ?Sized + Send + Sync + 'static>(PhantomData<T>);
@@ -663,6 +784,6 @@ impl<T: ?Sized> ToSimValue<CanonicalType> for PhantomData<T> {
     fn to_sim_value(&self, ty: CanonicalType) -> SimValue<CanonicalType> {
         let ty = Bundle::from_canonical(ty);
         assert!(ty.fields().is_empty());
-        ToSimValue::into_sim_value(BitVec::new(), ty).into_canonical()
+        SimValue::into_canonical(ToSimValue::into_sim_value(BitVec::new(), ty))
     }
 }

crates/fayalite/src/clock.rs

@@ -8,6 +8,7 @@ use crate::{
     source_location::SourceLocation,
     ty::{impl_match_variant_as_self, CanonicalType, StaticType, Type, TypeProperties},
 };
+use bitvec::slice::BitSlice;
 
 #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug, Default)]
 pub struct Clock;
@@ -15,6 +16,7 @@ pub struct Clock;
 impl Type for Clock {
     type BaseType = Clock;
     type MaskType = Bool;
+    type SimValue = bool;
 
     impl_match_variant_as_self!();
 
@@ -36,6 +38,21 @@ impl Type for Clock {
         };
         retval
     }
+
+    fn sim_value_from_bits(&self, bits: &BitSlice) -> Self::SimValue {
+        assert_eq!(bits.len(), 1);
+        bits[0]
+    }
+
+    fn sim_value_clone_from_bits(&self, value: &mut Self::SimValue, bits: &BitSlice) {
+        assert_eq!(bits.len(), 1);
+        *value = bits[0];
+    }
+
+    fn sim_value_to_bits(&self, value: &Self::SimValue, bits: &mut BitSlice) {
+        assert_eq!(bits.len(), 1);
+        bits.set(0, *value);
+    }
 }
 
 impl Clock {

crates/fayalite/src/enum_.rs

@@ -7,17 +7,22 @@ use crate::{
         Expr, ToExpr,
     },
     hdl,
-    int::Bool,
+    int::{Bool, UIntValue},
     intern::{Intern, Interned},
     module::{
         connect, enum_match_variants_helper, incomplete_wire, wire,
         EnumMatchVariantAndInactiveScopeImpl, EnumMatchVariantsIterImpl, Scope,
     },
+    sim::value::{SimValue, SimValuePartialEq},
     source_location::SourceLocation,
-    ty::{CanonicalType, MatchVariantAndInactiveScope, StaticType, Type, TypeProperties},
+    ty::{
+        CanonicalType, MatchVariantAndInactiveScope, OpaqueSimValue, StaticType, Type,
+        TypeProperties,
+    },
 };
+use bitvec::{order::Lsb0, slice::BitSlice, view::BitView};
 use hashbrown::HashMap;
-use std::{convert::Infallible, fmt, iter::FusedIterator};
+use std::{convert::Infallible, fmt, iter::FusedIterator, sync::Arc};
 
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
 pub struct EnumVariant {
@@ -152,6 +157,12 @@ impl EnumTypePropertiesBuilder {
             variant_count: variant_count + 1,
         }
     }
+    #[must_use]
+    pub fn variants(self, variants: impl IntoIterator<Item = EnumVariant>) -> Self {
+        variants.into_iter().fold(self, |this, variant| {
+            this.variant(variant.ty.map(CanonicalType::type_properties))
+        })
+    }
     pub const fn finish(self) -> TypeProperties {
         assert!(
             self.variant_count != 0,
@@ -325,6 +336,7 @@ impl EnumType for Enum {
 impl Type for Enum {
     type BaseType = Enum;
     type MaskType = Bool;
+    type SimValue = OpaqueSimValue;
     type MatchVariant = Option<Expr<CanonicalType>>;
     type MatchActiveScope = Scope;
     type MatchVariantAndInactiveScope = EnumMatchVariantAndInactiveScope<Self>;
@@ -355,6 +367,296 @@ impl Type for Enum {
     fn source_location() -> SourceLocation {
         SourceLocation::builtin()
     }
+    fn sim_value_from_bits(&self, bits: &BitSlice) -> Self::SimValue {
+        assert_eq!(bits.len(), self.type_properties().bit_width);
+        OpaqueSimValue::from_bitslice(bits)
+    }
+    fn sim_value_clone_from_bits(&self, value: &mut Self::SimValue, bits: &BitSlice) {
+        assert_eq!(bits.len(), self.type_properties().bit_width);
+        assert_eq!(value.bit_width(), self.type_properties().bit_width);
+        value.bits_mut().bits_mut().copy_from_bitslice(bits);
+    }
+    fn sim_value_to_bits(&self, value: &Self::SimValue, bits: &mut BitSlice) {
+        assert_eq!(bits.len(), self.type_properties().bit_width);
+        assert_eq!(value.bit_width(), self.type_properties().bit_width);
+        bits.copy_from_bitslice(value.bits().bits());
+    }
+}
+
+#[derive(Clone, PartialEq, Eq, Hash, Debug, Default)]
+pub struct EnumPaddingSimValue {
+    bits: Option<UIntValue>,
+}
+
+impl EnumPaddingSimValue {
+    pub fn bit_width(&self) -> Option<usize> {
+        self.bits.as_ref().map(UIntValue::width)
+    }
+    pub fn bits(&self) -> &Option<UIntValue> {
+        &self.bits
+    }
+    pub fn bits_mut(&mut self) -> &mut Option<UIntValue> {
+        &mut self.bits
+    }
+    pub fn into_bits(self) -> Option<UIntValue> {
+        self.bits
+    }
+    pub fn from_bits(bits: Option<UIntValue>) -> Self {
+        Self { bits }
+    }
+    pub fn from_bitslice(v: &BitSlice) -> Self {
+        Self {
+            bits: Some(UIntValue::new(Arc::new(v.to_bitvec()))),
+        }
+    }
+}
+
+#[derive(Clone, PartialEq, Eq, Hash, Debug)]
+pub struct UnknownVariantSimValue {
+    discriminant: usize,
+    body_bits: UIntValue,
+}
+
+impl UnknownVariantSimValue {
+    pub fn discriminant(&self) -> usize {
+        self.discriminant
+    }
+    pub fn body_bits(&self) -> &UIntValue {
+        &self.body_bits
+    }
+    pub fn body_bits_mut(&mut self) -> &mut UIntValue {
+        &mut self.body_bits
+    }
+    pub fn into_body_bits(self) -> UIntValue {
+        self.body_bits
+    }
+    pub fn into_parts(self) -> (usize, UIntValue) {
+        (self.discriminant, self.body_bits)
+    }
+    pub fn new(discriminant: usize, body_bits: UIntValue) -> Self {
+        Self {
+            discriminant,
+            body_bits,
+        }
+    }
+}
+
+pub struct EnumSimValueFromBits<'a> {
+    variants: Interned<[EnumVariant]>,
+    discriminant: usize,
+    body_bits: &'a BitSlice,
+}
+
+impl<'a> EnumSimValueFromBits<'a> {
+    #[track_caller]
+    pub fn new<T: EnumType>(ty: T, bits: &'a BitSlice) -> Self {
+        let variants = ty.variants();
+        let bit_width = EnumTypePropertiesBuilder::new()
+            .variants(variants)
+            .finish()
+            .bit_width;
+        assert_eq!(bit_width, bits.len());
+        let (discriminant_bits, body_bits) =
+            bits.split_at(discriminant_bit_width_impl(variants.len()));
+        let mut discriminant = 0usize;
+        discriminant.view_bits_mut::<Lsb0>()[..discriminant_bits.len()]
+            .copy_from_bitslice(discriminant_bits);
+        Self {
+            variants,
+            discriminant,
+            body_bits,
+        }
+    }
+    pub fn discriminant(&self) -> usize {
+        self.discriminant
+    }
+    #[track_caller]
+    #[cold]
+    fn usage_error(&self, clone: bool) -> ! {
+        let clone = if clone { "clone_" } else { "" };
+        match self.variants.get(self.discriminant) {
+            None => {
+                panic!("should have called EnumSimValueFromBits::unknown_variant_{clone}from_bits");
+            }
+            Some(EnumVariant { ty: None, .. }) => {
+                panic!(
+                    "should have called EnumSimValueFromBits::variant_no_field_{clone}from_bits"
+                );
+            }
+            Some(EnumVariant { ty: Some(_), .. }) => {
+                panic!(
+                    "should have called EnumSimValueFromBits::variant_with_field_{clone}from_bits"
+                );
+            }
+        }
+    }
+    #[track_caller]
+    fn known_variant(&self, clone: bool) -> (Option<CanonicalType>, &'a BitSlice, &'a BitSlice) {
+        let Some(EnumVariant { ty, .. }) = self.variants.get(self.discriminant) else {
+            self.usage_error(clone);
+        };
+        let variant_bit_width = ty.map_or(0, CanonicalType::bit_width);
+        let (variant_bits, padding_bits) = self.body_bits.split_at(variant_bit_width);
+        (*ty, variant_bits, padding_bits)
+    }
+    #[track_caller]
+    pub fn unknown_variant_from_bits(self) -> UnknownVariantSimValue {
+        let None = self.variants.get(self.discriminant) else {
+            self.usage_error(false);
+        };
+        UnknownVariantSimValue::new(
+            self.discriminant,
+            UIntValue::new(Arc::new(self.body_bits.to_bitvec())),
+        )
+    }
+    #[track_caller]
+    pub fn unknown_variant_clone_from_bits(self, value: &mut UnknownVariantSimValue) {
+        let None = self.variants.get(self.discriminant) else {
+            self.usage_error(true);
+        };
+        value.discriminant = self.discriminant;
+        assert_eq!(value.body_bits.width(), self.body_bits.len());
+        value
+            .body_bits
+            .bits_mut()
+            .copy_from_bitslice(self.body_bits);
+    }
+    #[track_caller]
+    pub fn variant_no_field_from_bits(self) -> EnumPaddingSimValue {
+        let (None, _variant_bits, padding_bits) = self.known_variant(false) else {
+            self.usage_error(false);
+        };
+        EnumPaddingSimValue::from_bitslice(padding_bits)
+    }
+    #[track_caller]
+    pub fn variant_with_field_from_bits<T: Type>(self) -> (SimValue<T>, EnumPaddingSimValue) {
+        let (Some(variant_ty), variant_bits, padding_bits) = self.known_variant(false) else {
+            self.usage_error(false);
+        };
+        (
+            SimValue::from_bitslice(T::from_canonical(variant_ty), variant_bits),
+            EnumPaddingSimValue::from_bitslice(padding_bits),
+        )
+    }
+    #[track_caller]
+    fn clone_padding_from_bits(padding: &mut EnumPaddingSimValue, padding_bits: &BitSlice) {
+        match padding.bits_mut() {
+            None => *padding = EnumPaddingSimValue::from_bitslice(padding_bits),
+            Some(padding) => {
+                assert_eq!(padding.width(), padding_bits.len());
+                padding.bits_mut().copy_from_bitslice(padding_bits);
+            }
+        }
+    }
+    #[track_caller]
+    pub fn variant_no_field_clone_from_bits(self, padding: &mut EnumPaddingSimValue) {
+        let (None, _variant_bits, padding_bits) = self.known_variant(true) else {
+            self.usage_error(true);
+        };
+        Self::clone_padding_from_bits(padding, padding_bits);
+    }
+    #[track_caller]
+    pub fn variant_with_field_clone_from_bits<T: Type>(
+        self,
+        value: &mut SimValue<T>,
+        padding: &mut EnumPaddingSimValue,
+    ) {
+        let (Some(variant_ty), variant_bits, padding_bits) = self.known_variant(true) else {
+            self.usage_error(true);
+        };
+        assert_eq!(SimValue::ty(value), T::from_canonical(variant_ty));
+        SimValue::bits_mut(value)
+            .bits_mut()
+            .copy_from_bitslice(variant_bits);
+        Self::clone_padding_from_bits(padding, padding_bits);
+    }
+}
+
+pub struct EnumSimValueToBits<'a> {
+    variants: Interned<[EnumVariant]>,
+    bit_width: usize,
+    discriminant_bit_width: usize,
+    bits: &'a mut BitSlice,
+}
+
+impl<'a> EnumSimValueToBits<'a> {
+    #[track_caller]
+    pub fn new<T: EnumType>(ty: T, bits: &'a mut BitSlice) -> Self {
+        let variants = ty.variants();
+        let bit_width = EnumTypePropertiesBuilder::new()
+            .variants(variants)
+            .finish()
+            .bit_width;
+        assert_eq!(bit_width, bits.len());
+        Self {
+            variants,
+            bit_width,
+            discriminant_bit_width: discriminant_bit_width_impl(variants.len()),
+            bits,
+        }
+    }
+    #[track_caller]
+    fn discriminant_to_bits(&mut self, mut discriminant: usize) {
+        let orig_discriminant = discriminant;
+        let discriminant_bits =
+            &mut discriminant.view_bits_mut::<Lsb0>()[..self.discriminant_bit_width];
+        self.bits[..self.discriminant_bit_width].copy_from_bitslice(discriminant_bits);
+        discriminant_bits.fill(false);
+        assert!(
+            discriminant == 0,
+            "{orig_discriminant:#x} is too big to fit in enum discriminant bits",
+        );
+    }
+    #[track_caller]
+    pub fn unknown_variant_to_bits(mut self, value: &UnknownVariantSimValue) {
+        self.discriminant_to_bits(value.discriminant);
+        let None = self.variants.get(value.discriminant) else {
+            panic!("can't use UnknownVariantSimValue to set known discriminant");
+        };
+        assert_eq!(
+            self.bit_width - self.discriminant_bit_width,
+            value.body_bits.width()
+        );
+        self.bits[self.discriminant_bit_width..].copy_from_bitslice(value.body_bits.bits());
+    }
+    #[track_caller]
+    fn known_variant(
+        mut self,
+        discriminant: usize,
+        padding: &EnumPaddingSimValue,
+    ) -> (Option<CanonicalType>, &'a mut BitSlice) {
+        self.discriminant_to_bits(discriminant);
+        let variant_ty = self.variants[discriminant].ty;
+        let variant_bit_width = variant_ty.map_or(0, CanonicalType::bit_width);
+        let padding_bits = &mut self.bits[self.discriminant_bit_width..][variant_bit_width..];
+        if let Some(padding) = padding.bits() {
+            assert_eq!(padding.width(), padding_bits.len());
+            padding_bits.copy_from_bitslice(padding.bits());
+        } else {
+            padding_bits.fill(false);
+        }
+        let variant_bits = &mut self.bits[self.discriminant_bit_width..][..variant_bit_width];
+        (variant_ty, variant_bits)
+    }
+    #[track_caller]
+    pub fn variant_no_field_to_bits(self, discriminant: usize, padding: &EnumPaddingSimValue) {
+        let (None, _variant_bits) = self.known_variant(discriminant, padding) else {
+            panic!("expected variant to have no field");
+        };
+    }
+    #[track_caller]
+    pub fn variant_with_field_to_bits<T: Type>(
+        self,
+        discriminant: usize,
+        value: &SimValue<T>,
+        padding: &EnumPaddingSimValue,
+    ) {
+        let (Some(variant_ty), variant_bits) = self.known_variant(discriminant, padding) else {
+            panic!("expected variant to have a field");
+        };
+        assert_eq!(SimValue::ty(value), T::from_canonical(variant_ty));
+        variant_bits.copy_from_bitslice(SimValue::bits(value).bits());
+    }
 }
 
 #[hdl]
@@ -417,6 +719,25 @@ impl<Lhs: Type + ExprPartialEq<Rhs>, Rhs: Type> ExprPartialEq<HdlOption<Rhs>> fo
     }
 }
 
+impl<Lhs: SimValuePartialEq<Rhs>, Rhs: Type> SimValuePartialEq<HdlOption<Rhs>> for HdlOption<Lhs> {
+    fn sim_value_eq(this: &SimValue<Self>, other: &SimValue<HdlOption<Rhs>>) -> bool {
+        type SimValueMatch<T> = <T as Type>::SimValue;
+        match (&**this, &**other) {
+            (SimValueMatch::<Self>::HdlNone(_), SimValueMatch::<HdlOption<Rhs>>::HdlNone(_)) => {
+                true
+            }
+            (SimValueMatch::<Self>::HdlSome(..), SimValueMatch::<HdlOption<Rhs>>::HdlNone(_))
+            | (SimValueMatch::<Self>::HdlNone(_), SimValueMatch::<HdlOption<Rhs>>::HdlSome(..)) => {
+                false
+            }
+            (
+                SimValueMatch::<Self>::HdlSome(l, _),
+                SimValueMatch::<HdlOption<Rhs>>::HdlSome(r, _),
+            ) => l == r,
+        }
+    }
+}
+
 #[allow(non_snake_case)]
 pub fn HdlNone<T: StaticType>() -> Expr<HdlOption<T>> {
     HdlOption[T::TYPE].HdlNone()

crates/fayalite/src/expr.rs

@@ -700,6 +700,7 @@ impl<T: ToExpr + ?Sized> CastToBits for T {
 }
 
 pub trait CastBitsTo {
+    #[track_caller]
     fn cast_bits_to<T: Type>(&self, ty: T) -> Expr<T>;
 }
 

crates/fayalite/src/int.rs

@@ -7,6 +7,7 @@ use crate::{
         Expr, NotALiteralExpr, ToExpr, ToLiteralBits,
     },
     intern::{Intern, Interned, Memoize},
+    sim::value::SimValue,
     source_location::SourceLocation,
     ty::{impl_match_variant_as_self, CanonicalType, StaticType, Type, TypeProperties},
     util::{interned_bit, ConstBool, ConstUsize, GenericConstBool, GenericConstUsize},
@@ -49,6 +50,15 @@ pub trait KnownSize:
         + IntoIterator<Item = Expr<Element>>
         + TryFrom<Vec<Expr<Element>>>
         + Into<Vec<Expr<Element>>>;
+    type ArraySimValue<Element: Type>: AsRef<[SimValue<Element>]>
+        + AsMut<[SimValue<Element>]>
+        + BorrowMut<[SimValue<Element>]>
+        + 'static
+        + Clone
+        + std::fmt::Debug
+        + IntoIterator<Item = SimValue<Element>>
+        + TryFrom<Vec<SimValue<Element>>>
+        + Into<Vec<SimValue<Element>>>;
 }
 
 macro_rules! known_widths {
@@ -60,6 +70,7 @@ macro_rules! known_widths {
         }> {
             const SIZE: Self = Self;
             type ArrayMatch<Element: Type> = [Expr<Element>; Self::VALUE];
+            type ArraySimValue<Element: Type> = [SimValue<Element>; Self::VALUE];
         }
     };
     ([2 $($rest:tt)*] $($bits:literal)+) => {
@@ -72,6 +83,7 @@ macro_rules! known_widths {
         impl KnownSize for ConstUsize<{2 $(* $rest)*}> {
             const SIZE: Self = Self;
             type ArrayMatch<Element: Type> = [Expr<Element>; Self::VALUE];
+            type ArraySimValue<Element: Type> = [SimValue<Element>; Self::VALUE];
         }
     };
 }
@@ -100,6 +112,15 @@ pub trait Size:
         + IntoIterator<Item = Expr<Element>>
         + TryFrom<Vec<Expr<Element>>>
         + Into<Vec<Expr<Element>>>;
+    type ArraySimValue<Element: Type>: AsRef<[SimValue<Element>]>
+        + AsMut<[SimValue<Element>]>
+        + BorrowMut<[SimValue<Element>]>
+        + 'static
+        + Clone
+        + std::fmt::Debug
+        + IntoIterator<Item = SimValue<Element>>
+        + TryFrom<Vec<SimValue<Element>>>
+        + Into<Vec<SimValue<Element>>>;
     const KNOWN_VALUE: Option<usize>;
     type SizeType: SizeType<Size = Self>
         + Copy
@@ -125,6 +146,7 @@ impl SizeType for usize {
 
 impl Size for DynSize {
     type ArrayMatch<Element: Type> = Box<[Expr<Element>]>;
+    type ArraySimValue<Element: Type> = Box<[SimValue<Element>]>;
     const KNOWN_VALUE: Option<usize> = None;
     type SizeType = usize;
 
@@ -147,6 +169,7 @@ impl<T: KnownSize> SizeType for T {
 
 impl<T: KnownSize> Size for T {
     type ArrayMatch<Element: Type> = <T as KnownSize>::ArrayMatch<Element>;
+    type ArraySimValue<Element: Type> = <T as KnownSize>::ArraySimValue<Element>;
 
     const KNOWN_VALUE: Option<usize> = Some(T::VALUE);
 
@@ -287,6 +310,7 @@ macro_rules! impl_int {
         impl<Width: Size> Type for $name<Width> {
             type BaseType = $pretty_name;
             type MaskType = Bool;
+            type SimValue = $value<Width>;
             impl_match_variant_as_self!();
             fn mask_type(&self) -> Self::MaskType {
                 Bool
@@ -306,6 +330,20 @@ macro_rules! impl_int {
             fn source_location() -> SourceLocation {
                 SourceLocation::builtin()
             }
+            fn sim_value_from_bits(&self, bits: &BitSlice) -> Self::SimValue {
+                assert_eq!(bits.len(), self.width());
+                $value::new(Arc::new(bits.to_bitvec()))
+            }
+            fn sim_value_clone_from_bits(&self, value: &mut Self::SimValue, bits: &BitSlice) {
+                assert_eq!(bits.len(), self.width());
+                assert_eq!(value.width(), self.width());
+                value.bits_mut().copy_from_bitslice(bits);
+            }
+            fn sim_value_to_bits(&self, value: &Self::SimValue, bits: &mut BitSlice) {
+                assert_eq!(bits.len(), self.width());
+                assert_eq!(value.width(), self.width());
+                bits.copy_from_bitslice(value.bits());
+            }
         }
 
         impl<Width: KnownSize> StaticType for $name<Width> {
@@ -331,7 +369,7 @@ macro_rules! impl_int {
             }
         }
 
-        #[derive(Clone, PartialEq, Eq, Hash)]
+        #[derive(Clone, Eq, Hash)]
         pub struct $value<Width: Size = DynSize> {
             bits: Arc<BitVec>,
             _phantom: PhantomData<Width>,
@@ -351,9 +389,15 @@ macro_rules! impl_int {
             }
         }
 
-        impl<Width: Size> PartialOrd for $value<Width> {
+        impl<LhsWidth: Size, RhsWidth: Size> PartialEq<$value<RhsWidth>> for $value<LhsWidth> {
-            fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+            fn eq(&self, other: &$value<RhsWidth>) -> bool {
-                Some(self.cmp(other))
+                self.to_bigint() == other.to_bigint()
+            }
+        }
+
+        impl<LhsWidth: Size, RhsWidth: Size> PartialOrd<$value<RhsWidth>> for $value<LhsWidth> {
+            fn partial_cmp(&self, other: &$value<RhsWidth>) -> Option<std::cmp::Ordering> {
+                Some(self.to_bigint().cmp(&other.to_bigint()))
             }
         }
 
@@ -401,6 +445,9 @@ macro_rules! impl_int {
             pub fn bits(&self) -> &Arc<BitVec> {
                 &self.bits
             }
+            pub fn bits_mut(&mut self) -> &mut BitSlice {
+                Arc::<BitVec>::make_mut(&mut self.bits)
+            }
         }
 
         impl<Width: Size> ToLiteralBits for $value<Width> {
@@ -748,6 +795,7 @@ impl Bool {
 impl Type for Bool {
     type BaseType = Bool;
     type MaskType = Bool;
+    type SimValue = bool;
     impl_match_variant_as_self!();
     fn mask_type(&self) -> Self::MaskType {
         Bool
@@ -765,6 +813,18 @@ impl Type for Bool {
     fn source_location() -> SourceLocation {
         SourceLocation::builtin()
     }
+    fn sim_value_from_bits(&self, bits: &BitSlice) -> Self::SimValue {
+        assert_eq!(bits.len(), 1);
+        bits[0]
+    }
+    fn sim_value_clone_from_bits(&self, value: &mut Self::SimValue, bits: &BitSlice) {
+        assert_eq!(bits.len(), 1);
+        *value = bits[0];
+    }
+    fn sim_value_to_bits(&self, value: &Self::SimValue, bits: &mut BitSlice) {
+        assert_eq!(bits.len(), 1);
+        bits.set(0, *value);
+    }
 }
 
 impl StaticType for Bool {

crates/fayalite/src/lib.rs

@@ -8,6 +8,8 @@
 
 extern crate self as fayalite;
 
+#[doc(hidden)]
+pub use bitvec as __bitvec;
 #[doc(hidden)]
 pub use std as __std;
 

crates/fayalite/src/phantom_const.rs

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: LGPL-3.0-or-later
 // See Notices.txt for copyright information
 
+use bitvec::slice::BitSlice;
 use serde::{de::DeserializeOwned, Deserialize, Serialize};
 
 use crate::{
@@ -10,6 +11,7 @@ use crate::{
     },
     int::Bool,
     intern::{Intern, Interned, InternedCompare, LazyInterned, Memoize},
+    sim::value::{SimValue, SimValuePartialEq},
     source_location::SourceLocation,
     ty::{impl_match_variant_as_self, CanonicalType, StaticType, Type, TypeProperties},
 };
@@ -246,6 +248,7 @@ impl<T: ?Sized + PhantomConstValue> PhantomConst<T> {
 impl<T: ?Sized + PhantomConstValue> Type for PhantomConst<T> {
     type BaseType = PhantomConst;
     type MaskType = ();
+    type SimValue = ();
     impl_match_variant_as_self!();
 
     fn mask_type(&self) -> Self::MaskType {
@@ -266,6 +269,19 @@ impl<T: ?Sized + PhantomConstValue> Type for PhantomConst<T> {
     fn source_location() -> SourceLocation {
         SourceLocation::builtin()
     }
+
+    fn sim_value_from_bits(&self, bits: &BitSlice) -> Self::SimValue {
+        assert!(bits.is_empty());
+        ()
+    }
+
+    fn sim_value_clone_from_bits(&self, _value: &mut Self::SimValue, bits: &BitSlice) {
+        assert!(bits.is_empty());
+    }
+
+    fn sim_value_to_bits(&self, _value: &Self::SimValue, bits: &mut BitSlice) {
+        assert!(bits.is_empty());
+    }
 }
 
 impl<T: ?Sized + PhantomConstValue> StaticType for PhantomConst<T>
@@ -311,3 +327,10 @@ impl<T: ?Sized + PhantomConstValue> ExprPartialOrd<Self> for PhantomConst<T> {
         true.to_expr()
     }
 }
+
+impl<T: ?Sized + PhantomConstValue> SimValuePartialEq<Self> for PhantomConst<T> {
+    fn sim_value_eq(this: &SimValue<Self>, other: &SimValue<Self>) -> bool {
+        assert_eq!(SimValue::ty(this), SimValue::ty(other));
+        true
+    }
+}

crates/fayalite/src/reset.rs

@@ -7,6 +7,7 @@ use crate::{
     source_location::SourceLocation,
     ty::{impl_match_variant_as_self, CanonicalType, StaticType, Type, TypeProperties},
 };
+use bitvec::slice::BitSlice;
 
 mod sealed {
     pub trait ResetTypeSealed {}
@@ -45,6 +46,7 @@ macro_rules! reset_type {
         impl Type for $name {
             type BaseType = $name;
             type MaskType = Bool;
+            type SimValue = bool;
 
             impl_match_variant_as_self!();
 
@@ -66,6 +68,21 @@ macro_rules! reset_type {
                 };
                 retval
             }
+
+            fn sim_value_from_bits(&self, bits: &BitSlice) -> Self::SimValue {
+                assert_eq!(bits.len(), 1);
+                bits[0]
+            }
+
+            fn sim_value_clone_from_bits(&self, value: &mut Self::SimValue, bits: &BitSlice) {
+                assert_eq!(bits.len(), 1);
+                *value = bits[0];
+            }
+
+            fn sim_value_to_bits(&self, value: &Self::SimValue, bits: &mut BitSlice) {
+                assert_eq!(bits.len(), 1);
+                bits.set(0, *value);
+            }
         }
 
         impl $name {

crates/fayalite/src/sim.rs

@@ -14,7 +14,7 @@ use crate::{
         },
         ExprEnum, Flow, ToLiteralBits,
     },
-    int::{BoolOrIntType, IntType, SIntValue, UIntValue},
+    int::{BoolOrIntType, UIntValue},
     intern::{
         Intern, Interned, InternedCompare, Memoize, PtrEqWithTypeId, SupportsPtrEqWithTypeId,
     },
@@ -38,6 +38,7 @@ use crate::{
             TypeIndexRange, TypeLayout, TypeLen, TypeParts,
         },
         time::{SimDuration, SimInstant},
+        value::SimValue,
     },
     ty::StaticType,
     util::{BitSliceWriteWithBase, DebugAsDisplay},
@@ -72,6 +73,7 @@ use std::{
 
 mod interpreter;
 pub mod time;
+pub mod value;
 pub mod vcd;
 
 #[derive(Debug, PartialEq, Eq, Hash, Clone, Copy)]
@@ -5904,635 +5906,6 @@ impl SimTraceKind {
     }
 }
 
-#[derive(Clone, PartialEq, Eq, Hash)]
-pub struct SimValue<T: Type> {
-    ty: T,
-    bits: BitVec,
-}
-
-impl<T: Type> fmt::Debug for SimValue<T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_struct("SimValue")
-            .field("ty", &self.ty)
-            .field("bits", &BitSliceWriteWithBase(&self.bits))
-            .finish()
-    }
-}
-
-impl SimValue<CanonicalType> {
-    #[track_caller]
-    fn to_expr_impl(ty: CanonicalType, bits: &BitSlice) -> Expr<CanonicalType> {
-        match ty {
-            CanonicalType::UInt(_) => Expr::canonical(<UInt>::bits_to_expr(Cow::Borrowed(bits))),
-            CanonicalType::SInt(_) => Expr::canonical(<SInt>::bits_to_expr(Cow::Borrowed(bits))),
-            CanonicalType::Bool(_) => Expr::canonical(Bool::bits_to_expr(Cow::Borrowed(bits))),
-            CanonicalType::Array(ty) => {
-                let element_bit_width = ty.element().bit_width();
-                Expr::<Array>::canonical(
-                    crate::expr::ops::ArrayLiteral::new(
-                        ty.element(),
-                        (0..ty.len())
-                            .map(|array_index| {
-                                let start = element_bit_width * array_index;
-                                let end = start + element_bit_width;
-                                Self::to_expr_impl(ty.element(), &bits[start..end])
-                            })
-                            .collect(),
-                    )
-                    .to_expr(),
-                )
-            }
-            CanonicalType::Enum(ty) => {
-                let discriminant_bit_width = ty.discriminant_bit_width();
-                let mut variant_index = [0; mem::size_of::<usize>()];
-                variant_index.view_bits_mut::<Lsb0>()[0..discriminant_bit_width]
-                    .clone_from_bitslice(&bits[..discriminant_bit_width]);
-                let variant_index = usize::from_le_bytes(variant_index);
-                if let Some(variant) = ty.variants().get(variant_index) {
-                    let data_bit_width = variant.ty.map_or(0, CanonicalType::bit_width);
-                    Expr::canonical(
-                        crate::expr::ops::EnumLiteral::new_by_index(
-                            ty,
-                            variant_index,
-                            variant.ty.map(|ty| {
-                                Self::to_expr_impl(
-                                    ty,
-                                    &bits[discriminant_bit_width
-                                        ..discriminant_bit_width + data_bit_width],
-                                )
-                            }),
-                        )
-                        .to_expr(),
-                    )
-                } else {
-                    Expr::canonical(<UInt>::bits_to_expr(Cow::Borrowed(bits)).cast_bits_to(ty))
-                }
-            }
-            CanonicalType::Bundle(ty) => Expr::canonical(
-                crate::expr::ops::BundleLiteral::new(
-                    ty,
-                    ty.fields()
-                        .iter()
-                        .zip(ty.field_offsets().iter())
-                        .map(|(field, &field_offset)| {
-                            Self::to_expr_impl(
-                                field.ty,
-                                &bits[field_offset..field_offset + field.ty.bit_width()],
-                            )
-                        })
-                        .collect(),
-                )
-                .to_expr(),
-            ),
-            CanonicalType::AsyncReset(ty) => {
-                Expr::canonical(Bool::bits_to_expr(Cow::Borrowed(bits)).cast_to(ty))
-            }
-            CanonicalType::SyncReset(ty) => {
-                Expr::canonical(Bool::bits_to_expr(Cow::Borrowed(bits)).cast_to(ty))
-            }
-            CanonicalType::Reset(_) => panic!(
-                "can't convert SimValue<Reset> to Expr<Reset> -- \
-                can't deduce whether reset value should be sync or async"
-            ),
-            CanonicalType::Clock(ty) => {
-                Expr::canonical(Bool::bits_to_expr(Cow::Borrowed(bits)).cast_to(ty))
-            }
-            CanonicalType::PhantomConst(ty) => Expr::canonical(ty.to_expr()),
-        }
-    }
-}
-
-impl<T: Type> ToExpr for SimValue<T> {
-    type Type = T;
-
-    #[track_caller]
-    fn to_expr(&self) -> Expr<Self::Type> {
-        Expr::from_canonical(SimValue::to_expr_impl(self.ty.canonical(), &self.bits))
-    }
-}
-
-impl<T: Type> ToSimValue<T> for SimValue<T> {
-    #[track_caller]
-    fn to_sim_value(&self, ty: T) -> SimValue<T> {
-        assert_eq!(self.ty, ty);
-        self.clone()
-    }
-
-    #[track_caller]
-    fn into_sim_value(self, ty: T) -> SimValue<T> {
-        assert_eq!(self.ty, ty);
-        self
-    }
-
-    #[track_caller]
-    fn box_into_sim_value(self: Box<Self>, ty: T) -> SimValue<T> {
-        assert_eq!(self.ty, ty);
-        *self
-    }
-}
-
-impl<T: Type> ToSimValue<T> for BitVec {
-    #[track_caller]
-    fn to_sim_value(&self, ty: T) -> SimValue<T> {
-        self.clone().into_sim_value(ty)
-    }
-
-    #[track_caller]
-    fn into_sim_value(self, ty: T) -> SimValue<T> {
-        assert_eq!(ty.canonical().bit_width(), self.len());
-        SimValue { ty, bits: self }
-    }
-
-    #[track_caller]
-    fn box_into_sim_value(self: Box<Self>, ty: T) -> SimValue<T> {
-        Self::into_sim_value(*self, ty)
-    }
-}
-
-impl<T: Type> ToSimValue<T> for bitvec::boxed::BitBox {
-    #[track_caller]
-    fn to_sim_value(&self, ty: T) -> SimValue<T> {
-        self.clone().into_sim_value(ty)
-    }
-
-    #[track_caller]
-    fn into_sim_value(self, ty: T) -> SimValue<T> {
-        assert_eq!(ty.canonical().bit_width(), self.len());
-        SimValue {
-            ty,
-            bits: self.into_bitvec(),
-        }
-    }
-
-    #[track_caller]
-    fn box_into_sim_value(self: Box<Self>, ty: T) -> SimValue<T> {
-        Self::into_sim_value(*self, ty)
-    }
-}
-
-impl<T: Type> ToSimValue<T> for BitSlice {
-    #[track_caller]
-    fn to_sim_value(&self, ty: T) -> SimValue<T> {
-        assert_eq!(ty.canonical().bit_width(), self.len());
-        SimValue {
-            ty,
-            bits: self.to_bitvec(),
-        }
-    }
-}
-
-impl<T: Type> SimValue<T> {
-    pub fn ty(&self) -> T {
-        self.ty
-    }
-    pub fn bits(&self) -> &BitSlice {
-        &self.bits
-    }
-    pub fn into_bits(self) -> BitVec {
-        self.bits
-    }
-    #[track_caller]
-    pub fn from_canonical(v: SimValue<CanonicalType>) -> Self {
-        Self {
-            ty: T::from_canonical(v.ty),
-            bits: v.bits,
-        }
-    }
-    pub fn into_canonical(self) -> SimValue<CanonicalType> {
-        SimValue {
-            ty: self.ty.canonical(),
-            bits: self.bits,
-        }
-    }
-    #[track_caller]
-    pub fn from_dyn_int(v: SimValue<T::Dyn>) -> Self
-    where
-        T: IntType,
-    {
-        Self {
-            ty: T::from_dyn_int(v.ty),
-            bits: v.bits,
-        }
-    }
-    pub fn into_dyn_int(self) -> SimValue<T::Dyn>
-    where
-        T: IntType,
-    {
-        SimValue {
-            ty: self.ty.as_dyn_int(),
-            bits: self.bits,
-        }
-    }
-    #[track_caller]
-    pub fn from_bundle(v: SimValue<Bundle>) -> Self
-    where
-        T: BundleType,
-    {
-        Self {
-            ty: T::from_canonical(CanonicalType::Bundle(v.ty)),
-            bits: v.bits,
-        }
-    }
-    pub fn into_bundle(self) -> SimValue<Bundle>
-    where
-        T: BundleType,
-    {
-        SimValue {
-            ty: Bundle::from_canonical(self.ty.canonical()),
-            bits: self.bits,
-        }
-    }
-    #[track_caller]
-    pub fn from_enum(v: SimValue<Enum>) -> Self
-    where
-        T: EnumType,
-    {
-        Self {
-            ty: T::from_canonical(CanonicalType::Enum(v.ty)),
-            bits: v.bits,
-        }
-    }
-    pub fn into_enum(self) -> SimValue<Enum>
-    where
-        T: EnumType,
-    {
-        SimValue {
-            ty: Enum::from_canonical(self.ty.canonical()),
-            bits: self.bits,
-        }
-    }
-}
-
-pub trait ToSimValue<T: Type> {
-    #[track_caller]
-    fn to_sim_value(&self, ty: T) -> SimValue<T>;
-    #[track_caller]
-    fn into_sim_value(self, ty: T) -> SimValue<T>
-    where
-        Self: Sized,
-    {
-        self.to_sim_value(ty)
-    }
-    #[track_caller]
-    fn box_into_sim_value(self: Box<Self>, ty: T) -> SimValue<T> {
-        self.to_sim_value(ty)
-    }
-}
-
-impl<This: ?Sized + ToSimValue<T>, T: Type> ToSimValue<T> for &'_ This {
-    #[track_caller]
-    fn to_sim_value(&self, ty: T) -> SimValue<T> {
-        This::to_sim_value(self, ty)
-    }
-}
-
-impl<This: ?Sized + ToSimValue<T>, T: Type> ToSimValue<T> for &'_ mut This {
-    #[track_caller]
-    fn to_sim_value(&self, ty: T) -> SimValue<T> {
-        This::to_sim_value(self, ty)
-    }
-}
-
-impl<This: ?Sized + ToSimValue<T>, T: Type> ToSimValue<T> for Box<This> {
-    #[track_caller]
-    fn to_sim_value(&self, ty: T) -> SimValue<T> {
-        This::to_sim_value(self, ty)
-    }
-    #[track_caller]
-    fn into_sim_value(self, ty: T) -> SimValue<T> {
-        This::box_into_sim_value(self, ty)
-    }
-    #[track_caller]
-    fn box_into_sim_value(self: Box<Self>, ty: T) -> SimValue<T> {
-        This::box_into_sim_value(*self, ty)
-    }
-}
-
-impl<This: ?Sized + ToSimValue<T> + Send + Sync + 'static, T: Type> ToSimValue<T>
-    for Interned<This>
-{
-    #[track_caller]
-    fn to_sim_value(&self, ty: T) -> SimValue<T> {
-        This::to_sim_value(self, ty)
-    }
-}
-
-impl<T: Type, Len: Size> SimValue<ArrayType<T, Len>> {
-    #[track_caller]
-    pub fn from_array_elements<
-        I: IntoIterator<Item: ToSimValue<T>, IntoIter: ExactSizeIterator>,
-    >(
-        elements: I,
-        ty: ArrayType<T, Len>,
-    ) -> Self {
-        let mut iter = elements.into_iter();
-        assert_eq!(iter.len(), ty.len());
-        let Some(first) = iter.next() else {
-            return SimValue {
-                ty,
-                bits: BitVec::new(),
-            };
-        };
-        let SimValue {
-            ty: element_ty,
-            mut bits,
-        } = first.into_sim_value(ty.element());
-        assert_eq!(element_ty, ty.element());
-        bits.reserve(ty.type_properties().bit_width - bits.len());
-        for element in iter {
-            let SimValue {
-                ty: element_ty,
-                bits: element_bits,
-            } = element.into_sim_value(ty.element());
-            assert_eq!(element_ty, ty.element());
-            bits.extend_from_bitslice(&element_bits);
-        }
-        SimValue { ty, bits }
-    }
-}
-
-impl<Element: ToSimValue<T>, T: Type> ToSimValue<Array<T>> for [Element] {
-    #[track_caller]
-    fn to_sim_value(&self, ty: Array<T>) -> SimValue<Array<T>> {
-        SimValue::from_array_elements(self, ty)
-    }
-    #[track_caller]
-    fn box_into_sim_value(self: Box<Self>, ty: Array<T>) -> SimValue<Array<T>> {
-        SimValue::from_array_elements(self, ty)
-    }
-}
-
-impl<Element: ToSimValue<CanonicalType>> ToSimValue<CanonicalType> for [Element] {
-    #[track_caller]
-    fn to_sim_value(&self, ty: CanonicalType) -> SimValue<CanonicalType> {
-        SimValue::from_array_elements(self, <Array>::from_canonical(ty)).into_canonical()
-    }
-    #[track_caller]
-    fn box_into_sim_value(self: Box<Self>, ty: CanonicalType) -> SimValue<CanonicalType> {
-        SimValue::from_array_elements(self, <Array>::from_canonical(ty)).into_canonical()
-    }
-}
-
-impl<Element: ToSimValue<T>, T: Type, const N: usize> ToSimValue<Array<T, N>> for [Element; N]
-where
-    ConstUsize<N>: KnownSize,
-{
-    #[track_caller]
-    fn to_sim_value(&self, ty: Array<T, N>) -> SimValue<Array<T, N>> {
-        SimValue::from_array_elements(self, ty)
-    }
-    #[track_caller]
-    fn into_sim_value(self, ty: Array<T, N>) -> SimValue<Array<T, N>> {
-        SimValue::from_array_elements(self, ty)
-    }
-    #[track_caller]
-    fn box_into_sim_value(self: Box<Self>, ty: Array<T, N>) -> SimValue<Array<T, N>> {
-        SimValue::from_array_elements(<Vec<Element> as From<Box<[Element]>>>::from(self), ty)
-    }
-}
-
-impl<Element: ToSimValue<T>, T: Type, const N: usize> ToSimValue<Array<T>> for [Element; N]
-where
-    ConstUsize<N>: KnownSize,
-{
-    #[track_caller]
-    fn to_sim_value(&self, ty: Array<T>) -> SimValue<Array<T>> {
-        SimValue::from_array_elements(self, ty)
-    }
-    #[track_caller]
-    fn into_sim_value(self, ty: Array<T>) -> SimValue<Array<T>> {
-        SimValue::from_array_elements(self, ty)
-    }
-    #[track_caller]
-    fn box_into_sim_value(self: Box<Self>, ty: Array<T>) -> SimValue<Array<T>> {
-        SimValue::from_array_elements(<Vec<Element> as From<Box<[Element]>>>::from(self), ty)
-    }
-}
-
-impl<Element: ToSimValue<CanonicalType>, const N: usize> ToSimValue<CanonicalType>
-    for [Element; N]
-{
-    #[track_caller]
-    fn to_sim_value(&self, ty: CanonicalType) -> SimValue<CanonicalType> {
-        SimValue::from_array_elements(self, <Array>::from_canonical(ty)).into_canonical()
-    }
-    #[track_caller]
-    fn into_sim_value(self, ty: CanonicalType) -> SimValue<CanonicalType> {
-        SimValue::from_array_elements(self, <Array>::from_canonical(ty)).into_canonical()
-    }
-    #[track_caller]
-    fn box_into_sim_value(self: Box<Self>, ty: CanonicalType) -> SimValue<CanonicalType> {
-        SimValue::from_array_elements(
-            <Vec<Element> as From<Box<[Element]>>>::from(self),
-            <Array>::from_canonical(ty),
-        )
-        .into_canonical()
-    }
-}
-
-impl<Element: ToSimValue<T>, T: Type> ToSimValue<Array<T>> for Vec<Element> {
-    #[track_caller]
-    fn to_sim_value(&self, ty: Array<T>) -> SimValue<Array<T>> {
-        SimValue::from_array_elements(self, ty)
-    }
-    #[track_caller]
-    fn into_sim_value(self, ty: Array<T>) -> SimValue<Array<T>> {
-        SimValue::from_array_elements(self, ty)
-    }
-    #[track_caller]
-    fn box_into_sim_value(self: Box<Self>, ty: Array<T>) -> SimValue<Array<T>> {
-        SimValue::from_array_elements(*self, ty)
-    }
-}
-
-impl<Element: ToSimValue<CanonicalType>> ToSimValue<CanonicalType> for Vec<Element> {
-    #[track_caller]
-    fn to_sim_value(&self, ty: CanonicalType) -> SimValue<CanonicalType> {
-        SimValue::from_array_elements(self, <Array>::from_canonical(ty)).into_canonical()
-    }
-    #[track_caller]
-    fn into_sim_value(self, ty: CanonicalType) -> SimValue<CanonicalType> {
-        SimValue::from_array_elements(self, <Array>::from_canonical(ty)).into_canonical()
-    }
-    #[track_caller]
-    fn box_into_sim_value(self: Box<Self>, ty: CanonicalType) -> SimValue<CanonicalType> {
-        SimValue::from_array_elements(*self, <Array>::from_canonical(ty)).into_canonical()
-    }
-}
-
-impl<T: Type> ToSimValue<T> for Expr<T> {
-    #[track_caller]
-    fn to_sim_value(&self, ty: T) -> SimValue<T> {
-        assert_eq!(Expr::ty(*self), ty);
-        SimValue {
-            ty,
-            bits: self
-                .to_literal_bits()
-                .expect("must be a literal expression")
-                .to_bitvec(),
-        }
-    }
-}
-
-macro_rules! impl_to_sim_value_for_bool_like {
-    ($ty:ident) => {
-        impl ToSimValue<$ty> for bool {
-            fn to_sim_value(&self, ty: $ty) -> SimValue<$ty> {
-                SimValue {
-                    ty,
-                    bits: BitVec::repeat(*self, 1),
-                }
-            }
-        }
-    };
-}
-
-impl_to_sim_value_for_bool_like!(Bool);
-impl_to_sim_value_for_bool_like!(AsyncReset);
-impl_to_sim_value_for_bool_like!(SyncReset);
-impl_to_sim_value_for_bool_like!(Reset);
-impl_to_sim_value_for_bool_like!(Clock);
-
-impl ToSimValue<CanonicalType> for bool {
-    #[track_caller]
-    fn to_sim_value(&self, ty: CanonicalType) -> SimValue<CanonicalType> {
-        match ty {
-            CanonicalType::UInt(_)
-            | CanonicalType::SInt(_)
-            | CanonicalType::Array(_)
-            | CanonicalType::Enum(_)
-            | CanonicalType::Bundle(_)
-            | CanonicalType::PhantomConst(_) => {
-                panic!("can't create SimValue from bool: expected value of type: {ty:?}");
-            }
-            CanonicalType::Bool(_)
-            | CanonicalType::AsyncReset(_)
-            | CanonicalType::SyncReset(_)
-            | CanonicalType::Reset(_)
-            | CanonicalType::Clock(_) => SimValue {
-                ty,
-                bits: BitVec::repeat(*self, 1),
-            },
-        }
-    }
-}
-
-macro_rules! impl_to_sim_value_for_primitive_int {
-    ($prim:ident) => {
-        impl ToSimValue<<$prim as ToExpr>::Type> for $prim {
-            #[track_caller]
-            fn to_sim_value(
-                &self,
-                ty: <$prim as ToExpr>::Type,
-            ) -> SimValue<<$prim as ToExpr>::Type> {
-                SimValue {
-                    ty,
-                    bits: <<$prim as ToExpr>::Type as BoolOrIntType>::le_bytes_to_bits_wrapping(
-                        &self.to_le_bytes(),
-                        ty.width(),
-                    ),
-                }
-            }
-        }
-
-        impl ToSimValue<<<$prim as ToExpr>::Type as IntType>::Dyn> for $prim {
-            #[track_caller]
-            fn to_sim_value(
-                &self,
-                ty: <<$prim as ToExpr>::Type as IntType>::Dyn,
-            ) -> SimValue<<<$prim as ToExpr>::Type as IntType>::Dyn> {
-                SimValue {
-                    ty,
-                    bits: <<$prim as ToExpr>::Type as BoolOrIntType>::le_bytes_to_bits_wrapping(
-                        &self.to_le_bytes(),
-                        ty.width(),
-                    ),
-                }
-            }
-        }
-
-        impl ToSimValue<CanonicalType> for $prim {
-            #[track_caller]
-            fn to_sim_value(&self, ty: CanonicalType) -> SimValue<CanonicalType> {
-                let ty: <<$prim as ToExpr>::Type as IntType>::Dyn = Type::from_canonical(ty);
-                self.to_sim_value(ty).into_canonical()
-            }
-        }
-    };
-}
-
-impl_to_sim_value_for_primitive_int!(u8);
-impl_to_sim_value_for_primitive_int!(u16);
-impl_to_sim_value_for_primitive_int!(u32);
-impl_to_sim_value_for_primitive_int!(u64);
-impl_to_sim_value_for_primitive_int!(u128);
-impl_to_sim_value_for_primitive_int!(usize);
-impl_to_sim_value_for_primitive_int!(i8);
-impl_to_sim_value_for_primitive_int!(i16);
-impl_to_sim_value_for_primitive_int!(i32);
-impl_to_sim_value_for_primitive_int!(i64);
-impl_to_sim_value_for_primitive_int!(i128);
-impl_to_sim_value_for_primitive_int!(isize);
-
-macro_rules! impl_to_sim_value_for_int_value {
-    ($IntValue:ident, $Int:ident, $IntType:ident) => {
-        impl<Width: KnownSize> ToSimValue<$IntType<Width>> for $IntValue<Width> {
-            fn to_sim_value(&self, ty: $IntType<Width>) -> SimValue<$IntType<Width>> {
-                self.bits().to_bitvec().into_sim_value(ty)
-            }
-
-            fn into_sim_value(self, ty: $IntType<Width>) -> SimValue<$IntType<Width>> {
-                Arc::try_unwrap(self.into_bits())
-                    .unwrap_or_else(|v: Arc<BitVec>| v.to_bitvec())
-                    .into_sim_value(ty)
-            }
-
-            fn box_into_sim_value(
-                self: Box<Self>,
-                ty: $IntType<Width>,
-            ) -> SimValue<$IntType<Width>> {
-                Self::into_sim_value(*self, ty)
-            }
-        }
-
-        impl<Width: Size> ToSimValue<$Int> for $IntValue<Width> {
-            fn to_sim_value(&self, ty: $Int) -> SimValue<$Int> {
-                self.bits().to_bitvec().into_sim_value(ty)
-            }
-
-            fn into_sim_value(self, ty: $Int) -> SimValue<$Int> {
-                Arc::try_unwrap(self.into_bits())
-                    .unwrap_or_else(|v: Arc<BitVec>| v.to_bitvec())
-                    .into_sim_value(ty)
-            }
-
-            fn box_into_sim_value(self: Box<Self>, ty: $Int) -> SimValue<$Int> {
-                Self::into_sim_value(*self, ty)
-            }
-        }
-
-        impl<Width: Size> ToSimValue<CanonicalType> for $IntValue<Width> {
-            #[track_caller]
-            fn to_sim_value(&self, ty: CanonicalType) -> SimValue<CanonicalType> {
-                ToSimValue::<$Int>::to_sim_value(self, $Int::from_canonical(ty)).into_canonical()
-            }
-
-            #[track_caller]
-            fn into_sim_value(self, ty: CanonicalType) -> SimValue<CanonicalType> {
-                ToSimValue::<$Int>::into_sim_value(self, $Int::from_canonical(ty)).into_canonical()
-            }
-
-            #[track_caller]
-            fn box_into_sim_value(self: Box<Self>, ty: CanonicalType) -> SimValue<CanonicalType> {
-                Self::into_sim_value(*self, ty)
-            }
-        }
-    };
-}
-
-impl_to_sim_value_for_int_value!(UIntValue, UInt, UIntType);
-impl_to_sim_value_for_int_value!(SIntValue, SInt, SIntType);
-
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
 enum MaybeNeedsSettle<S, N = S> {
     NeedsSettle(S),
@@ -7024,19 +6397,19 @@ impl<I: BoolOrIntType> MaybeNeedsSettleFn<&'_ mut interpreter::State> for ReadBo
 struct ReadFn {
     compiled_value: CompiledValue<CanonicalType>,
     io: Expr<CanonicalType>,
-    bits: BitVec,
 }
 
 impl MaybeNeedsSettleFn<&'_ mut interpreter::State> for ReadFn {
     type Output = SimValue<CanonicalType>;
 
     fn call(self, state: &mut interpreter::State) -> Self::Output {
-        let Self {
+        let Self { compiled_value, io } = self;
-            compiled_value,
+        SimulationImpl::read_no_settle_helper(
+            state,
             io,
-            bits,
+            compiled_value,
-        } = self;
+            UIntValue::new(Arc::new(BitVec::repeat(false, Expr::ty(io).bit_width()))),
-        SimulationImpl::read_no_settle_helper(state, io, compiled_value, bits)
+        )
     }
 }
 
@@ -7390,7 +6763,7 @@ impl SimulationImpl {
                 (WaitTarget::Instant(instant), None) => Some(instant),
                 (WaitTarget::Instant(instant), Some(retval)) => Some(instant.min(retval)),
                 (WaitTarget::Change { key, value }, retval) => {
-                    if Self::value_changed(&mut self.state, key, &value.bits) {
+                    if Self::value_changed(&mut self.state, key, SimValue::bits(value).bits()) {
                         Some(self.instant)
                     } else {
                         retval
@@ -7688,7 +7061,7 @@ impl SimulationImpl {
         }
     }
     #[track_caller]
-    fn read_write_sim_value_helper<Bits>(
+    fn read_write_sim_value_helper<Bits: ?Sized>(
         state: &mut interpreter::State,
         compiled_value: CompiledValue<CanonicalType>,
         start_bit_index: usize,
@@ -7783,15 +7156,13 @@ impl SimulationImpl {
         state: &mut interpreter::State,
         io: Expr<CanonicalType>,
         compiled_value: CompiledValue<CanonicalType>,
-        mut bits: BitVec,
+        mut bits: UIntValue,
     ) -> SimValue<CanonicalType> {
-        bits.clear();
-        bits.resize(compiled_value.layout.ty.bit_width(), false);
         SimulationImpl::read_write_sim_value_helper(
             state,
             compiled_value,
             0,
-            &mut bits,
+            bits.bits_mut(),
             |_signed, bit_range, bits, value| {
                 <UInt>::copy_bits_from_bigint_wrapping(value, &mut bits[bit_range]);
             },
@@ -7802,10 +7173,7 @@ impl SimulationImpl {
                 bits[bit_range].clone_from_bitslice(bitslice);
             },
         );
-        SimValue {
+        SimValue::from_bits(Expr::ty(io), bits)
-            ty: Expr::ty(io),
-            bits,
-        }
     }
     /// doesn't modify `bits`
     fn value_changed(
@@ -7847,11 +7215,7 @@ impl SimulationImpl {
     ) {
         let compiled_value = self.get_module(which_module).read_helper(io, which_module);
         let value = compiled_value
-            .map(|compiled_value| ReadFn {
+            .map(|compiled_value| ReadFn { compiled_value, io })
-                compiled_value,
-                io,
-                bits: BitVec::new(),
-            })
             .apply_no_settle(&mut self.state);
         let (MaybeNeedsSettle::NeedsSettle(compiled_value)
         | MaybeNeedsSettle::NoSettleNeeded(compiled_value)) = compiled_value;
@@ -7868,12 +7232,12 @@ impl SimulationImpl {
             .get_module_mut(which_module)
             .write_helper(io, which_module);
         self.state_ready_to_run = true;
-        assert_eq!(Expr::ty(io), value.ty());
+        assert_eq!(Expr::ty(io), SimValue::ty(value));
         Self::read_write_sim_value_helper(
             &mut self.state,
             compiled_value,
             0,
-            &mut value.bits(),
+            &mut value.bits().bits(),
             |signed, bit_range, bits, value| {
                 if signed {
                     *value = SInt::bits_to_bigint(&bits[bit_range]);
@@ -8167,10 +7531,10 @@ macro_rules! impl_simulation_methods {
             SimValue::from_canonical($self.settle_if_needed(retval)$(.$await)?)
         }
         $(#[$track_caller])?
-        pub $($async)? fn write<IO: Type, V: ToSimValue<IO>>(&mut $self, io: Expr<IO>, value: V) {
+        pub $($async)? fn write<IO: Type, V: value::ToSimValue<IO>>(&mut $self, io: Expr<IO>, value: V) {
             $self.sim_impl.borrow_mut().write(
                 Expr::canonical(io),
-                &value.into_sim_value(Expr::ty(io)).into_canonical(),
+                &SimValue::into_canonical(value.into_sim_value(Expr::ty(io))),
                 $which_module,
             );
         }

crates/fayalite/src/sim/value.rs

@@ -0,0 +1,665 @@
+// SPDX-License-Identifier: LGPL-3.0-or-later
+// See Notices.txt for copyright information
+
+use crate::{
+    array::{Array, ArrayType},
+    bundle::{Bundle, BundleType},
+    clock::Clock,
+    enum_::{Enum, EnumType},
+    expr::{CastBitsTo, Expr, ToExpr},
+    int::{Bool, IntType, KnownSize, SInt, SIntType, SIntValue, Size, UInt, UIntType, UIntValue},
+    reset::{AsyncReset, Reset, SyncReset},
+    ty::{CanonicalType, Type},
+    util::{
+        alternating_cell::{AlternatingCell, AlternatingCellMethods},
+        ConstUsize,
+    },
+};
+use bitvec::{slice::BitSlice, vec::BitVec};
+use std::{
+    fmt,
+    ops::{Deref, DerefMut},
+    sync::Arc,
+};
+
+#[derive(Copy, Clone, Eq, PartialEq)]
+enum ValidFlags {
+    BothValid = 0,
+    OnlyValueValid = 1,
+    OnlyBitsValid = 2,
+}
+
+#[derive(Clone)]
+struct SimValueInner<T: Type> {
+    value: T::SimValue,
+    bits: UIntValue,
+    valid_flags: ValidFlags,
+    ty: T,
+}
+
+impl<T: Type> SimValueInner<T> {
+    fn fill_bits(&mut self) {
+        match self.valid_flags {
+            ValidFlags::BothValid | ValidFlags::OnlyBitsValid => {}
+            ValidFlags::OnlyValueValid => {
+                self.ty.sim_value_to_bits(&self.value, self.bits.bits_mut());
+                self.valid_flags = ValidFlags::BothValid;
+            }
+        }
+    }
+    fn into_bits(mut self) -> UIntValue {
+        self.fill_bits();
+        self.bits
+    }
+    fn bits_mut(&mut self) -> &mut UIntValue {
+        self.fill_bits();
+        self.valid_flags = ValidFlags::OnlyBitsValid;
+        &mut self.bits
+    }
+    fn fill_value(&mut self) {
+        match self.valid_flags {
+            ValidFlags::BothValid | ValidFlags::OnlyValueValid => {}
+            ValidFlags::OnlyBitsValid => {
+                self.ty
+                    .sim_value_clone_from_bits(&mut self.value, self.bits.bits());
+                self.valid_flags = ValidFlags::BothValid;
+            }
+        }
+    }
+    fn into_value(mut self) -> T::SimValue {
+        self.fill_value();
+        self.value
+    }
+    fn value_mut(&mut self) -> &mut T::SimValue {
+        self.fill_value();
+        self.valid_flags = ValidFlags::OnlyValueValid;
+        &mut self.value
+    }
+}
+
+impl<T: Type> AlternatingCellMethods for SimValueInner<T> {
+    fn unique_to_shared(&mut self) {
+        match self.valid_flags {
+            ValidFlags::BothValid => return,
+            ValidFlags::OnlyValueValid => {
+                self.ty.sim_value_to_bits(&self.value, self.bits.bits_mut())
+            }
+            ValidFlags::OnlyBitsValid => self
+                .ty
+                .sim_value_clone_from_bits(&mut self.value, self.bits.bits()),
+        }
+        self.valid_flags = ValidFlags::BothValid;
+    }
+
+    fn shared_to_unique(&mut self) {}
+}
+
+pub struct SimValue<T: Type> {
+    inner: AlternatingCell<SimValueInner<T>>,
+}
+
+impl<T: Type + Clone> Clone for SimValue<T> {
+    fn clone(&self) -> Self {
+        Self {
+            inner: AlternatingCell::new_unique(self.inner.share().clone()),
+        }
+    }
+}
+
+impl<T: Type> SimValue<T> {
+    #[track_caller]
+    pub fn from_bits(ty: T, bits: UIntValue) -> Self {
+        assert_eq!(ty.canonical().bit_width(), bits.width());
+        let inner = SimValueInner {
+            value: ty.sim_value_from_bits(bits.bits()),
+            bits,
+            valid_flags: ValidFlags::BothValid,
+            ty,
+        };
+        Self {
+            inner: AlternatingCell::new_shared(inner),
+        }
+    }
+    #[track_caller]
+    pub fn from_bitslice(ty: T, bits: &BitSlice) -> Self {
+        Self::from_bits(ty, UIntValue::new(Arc::new(bits.to_bitvec())))
+    }
+    pub fn from_value(ty: T, value: T::SimValue) -> Self {
+        let inner = SimValueInner {
+            bits: UIntValue::new_dyn(Arc::new(BitVec::repeat(false, ty.canonical().bit_width()))),
+            value,
+            valid_flags: ValidFlags::OnlyValueValid,
+            ty,
+        };
+        Self {
+            inner: AlternatingCell::new_unique(inner),
+        }
+    }
+    pub fn ty(this: &Self) -> T {
+        this.inner.share().ty
+    }
+    pub fn into_bits(this: Self) -> UIntValue {
+        this.inner.into_inner().into_bits()
+    }
+    pub fn into_ty_and_bits(this: Self) -> (T, UIntValue) {
+        let inner = this.inner.into_inner();
+        (inner.ty, inner.into_bits())
+    }
+    pub fn bits(this: &Self) -> &UIntValue {
+        &this.inner.share().bits
+    }
+    pub fn bits_mut(this: &mut Self) -> &mut UIntValue {
+        this.inner.unique().bits_mut()
+    }
+    pub fn into_value(this: Self) -> T::SimValue {
+        this.inner.into_inner().into_value()
+    }
+    pub fn value(this: &Self) -> &T::SimValue {
+        &this.inner.share().value
+    }
+    pub fn value_mut(this: &mut Self) -> &mut T::SimValue {
+        this.inner.unique().value_mut()
+    }
+    #[track_caller]
+    pub fn from_canonical(v: SimValue<CanonicalType>) -> Self {
+        let (ty, bits) = SimValue::into_ty_and_bits(v);
+        Self::from_bits(T::from_canonical(ty), bits)
+    }
+    pub fn into_canonical(this: Self) -> SimValue<CanonicalType> {
+        let (ty, bits) = Self::into_ty_and_bits(this);
+        SimValue::from_bits(ty.canonical(), bits)
+    }
+    pub fn canonical(this: &Self) -> SimValue<CanonicalType> {
+        SimValue::from_bits(Self::ty(this).canonical(), Self::bits(this).clone())
+    }
+    #[track_caller]
+    pub fn from_dyn_int(v: SimValue<T::Dyn>) -> Self
+    where
+        T: IntType,
+    {
+        let (ty, bits) = SimValue::into_ty_and_bits(v);
+        SimValue::from_bits(T::from_dyn_int(ty), bits)
+    }
+    pub fn into_dyn_int(this: Self) -> SimValue<T::Dyn>
+    where
+        T: IntType,
+    {
+        let (ty, bits) = Self::into_ty_and_bits(this);
+        SimValue::from_bits(ty.as_dyn_int(), bits)
+    }
+    pub fn to_dyn_int(this: &Self) -> SimValue<T::Dyn>
+    where
+        T: IntType,
+    {
+        SimValue::from_bits(Self::ty(this).as_dyn_int(), Self::bits(&this).clone())
+    }
+    #[track_caller]
+    pub fn from_bundle(v: SimValue<Bundle>) -> Self
+    where
+        T: BundleType,
+    {
+        let (ty, bits) = SimValue::into_ty_and_bits(v);
+        SimValue::from_bits(T::from_canonical(CanonicalType::Bundle(ty)), bits)
+    }
+    pub fn into_bundle(this: Self) -> SimValue<Bundle>
+    where
+        T: BundleType,
+    {
+        let (ty, bits) = Self::into_ty_and_bits(this);
+        SimValue::from_bits(Bundle::from_canonical(ty.canonical()), bits)
+    }
+    pub fn to_bundle(this: &Self) -> SimValue<Bundle>
+    where
+        T: BundleType,
+    {
+        SimValue::from_bits(
+            Bundle::from_canonical(Self::ty(this).canonical()),
+            Self::bits(&this).clone(),
+        )
+    }
+    #[track_caller]
+    pub fn from_enum(v: SimValue<Enum>) -> Self
+    where
+        T: EnumType,
+    {
+        let (ty, bits) = SimValue::into_ty_and_bits(v);
+        SimValue::from_bits(T::from_canonical(CanonicalType::Enum(ty)), bits)
+    }
+    pub fn into_enum(this: Self) -> SimValue<Enum>
+    where
+        T: EnumType,
+    {
+        let (ty, bits) = Self::into_ty_and_bits(this);
+        SimValue::from_bits(Enum::from_canonical(ty.canonical()), bits)
+    }
+    pub fn to_enum(this: &Self) -> SimValue<Enum>
+    where
+        T: EnumType,
+    {
+        SimValue::from_bits(
+            Enum::from_canonical(Self::ty(this).canonical()),
+            Self::bits(&this).clone(),
+        )
+    }
+}
+
+impl<T: Type> Deref for SimValue<T> {
+    type Target = T::SimValue;
+
+    fn deref(&self) -> &Self::Target {
+        Self::value(self)
+    }
+}
+
+impl<T: Type> DerefMut for SimValue<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        Self::value_mut(self)
+    }
+}
+
+impl<T: Type> fmt::Debug for SimValue<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let inner = self.inner.share();
+        f.debug_struct("SimValue")
+            .field("ty", &inner.ty)
+            .field("value", &inner.value)
+            .finish()
+    }
+}
+
+impl<T: Type> ToExpr for SimValue<T> {
+    type Type = T;
+
+    #[track_caller]
+    fn to_expr(&self) -> Expr<Self::Type> {
+        let inner = self.inner.share();
+        inner.bits.cast_bits_to(inner.ty)
+    }
+}
+
+pub trait SimValuePartialEq<T: Type = Self>: Type {
+    #[track_caller]
+    fn sim_value_eq(this: &SimValue<Self>, other: &SimValue<T>) -> bool;
+}
+
+impl<T: SimValuePartialEq<U>, U: Type> PartialEq<SimValue<U>> for SimValue<T> {
+    #[track_caller]
+    fn eq(&self, other: &SimValue<U>) -> bool {
+        T::sim_value_eq(self, other)
+    }
+}
+
+impl<LhsWidth: Size, RhsWidth: Size> SimValuePartialEq<UIntType<RhsWidth>> for UIntType<LhsWidth> {
+    fn sim_value_eq(this: &SimValue<Self>, other: &SimValue<UIntType<RhsWidth>>) -> bool {
+        **this == **other
+    }
+}
+
+impl<LhsWidth: Size, RhsWidth: Size> SimValuePartialEq<SIntType<RhsWidth>> for SIntType<LhsWidth> {
+    fn sim_value_eq(this: &SimValue<Self>, other: &SimValue<SIntType<RhsWidth>>) -> bool {
+        **this == **other
+    }
+}
+
+impl SimValuePartialEq<Bool> for Bool {
+    fn sim_value_eq(this: &SimValue<Self>, other: &SimValue<Bool>) -> bool {
+        **this == **other
+    }
+}
+
+pub trait ToSimValue<T: Type> {
+    #[track_caller]
+    fn to_sim_value(&self, ty: T) -> SimValue<T>;
+    #[track_caller]
+    fn into_sim_value(self, ty: T) -> SimValue<T>
+    where
+        Self: Sized,
+    {
+        self.to_sim_value(ty)
+    }
+    #[track_caller]
+    fn arc_into_sim_value(self: Arc<Self>, ty: T) -> SimValue<T> {
+        self.to_sim_value(ty)
+    }
+    #[track_caller]
+    fn arc_to_sim_value(self: &Arc<Self>, ty: T) -> SimValue<T> {
+        self.to_sim_value(ty)
+    }
+}
+
+impl<T: Type> ToSimValue<T> for SimValue<T> {
+    #[track_caller]
+    fn to_sim_value(&self, ty: T) -> SimValue<T> {
+        assert_eq!(SimValue::ty(self), ty);
+        self.clone()
+    }
+
+    #[track_caller]
+    fn into_sim_value(self, ty: T) -> SimValue<T> {
+        assert_eq!(SimValue::ty(&self), ty);
+        self
+    }
+}
+
+impl<T: Type> ToSimValue<T> for BitVec {
+    #[track_caller]
+    fn to_sim_value(&self, ty: T) -> SimValue<T> {
+        self.clone().into_sim_value(ty)
+    }
+
+    #[track_caller]
+    fn into_sim_value(self, ty: T) -> SimValue<T> {
+        Arc::new(self).arc_into_sim_value(ty)
+    }
+
+    #[track_caller]
+    fn arc_into_sim_value(self: Arc<Self>, ty: T) -> SimValue<T> {
+        SimValue::from_bits(ty, UIntValue::new_dyn(self))
+    }
+
+    #[track_caller]
+    fn arc_to_sim_value(self: &Arc<Self>, ty: T) -> SimValue<T> {
+        SimValue::from_bits(ty, UIntValue::new_dyn(self.clone()))
+    }
+}
+
+impl<T: Type> ToSimValue<T> for bitvec::boxed::BitBox {
+    #[track_caller]
+    fn to_sim_value(&self, ty: T) -> SimValue<T> {
+        self.clone().into_sim_value(ty)
+    }
+
+    #[track_caller]
+    fn into_sim_value(self, ty: T) -> SimValue<T> {
+        self.into_bitvec().into_sim_value(ty)
+    }
+}
+
+impl<T: Type> ToSimValue<T> for BitSlice {
+    #[track_caller]
+    fn to_sim_value(&self, ty: T) -> SimValue<T> {
+        self.to_bitvec().into_sim_value(ty)
+    }
+}
+
+impl<This: ?Sized + ToSimValue<T>, T: Type> ToSimValue<T> for &'_ This {
+    fn to_sim_value(&self, ty: T) -> SimValue<T> {
+        This::to_sim_value(self, ty)
+    }
+}
+
+impl<This: ?Sized + ToSimValue<T>, T: Type> ToSimValue<T> for &'_ mut This {
+    fn to_sim_value(&self, ty: T) -> SimValue<T> {
+        This::to_sim_value(self, ty)
+    }
+}
+
+impl<This: ?Sized + ToSimValue<T>, T: Type> ToSimValue<T> for Arc<This> {
+    fn to_sim_value(&self, ty: T) -> SimValue<T> {
+        This::arc_to_sim_value(self, ty)
+    }
+    fn into_sim_value(self, ty: T) -> SimValue<T> {
+        This::arc_into_sim_value(self, ty)
+    }
+}
+
+impl<This: ?Sized + ToSimValue<T> + Send + Sync + 'static, T: Type> ToSimValue<T>
+    for crate::intern::Interned<This>
+{
+    fn to_sim_value(&self, ty: T) -> SimValue<T> {
+        This::to_sim_value(self, ty)
+    }
+}
+
+impl<This: ToSimValue<T>, T: Type> ToSimValue<T> for Box<This> {
+    fn to_sim_value(&self, ty: T) -> SimValue<T> {
+        This::to_sim_value(self, ty)
+    }
+    fn into_sim_value(self, ty: T) -> SimValue<T> {
+        This::into_sim_value(*self, ty)
+    }
+}
+
+impl<T: Type, Len: Size> SimValue<ArrayType<T, Len>> {
+    #[track_caller]
+    pub fn from_array_elements<I: IntoIterator<Item: ToSimValue<T>>>(
+        ty: ArrayType<T, Len>,
+        elements: I,
+    ) -> Self {
+        let element_ty = ty.element();
+        let elements = Vec::from_iter(
+            elements
+                .into_iter()
+                .map(|element| element.into_sim_value(element_ty)),
+        );
+        assert_eq!(elements.len(), ty.len());
+        SimValue::from_value(ty, elements.try_into().ok().expect("already checked len"))
+    }
+}
+
+impl<Element: ToSimValue<T>, T: Type> ToSimValue<Array<T>> for [Element] {
+    #[track_caller]
+    fn to_sim_value(&self, ty: Array<T>) -> SimValue<Array<T>> {
+        SimValue::from_array_elements(ty, self)
+    }
+}
+
+impl<Element: ToSimValue<CanonicalType>> ToSimValue<CanonicalType> for [Element] {
+    #[track_caller]
+    fn to_sim_value(&self, ty: CanonicalType) -> SimValue<CanonicalType> {
+        SimValue::into_canonical(SimValue::from_array_elements(
+            <Array>::from_canonical(ty),
+            self,
+        ))
+    }
+}
+
+impl<Element: ToSimValue<T>, T: Type, const N: usize> ToSimValue<Array<T, N>> for [Element; N]
+where
+    ConstUsize<N>: KnownSize,
+{
+    #[track_caller]
+    fn to_sim_value(&self, ty: Array<T, N>) -> SimValue<Array<T, N>> {
+        SimValue::from_array_elements(ty, self)
+    }
+    #[track_caller]
+    fn into_sim_value(self, ty: Array<T, N>) -> SimValue<Array<T, N>> {
+        SimValue::from_array_elements(ty, self)
+    }
+}
+
+impl<Element: ToSimValue<T>, T: Type, const N: usize> ToSimValue<Array<T>> for [Element; N] {
+    #[track_caller]
+    fn to_sim_value(&self, ty: Array<T>) -> SimValue<Array<T>> {
+        SimValue::from_array_elements(ty, self)
+    }
+    #[track_caller]
+    fn into_sim_value(self, ty: Array<T>) -> SimValue<Array<T>> {
+        SimValue::from_array_elements(ty, self)
+    }
+}
+
+impl<Element: ToSimValue<CanonicalType>, const N: usize> ToSimValue<CanonicalType>
+    for [Element; N]
+{
+    #[track_caller]
+    fn to_sim_value(&self, ty: CanonicalType) -> SimValue<CanonicalType> {
+        SimValue::into_canonical(SimValue::from_array_elements(
+            <Array>::from_canonical(ty),
+            self,
+        ))
+    }
+    #[track_caller]
+    fn into_sim_value(self, ty: CanonicalType) -> SimValue<CanonicalType> {
+        SimValue::into_canonical(SimValue::from_array_elements(
+            <Array>::from_canonical(ty),
+            self,
+        ))
+    }
+}
+
+impl<Element: ToSimValue<T>, T: Type> ToSimValue<Array<T>> for Vec<Element> {
+    #[track_caller]
+    fn to_sim_value(&self, ty: Array<T>) -> SimValue<Array<T>> {
+        SimValue::from_array_elements(ty, self)
+    }
+    #[track_caller]
+    fn into_sim_value(self, ty: Array<T>) -> SimValue<Array<T>> {
+        SimValue::from_array_elements(ty, self)
+    }
+}
+
+impl<Element: ToSimValue<CanonicalType>> ToSimValue<CanonicalType> for Vec<Element> {
+    #[track_caller]
+    fn to_sim_value(&self, ty: CanonicalType) -> SimValue<CanonicalType> {
+        SimValue::into_canonical(SimValue::from_array_elements(
+            <Array>::from_canonical(ty),
+            self,
+        ))
+    }
+    #[track_caller]
+    fn into_sim_value(self, ty: CanonicalType) -> SimValue<CanonicalType> {
+        SimValue::into_canonical(SimValue::from_array_elements(
+            <Array>::from_canonical(ty),
+            self,
+        ))
+    }
+}
+
+impl<T: Type> ToSimValue<T> for Expr<T> {
+    #[track_caller]
+    fn to_sim_value(&self, ty: T) -> SimValue<T> {
+        assert_eq!(Expr::ty(*self), ty);
+        SimValue::from_bitslice(
+            ty,
+            &crate::expr::ToLiteralBits::to_literal_bits(self)
+                .expect("must be a literal expression"),
+        )
+    }
+}
+
+macro_rules! impl_to_sim_value_for_bool_like {
+    ($ty:ident) => {
+        impl ToSimValue<$ty> for bool {
+            fn to_sim_value(&self, ty: $ty) -> SimValue<$ty> {
+                SimValue::from_value(ty, *self)
+            }
+        }
+    };
+}
+
+impl_to_sim_value_for_bool_like!(Bool);
+impl_to_sim_value_for_bool_like!(AsyncReset);
+impl_to_sim_value_for_bool_like!(SyncReset);
+impl_to_sim_value_for_bool_like!(Reset);
+impl_to_sim_value_for_bool_like!(Clock);
+
+impl ToSimValue<CanonicalType> for bool {
+    #[track_caller]
+    fn to_sim_value(&self, ty: CanonicalType) -> SimValue<CanonicalType> {
+        match ty {
+            CanonicalType::UInt(_)
+            | CanonicalType::SInt(_)
+            | CanonicalType::Array(_)
+            | CanonicalType::Enum(_)
+            | CanonicalType::Bundle(_)
+            | CanonicalType::PhantomConst(_) => {
+                panic!("can't create SimValue from bool: expected value of type: {ty:?}");
+            }
+            CanonicalType::Bool(_)
+            | CanonicalType::AsyncReset(_)
+            | CanonicalType::SyncReset(_)
+            | CanonicalType::Reset(_)
+            | CanonicalType::Clock(_) => {
+                SimValue::from_bits(ty, UIntValue::new(Arc::new(BitVec::repeat(*self, 1))))
+            }
+        }
+    }
+}
+
+macro_rules! impl_to_sim_value_for_primitive_int {
+    ($prim:ident) => {
+        impl ToSimValue<<$prim as ToExpr>::Type> for $prim {
+            #[track_caller]
+            fn to_sim_value(
+                &self,
+                ty: <$prim as ToExpr>::Type,
+            ) -> SimValue<<$prim as ToExpr>::Type> {
+                SimValue::from_value(ty, (*self).into())
+            }
+        }
+
+        impl ToSimValue<<<$prim as ToExpr>::Type as IntType>::Dyn> for $prim {
+            #[track_caller]
+            fn to_sim_value(
+                &self,
+                ty: <<$prim as ToExpr>::Type as IntType>::Dyn,
+            ) -> SimValue<<<$prim as ToExpr>::Type as IntType>::Dyn> {
+                SimValue::from_value(
+                    ty,
+                    <<$prim as ToExpr>::Type as Type>::SimValue::from(*self).as_dyn_int(),
+                )
+            }
+        }
+
+        impl ToSimValue<CanonicalType> for $prim {
+            #[track_caller]
+            fn to_sim_value(&self, ty: CanonicalType) -> SimValue<CanonicalType> {
+                let ty: <<$prim as ToExpr>::Type as IntType>::Dyn = Type::from_canonical(ty);
+                SimValue::into_canonical(self.to_sim_value(ty))
+            }
+        }
+    };
+}
+
+impl_to_sim_value_for_primitive_int!(u8);
+impl_to_sim_value_for_primitive_int!(u16);
+impl_to_sim_value_for_primitive_int!(u32);
+impl_to_sim_value_for_primitive_int!(u64);
+impl_to_sim_value_for_primitive_int!(u128);
+impl_to_sim_value_for_primitive_int!(usize);
+impl_to_sim_value_for_primitive_int!(i8);
+impl_to_sim_value_for_primitive_int!(i16);
+impl_to_sim_value_for_primitive_int!(i32);
+impl_to_sim_value_for_primitive_int!(i64);
+impl_to_sim_value_for_primitive_int!(i128);
+impl_to_sim_value_for_primitive_int!(isize);
+
+macro_rules! impl_to_sim_value_for_int_value {
+    ($IntValue:ident, $Int:ident, $IntType:ident) => {
+        impl<Width: KnownSize> ToSimValue<$IntType<Width>> for $IntValue<Width> {
+            fn to_sim_value(&self, ty: $IntType<Width>) -> SimValue<$IntType<Width>> {
+                self.bits().to_sim_value(ty)
+            }
+
+            fn into_sim_value(self, ty: $IntType<Width>) -> SimValue<$IntType<Width>> {
+                self.into_bits().into_sim_value(ty)
+            }
+        }
+
+        impl<Width: Size> ToSimValue<$Int> for $IntValue<Width> {
+            fn to_sim_value(&self, ty: $Int) -> SimValue<$Int> {
+                self.bits().to_sim_value(ty)
+            }
+
+            fn into_sim_value(self, ty: $Int) -> SimValue<$Int> {
+                self.into_bits().into_sim_value(ty)
+            }
+        }
+
+        impl<Width: Size> ToSimValue<CanonicalType> for $IntValue<Width> {
+            #[track_caller]
+            fn to_sim_value(&self, ty: CanonicalType) -> SimValue<CanonicalType> {
+                SimValue::into_canonical(self.to_sim_value($Int::from_canonical(ty)))
+            }
+
+            #[track_caller]
+            fn into_sim_value(self, ty: CanonicalType) -> SimValue<CanonicalType> {
+                SimValue::into_canonical(self.into_sim_value($Int::from_canonical(ty)))
+            }
+        }
+    };
+}
+
+impl_to_sim_value_for_int_value!(UIntValue, UInt, UIntType);
+impl_to_sim_value_for_int_value!(SIntValue, SInt, SIntType);

crates/fayalite/src/ty.rs

@@ -7,14 +7,15 @@ use crate::{
     clock::Clock,
     enum_::Enum,
     expr::Expr,
-    int::{Bool, SInt, UInt},
+    int::{Bool, SInt, UInt, UIntValue},
     intern::{Intern, Interned},
     phantom_const::PhantomConst,
     reset::{AsyncReset, Reset, SyncReset},
     source_location::SourceLocation,
     util::ConstUsize,
 };
-use std::{fmt, hash::Hash, iter::FusedIterator, ops::Index};
+use bitvec::slice::BitSlice;
+use std::{fmt, hash::Hash, iter::FusedIterator, ops::Index, sync::Arc};
 
 #[derive(Copy, Clone, Hash, PartialEq, Eq, Debug)]
 #[non_exhaustive]
@@ -268,6 +269,7 @@ pub trait Type:
 {
     type BaseType: BaseType;
     type MaskType: Type<MaskType = Self::MaskType>;
+    type SimValue: fmt::Debug + Clone + 'static;
     type MatchVariant: 'static + Send + Sync;
     type MatchActiveScope;
     type MatchVariantAndInactiveScope: MatchVariantAndInactiveScope<
@@ -285,6 +287,9 @@ pub trait Type:
     fn canonical(&self) -> CanonicalType;
     fn from_canonical(canonical_type: CanonicalType) -> Self;
     fn source_location() -> SourceLocation;
+    fn sim_value_from_bits(&self, bits: &BitSlice) -> Self::SimValue;
+    fn sim_value_clone_from_bits(&self, value: &mut Self::SimValue, bits: &BitSlice);
+    fn sim_value_to_bits(&self, value: &Self::SimValue, bits: &mut BitSlice);
 }
 
 pub trait BaseType: Type<BaseType = Self> + sealed::BaseTypeSealed + Into<CanonicalType> {}
@@ -314,6 +319,7 @@ pub trait TypeWithDeref: Type {
 impl Type for CanonicalType {
     type BaseType = CanonicalType;
     type MaskType = CanonicalType;
+    type SimValue = OpaqueSimValue;
     impl_match_variant_as_self!();
     fn mask_type(&self) -> Self::MaskType {
         match self {
@@ -339,6 +345,48 @@ impl Type for CanonicalType {
     fn source_location() -> SourceLocation {
         SourceLocation::builtin()
     }
+    fn sim_value_from_bits(&self, bits: &BitSlice) -> Self::SimValue {
+        assert_eq!(bits.len(), self.bit_width());
+        OpaqueSimValue::from_bitslice(bits)
+    }
+    fn sim_value_clone_from_bits(&self, value: &mut Self::SimValue, bits: &BitSlice) {
+        assert_eq!(bits.len(), self.bit_width());
+        assert_eq!(value.bit_width(), self.bit_width());
+        value.bits_mut().bits_mut().copy_from_bitslice(bits);
+    }
+    fn sim_value_to_bits(&self, value: &Self::SimValue, bits: &mut BitSlice) {
+        assert_eq!(bits.len(), self.bit_width());
+        assert_eq!(value.bit_width(), self.bit_width());
+        bits.copy_from_bitslice(value.bits().bits());
+    }
+}
+
+#[derive(Clone, PartialEq, Eq, Hash, Debug)]
+pub struct OpaqueSimValue {
+    bits: UIntValue,
+}
+
+impl OpaqueSimValue {
+    pub fn bit_width(&self) -> usize {
+        self.bits.width()
+    }
+    pub fn bits(&self) -> &UIntValue {
+        &self.bits
+    }
+    pub fn bits_mut(&mut self) -> &mut UIntValue {
+        &mut self.bits
+    }
+    pub fn into_bits(self) -> UIntValue {
+        self.bits
+    }
+    pub fn from_bits(bits: UIntValue) -> Self {
+        Self { bits }
+    }
+    pub fn from_bitslice(v: &BitSlice) -> Self {
+        Self {
+            bits: UIntValue::new(Arc::new(v.to_bitvec())),
+        }
+    }
 }
 
 pub trait StaticType: Type {

crates/fayalite/src/util.rs

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: LGPL-3.0-or-later
 // See Notices.txt for copyright information
 
+pub(crate) mod alternating_cell;
 mod const_bool;
 mod const_cmp;
 mod const_usize;

crates/fayalite/src/util/alternating_cell.rs

@@ -0,0 +1,122 @@
+// SPDX-License-Identifier: LGPL-3.0-or-later
+// See Notices.txt for copyright information
+
+use crate::util::DebugAsDisplay;
+use std::{
+    cell::{Cell, UnsafeCell},
+    fmt,
+};
+
+pub(crate) trait AlternatingCellMethods {
+    fn unique_to_shared(&mut self);
+    fn shared_to_unique(&mut self);
+}
+
+#[derive(Copy, Clone, Debug)]
+enum State {
+    Unique,
+    Shared,
+    Locked,
+}
+
+pub(crate) struct AlternatingCell<T: ?Sized> {
+    state: Cell<State>,
+    value: UnsafeCell<T>,
+}
+
+impl<T: ?Sized + fmt::Debug + AlternatingCellMethods> fmt::Debug for AlternatingCell<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("AlternatingCell")
+            .field(
+                self.try_share()
+                    .as_ref()
+                    .map(|v| -> &dyn fmt::Debug { v })
+                    .unwrap_or(&DebugAsDisplay("<...>")),
+            )
+            .finish()
+    }
+}
+
+impl<T: ?Sized> AlternatingCell<T> {
+    pub(crate) const fn new_shared(value: T) -> Self
+    where
+        T: Sized,
+    {
+        Self {
+            state: Cell::new(State::Shared),
+            value: UnsafeCell::new(value),
+        }
+    }
+    pub(crate) const fn new_unique(value: T) -> Self
+    where
+        T: Sized,
+    {
+        Self {
+            state: Cell::new(State::Unique),
+            value: UnsafeCell::new(value),
+        }
+    }
+    pub(crate) fn is_unique(&self) -> bool {
+        matches!(self.state.get(), State::Unique)
+    }
+    pub(crate) fn is_shared(&self) -> bool {
+        matches!(self.state.get(), State::Shared)
+    }
+    pub(crate) fn into_inner(self) -> T
+    where
+        T: Sized,
+    {
+        self.value.into_inner()
+    }
+    pub(crate) fn try_share(&self) -> Option<&T>
+    where
+        T: AlternatingCellMethods,
+    {
+        match self.state.get() {
+            State::Shared => {}
+            State::Unique => {
+                struct Locked<'a>(&'a Cell<State>);
+                impl Drop for Locked<'_> {
+                    fn drop(&mut self) {
+                        self.0.set(State::Shared);
+                    }
+                }
+                self.state.set(State::Locked);
+                let lock = Locked(&self.state);
+                // Safety: state is Locked, so nothing else will
+                // access value while calling unique_to_shared.
+                unsafe { &mut *self.value.get() }.unique_to_shared();
+                drop(lock);
+            }
+            State::Locked => return None,
+        }
+
+        // Safety: state is Shared so nothing will create any mutable
+        // references until the returned reference's lifetime expires.
+        Some(unsafe { &*self.value.get() })
+    }
+    #[track_caller]
+    pub(crate) fn share(&self) -> &T
+    where
+        T: AlternatingCellMethods,
+    {
+        let Some(retval) = self.try_share() else {
+            panic!("`share` called recursively");
+        };
+        retval
+    }
+    pub(crate) fn unique(&mut self) -> &mut T
+    where
+        T: AlternatingCellMethods,
+    {
+        match self.state.get() {
+            State::Shared => {
+                self.state.set(State::Unique);
+                self.value.get_mut().shared_to_unique();
+            }
+            State::Unique => {}
+            State::Locked => unreachable!(),
+        }
+        self.value.get_mut()
+    }
+}

crates/fayalite/tests/sim.rs

@@ -3,10 +3,16 @@
 
 use fayalite::{
     int::UIntValue,
+    memory::{ReadStruct, ReadWriteStruct, WriteStruct},
     module::{instance_with_loc, reg_builder_with_loc},
     prelude::*,
     reset::ResetType,
-    sim::{time::SimDuration, vcd::VcdWriterDecls, Simulation, ToSimValue},
+    sim::{
+        time::SimDuration,
+        value::{SimValue, ToSimValue},
+        vcd::VcdWriterDecls,
+        Simulation,
+    },
     ty::StaticType,
     util::RcWriter,
 };
@@ -396,32 +402,14 @@ fn test_enums() {
     sim.write_reset(sim.io().cd.rst, false);
     sim.advance_time(SimDuration::from_nanos(900));
     type BOutTy = HdlOption<(UInt<1>, Bool)>;
-    #[derive(Debug)]
+    #[derive(Debug, PartialEq)]
     struct IO {
         en: bool,
         which_in: u8,
         data_in: u8,
         which_out: u8,
         data_out: u8,
-        b_out: Expr<BOutTy>,
+        b_out: SimValue<BOutTy>,
-    }
-    impl PartialEq for IO {
-        fn eq(&self, other: &Self) -> bool {
-            let Self {
-                en,
-                which_in,
-                data_in,
-                which_out,
-                data_out,
-                b_out,
-            } = *self;
-            en == other.en
-                && which_in == other.which_in
-                && data_in == other.data_in
-                && which_out == other.which_out
-                && data_out == other.data_out
-                && b_out.to_sim_value(BOutTy::TYPE) == other.b_out.to_sim_value(BOutTy::TYPE)
-        }
     }
     let io_cycles = [
         IO {
@@ -430,7 +418,7 @@ fn test_enums() {
             data_in: 0,
             which_out: 0,
             data_out: 0,
-            b_out: HdlNone(),
+            b_out: HdlNone().to_sim_value(StaticType::TYPE),
         },
         IO {
             en: true,
@@ -438,7 +426,7 @@ fn test_enums() {
             data_in: 0,
             which_out: 0,
             data_out: 0,
-            b_out: HdlNone(),
+            b_out: HdlNone().to_sim_value(StaticType::TYPE),
         },
         IO {
             en: false,
@@ -446,7 +434,7 @@ fn test_enums() {
             data_in: 0,
             which_out: 1,
             data_out: 0,
-            b_out: HdlSome((0_hdl_u1, false)),
+            b_out: HdlSome((0_hdl_u1, false)).to_sim_value(StaticType::TYPE),
         },
         IO {
             en: true,
@@ -454,7 +442,7 @@ fn test_enums() {
             data_in: 0xF,
             which_out: 1,
             data_out: 0,
-            b_out: HdlSome((0_hdl_u1, false)),
+            b_out: HdlSome((0_hdl_u1, false)).to_sim_value(StaticType::TYPE),
         },
         IO {
             en: true,
@@ -462,7 +450,7 @@ fn test_enums() {
             data_in: 0xF,
             which_out: 1,
             data_out: 0x3,
-            b_out: HdlSome((1_hdl_u1, true)),
+            b_out: HdlSome((1_hdl_u1, true)).to_sim_value(StaticType::TYPE),
         },
         IO {
             en: true,
@@ -470,7 +458,7 @@ fn test_enums() {
             data_in: 0xF,
             which_out: 1,
             data_out: 0x3,
-            b_out: HdlSome((1_hdl_u1, true)),
+            b_out: HdlSome((1_hdl_u1, true)).to_sim_value(StaticType::TYPE),
         },
         IO {
             en: true,
@@ -478,7 +466,7 @@ fn test_enums() {
             data_in: 0xF,
             which_out: 2,
             data_out: 0xF,
-            b_out: HdlNone(),
+            b_out: HdlNone().to_sim_value(StaticType::TYPE),
         },
     ];
     for (
@@ -510,7 +498,7 @@ fn test_enums() {
                 .to_bigint()
                 .try_into()
                 .expect("known to be in range"),
-            b_out: sim.read(sim.io().b_out).to_expr(),
+            b_out: sim.read(sim.io().b_out),
         };
         assert_eq!(
             expected,
@@ -539,9 +527,9 @@ fn test_enums() {
 #[hdl_module(outline_generated)]
 pub fn memories() {
     #[hdl]
-    let r: fayalite::memory::ReadStruct<(UInt<8>, SInt<8>), ConstUsize<4>> = m.input();
+    let r: ReadStruct<(UInt<8>, SInt<8>), ConstUsize<4>> = m.input();
     #[hdl]
-    let w: fayalite::memory::WriteStruct<(UInt<8>, SInt<8>), ConstUsize<4>> = m.input();
+    let w: WriteStruct<(UInt<8>, SInt<8>), ConstUsize<4>> = m.input();
     #[hdl]
     let mut mem = memory_with_init([(0x01u8, 0x23i8); 16]);
     mem.read_latency(0);
@@ -560,120 +548,131 @@ fn test_memories() {
     sim.add_trace_writer(VcdWriterDecls::new(writer.clone()));
     sim.write_clock(sim.io().r.clk, false);
     sim.write_clock(sim.io().w.clk, false);
-    #[derive(Debug, PartialEq, Eq)]
+    #[hdl(cmp_eq)]
     struct IO {
-        r_addr: u8,
+        r_addr: UInt<4>,
-        r_en: bool,
+        r_en: Bool,
-        r_data: (u8, i8),
+        r_data: (UInt<8>, SInt<8>),
-        w_addr: u8,
+        w_addr: UInt<4>,
-        w_en: bool,
+        w_en: Bool,
-        w_data: (u8, i8),
+        w_data: (UInt<8>, SInt<8>),
-        w_mask: (bool, bool),
+        w_mask: (Bool, Bool),
     }
     let io_cycles = [
+        #[hdl]
         IO {
-            r_addr: 0,
+            r_addr: 0_hdl_u4,
             r_en: false,
-            r_data: (0, 0),
+            r_data: (0u8, 0i8),
-            w_addr: 0,
+            w_addr: 0_hdl_u4,
             w_en: false,
-            w_data: (0, 0),
+            w_data: (0u8, 0i8),
             w_mask: (false, false),
         },
+        #[hdl]
         IO {
-            r_addr: 0,
+            r_addr: 0_hdl_u4,
             r_en: true,
-            r_data: (0x1, 0x23),
+            r_data: (0x1u8, 0x23i8),
-            w_addr: 0,
+            w_addr: 0_hdl_u4,
             w_en: true,
-            w_data: (0x10, 0x20),
+            w_data: (0x10u8, 0x20i8),
             w_mask: (true, true),
         },
+        #[hdl]
         IO {
-            r_addr: 0,
+            r_addr: 0_hdl_u4,
             r_en: true,
-            r_data: (0x10, 0x20),
+            r_data: (0x10u8, 0x20i8),
-            w_addr: 0,
+            w_addr: 0_hdl_u4,
             w_en: true,
-            w_data: (0x30, 0x40),
+            w_data: (0x30u8, 0x40i8),
             w_mask: (false, true),
         },
+        #[hdl]
         IO {
-            r_addr: 0,
+            r_addr: 0_hdl_u4,
             r_en: true,
-            r_data: (0x10, 0x40),
+            r_data: (0x10u8, 0x40i8),
-            w_addr: 0,
+            w_addr: 0_hdl_u4,
             w_en: true,
-            w_data: (0x50, 0x60),
+            w_data: (0x50u8, 0x60i8),
             w_mask: (true, false),
         },
+        #[hdl]
         IO {
-            r_addr: 0,
+            r_addr: 0_hdl_u4,
             r_en: true,
-            r_data: (0x50, 0x40),
+            r_data: (0x50u8, 0x40i8),
-            w_addr: 0,
+            w_addr: 0_hdl_u4,
             w_en: true,
-            w_data: (0x70, -0x80),
+            w_data: (0x70u8, -0x80i8),
             w_mask: (false, false),
         },
+        #[hdl]
         IO {
-            r_addr: 0,
+            r_addr: 0_hdl_u4,
             r_en: true,
-            r_data: (0x50, 0x40),
+            r_data: (0x50u8, 0x40i8),
-            w_addr: 0,
+            w_addr: 0_hdl_u4,
             w_en: false,
-            w_data: (0x90, 0xA0u8 as i8),
+            w_data: (0x90u8, 0xA0u8 as i8),
             w_mask: (false, false),
         },
+        #[hdl]
         IO {
-            r_addr: 0,
+            r_addr: 0_hdl_u4,
             r_en: true,
-            r_data: (0x50, 0x40),
+            r_data: (0x50u8, 0x40i8),
-            w_addr: 1,
+            w_addr: 1_hdl_u4,
             w_en: true,
-            w_data: (0x90, 0xA0u8 as i8),
+            w_data: (0x90u8, 0xA0u8 as i8),
             w_mask: (true, true),
         },
+        #[hdl]
         IO {
-            r_addr: 0,
+            r_addr: 0_hdl_u4,
             r_en: true,
-            r_data: (0x50, 0x40),
+            r_data: (0x50u8, 0x40i8),
-            w_addr: 2,
+            w_addr: 2_hdl_u4,
             w_en: true,
-            w_data: (0xB0, 0xC0u8 as i8),
+            w_data: (0xB0u8, 0xC0u8 as i8),
             w_mask: (true, true),
         },
+        #[hdl]
         IO {
-            r_addr: 0,
+            r_addr: 0_hdl_u4,
             r_en: true,
-            r_data: (0x50, 0x40),
+            r_data: (0x50u8, 0x40i8),
-            w_addr: 2,
+            w_addr: 2_hdl_u4,
             w_en: false,
-            w_data: (0xD0, 0xE0u8 as i8),
+            w_data: (0xD0u8, 0xE0u8 as i8),
             w_mask: (true, true),
         },
+        #[hdl]
         IO {
-            r_addr: 1,
+            r_addr: 1_hdl_u4,
             r_en: true,
-            r_data: (0x90, 0xA0u8 as i8),
+            r_data: (0x90u8, 0xA0u8 as i8),
-            w_addr: 2,
+            w_addr: 2_hdl_u4,
             w_en: false,
-            w_data: (0xD0, 0xE0u8 as i8),
+            w_data: (0xD0u8, 0xE0u8 as i8),
             w_mask: (true, true),
         },
+        #[hdl]
         IO {
-            r_addr: 2,
+            r_addr: 2_hdl_u4,
             r_en: true,
-            r_data: (0xB0, 0xC0u8 as i8),
+            r_data: (0xB0u8, 0xC0u8 as i8),
-            w_addr: 2,
+            w_addr: 2_hdl_u4,
             w_en: false,
-            w_data: (0xD0, 0xE0u8 as i8),
+            w_data: (0xD0u8, 0xE0u8 as i8),
             w_mask: (true, true),
         },
     ];
-    for (
+    for (cycle, expected) in io_cycles.into_iter().enumerate() {
-        cycle,
+        #[hdl]
-        expected @ IO {
+        let IO {
             r_addr,
             r_en,
             r_data: _,
@@ -681,37 +680,26 @@ fn test_memories() {
             w_en,
             w_data,
             w_mask,
-        },
+        } = expected;
-    ) in io_cycles.into_iter().enumerate()
+        sim.write(sim.io().r.addr, r_addr);
-    {
+        sim.write(sim.io().r.en, r_en);
-        sim.write_bool_or_int(sim.io().r.addr, r_addr.cast_to_static());
+        sim.write(sim.io().w.addr, w_addr);
-        sim.write_bool(sim.io().r.en, r_en);
+        sim.write(sim.io().w.en, w_en);
-        sim.write_bool_or_int(sim.io().w.addr, w_addr.cast_to_static());
+        sim.write(sim.io().w.data, w_data);
-        sim.write_bool(sim.io().w.en, w_en);
+        sim.write(sim.io().w.mask, w_mask);
-        sim.write_bool_or_int(sim.io().w.data.0, w_data.0);
+        let io = (#[hdl]
-        sim.write_bool_or_int(sim.io().w.data.1, w_data.1);
+        IO {
-        sim.write_bool(sim.io().w.mask.0, w_mask.0);
-        sim.write_bool(sim.io().w.mask.1, w_mask.1);
-        let io = IO {
             r_addr,
             r_en,
-            r_data: (
+            r_data: sim.read(sim.io().r.data),
-                sim.read_bool_or_int(sim.io().r.data.0)
-                    .to_bigint()
-                    .try_into()
-                    .expect("known to be in range"),
-                sim.read_bool_or_int(sim.io().r.data.1)
-                    .to_bigint()
-                    .try_into()
-                    .expect("known to be in range"),
-            ),
             w_addr,
             w_en,
             w_data,
             w_mask,
-        };
+        })
+        .to_sim_value(StaticType::TYPE);
         assert_eq!(
-            expected,
+            expected.to_sim_value(StaticType::TYPE),
             io,
             "vcd:\n{}\ncycle: {cycle}",
             String::from_utf8(writer.take()).unwrap(),
@@ -739,7 +727,7 @@ fn test_memories() {
 #[hdl_module(outline_generated)]
 pub fn memories2() {
     #[hdl]
-    let rw: fayalite::memory::ReadWriteStruct<UInt<2>, ConstUsize<3>> = m.input();
+    let rw: ReadWriteStruct<UInt<2>, ConstUsize<3>> = m.input();
     #[hdl]
     let mut mem = memory_with_init([HdlSome(true); 5]);
     mem.read_latency(1);
@@ -1012,9 +1000,9 @@ fn test_memories2() {
 #[hdl_module(outline_generated)]
 pub fn memories3() {
     #[hdl]
-    let r: fayalite::memory::ReadStruct<Array<UInt<8>, 8>, ConstUsize<3>> = m.input();
+    let r: ReadStruct<Array<UInt<8>, 8>, ConstUsize<3>> = m.input();
     #[hdl]
-    let w: fayalite::memory::WriteStruct<Array<UInt<8>, 8>, ConstUsize<3>> = m.input();
+    let w: WriteStruct<Array<UInt<8>, 8>, ConstUsize<3>> = m.input();
     #[hdl]
     let mut mem: MemBuilder<Array<UInt<8>, 8>> = memory();
     mem.depth(8);

crates/fayalite/tests/sim/expected/extern_module2.txt

@@ -222,7 +222,9 @@ Simulation {
                     },
                     value: SimValue {
                         ty: Clock,
-                        bits: 0x1,
+                        value: OpaqueSimValue {
+                            bits: 0x1_u1,
+                        },
                     },
                 },
             },

crates/fayalite/tests/sim/expected/ripple_counter.txt

@@ -826,7 +826,9 @@ Simulation {
                     },
                     value: SimValue {
                         ty: Clock,
-                        bits: 0x0,
+                        value: OpaqueSimValue {
+                            bits: 0x0_u1,
+                        },
                     },
                 },
             },
@@ -921,7 +923,9 @@ Simulation {
                     },
                     value: SimValue {
                         ty: Clock,
-                        bits: 0x0,
+                        value: OpaqueSimValue {
+                            bits: 0x0_u1,
+                        },
                     },
                 },
             },
@@ -1016,7 +1020,9 @@ Simulation {
                     },
                     value: SimValue {
                         ty: Clock,
-                        bits: 0x0,
+                        value: OpaqueSimValue {
+                            bits: 0x0_u1,
+                        },
                     },
                 },
             },