verilog: support recursive functions using ternary expressions

This adds a mechanism for marking certain portions of elaboration as
occurring within unevaluated ternary branches. To enable elaboration of
the overall ternary, this also adds width detection for these
unelaborated function calls.

frontends/ast/ast.h

@@ -270,6 +270,9 @@ namespace AST
 		bool is_simple_const_expr();
 		std::string process_format_str(const std::string &sformat, int next_arg, int stage, int width_hint, bool sign_hint);
 
+		bool is_recursive_function() const;
+		std::pair<AstNode*, AstNode*> get_tern_choice();
+
 		// create a human-readable text representation of the AST (for debugging)
 		void dumpAst(FILE *f, std::string indent) const;
 		void dumpVlog(FILE *f, std::string indent) const;

frontends/ast/genrtlil.cc

@@ -944,6 +944,41 @@ void AstNode::detectSignWidthWorker(int &width_hint, bool &sign_hint, bool *foun
 			}
 			break;
 		}
+		if (current_scope.count(str))
+		{
+			// This width detection is needed for function calls which are
+			// unelaborated, which currently only applies to calls to recursive
+			// functions reached by unevaluated ternary branches.
+			const AstNode *func = current_scope.at(str);
+			if (func->type != AST_FUNCTION)
+				log_file_error(filename, location.first_line, "Function call to %s resolved to something that isn't a function!\n", RTLIL::unescape_id(str).c_str());
+			const AstNode *wire = nullptr;
+			for (const AstNode *child : func->children)
+				if (child->str == func->str) {
+					wire = child;
+					break;
+				}
+			log_assert(wire && wire->type == AST_WIRE);
+			sign_hint = wire->is_signed;
+			width_hint = 1;
+			if (!wire->children.empty())
+			{
+				log_assert(wire->children.size() == 1);
+				const AstNode *range = wire->children.at(0);
+				log_assert(range->type == AST_RANGE && range->children.size() == 2);
+				AstNode *left = range->children.at(0)->clone();
+				AstNode *right = range->children.at(1)->clone();
+				while (left->simplify(true, false, false, 1, -1, false, true)) { }
+				while (right->simplify(true, false, false, 1, -1, false, true)) { }
+				if (left->type != AST_CONSTANT || right->type != AST_CONSTANT)
+					log_file_error(filename, location.first_line, "Function %s has non-constant width!",
+							RTLIL::unescape_id(str).c_str());
+				width_hint = abs(int(left->asInt(true) - right->asInt(true)));
+				delete left;
+				delete right;
+			}
+			break;
+		}
 		YS_FALLTHROUGH
 
 	// everything should have been handled above -> print error if not.

frontends/ast/simplify.cc

@@ -575,6 +575,8 @@ bool AstNode::simplify(bool const_fold, bool at_zero, bool in_lvalue, int stage,
 		deep_recursion_warning = false;
 	}
 
+	static bool unevaluated_tern_branch = false;
+
 	AstNode *newNode = NULL;
 	bool did_something = false;
 
@@ -1091,7 +1093,6 @@ bool AstNode::simplify(bool const_fold, bool at_zero, bool in_lvalue, int stage,
 		break;
 
 	case AST_TERNARY:
-		detect_width_simple = true;
 		child_0_is_self_determined = true;
 		break;
 
@@ -1124,6 +1125,24 @@ bool AstNode::simplify(bool const_fold, bool at_zero, bool in_lvalue, int stage,
 		detectSignWidth(width_hint, sign_hint);
 
 	if (type == AST_TERNARY) {
+		if (width_hint < 0) {
+			while (!children[0]->basic_prep && children[0]->simplify(true, false, in_lvalue, stage, -1, false, in_param))
+				did_something = true;
+
+			bool backup_unevaluated_tern_branch = unevaluated_tern_branch;
+			AstNode *chosen = get_tern_choice().first;
+
+			unevaluated_tern_branch = backup_unevaluated_tern_branch || chosen == children[2];
+			while (!children[1]->basic_prep && children[1]->simplify(false, false, in_lvalue, stage, -1, false, in_param))
+				did_something = true;
+
+			unevaluated_tern_branch = backup_unevaluated_tern_branch || chosen == children[1];
+			while (!children[2]->basic_prep && children[2]->simplify(false, false, in_lvalue, stage, -1, false, in_param))
+				did_something = true;
+
+			unevaluated_tern_branch = backup_unevaluated_tern_branch;
+			detectSignWidth(width_hint, sign_hint);
+		}
 		int width_hint_left, width_hint_right;
 		bool sign_hint_left, sign_hint_right;
 		bool found_real_left, found_real_right;
@@ -1187,6 +1206,7 @@ bool AstNode::simplify(bool const_fold, bool at_zero, bool in_lvalue, int stage,
 	for (size_t i = 0; i < children.size(); i++) {
 		bool did_something_here = true;
 		bool backup_flag_autowire = flag_autowire;
+		bool backup_unevaluated_tern_branch = unevaluated_tern_branch;
 		if ((type == AST_GENFOR || type == AST_FOR) && i >= 3)
 			break;
 		if ((type == AST_GENIF || type == AST_GENCASE) && i >= 1)
@@ -1199,6 +1219,10 @@ bool AstNode::simplify(bool const_fold, bool at_zero, bool in_lvalue, int stage,
 			break;
 		if (type == AST_DEFPARAM && i == 0)
 			flag_autowire = true;
+		if (type == AST_TERNARY && i > 0 && !unevaluated_tern_branch) {
+			AstNode *chosen = get_tern_choice().first;
+			unevaluated_tern_branch = chosen && chosen != children[i];
+		}
 		while (did_something_here && i < children.size()) {
 			bool const_fold_here = const_fold, in_lvalue_here = in_lvalue;
 			int width_hint_here = width_hint;
@@ -1238,6 +1262,7 @@ bool AstNode::simplify(bool const_fold, bool at_zero, bool in_lvalue, int stage,
 			did_something = true;
 		}
 		flag_autowire = backup_flag_autowire;
+		unevaluated_tern_branch = backup_unevaluated_tern_branch;
 	}
 	for (auto &attr : attributes) {
 		while (attr.second->simplify(true, false, false, stage, -1, false, true))
@@ -3177,6 +3202,8 @@ skip_dynamic_range_lvalue_expansion:;
 		std::string prefix = sstr.str();
 
 		AstNode *decl = current_scope[str];
+		if (unevaluated_tern_branch && decl->is_recursive_function())
+			goto replace_fcall_later;
 		decl = decl->clone();
 		decl->replace_result_wire_name_in_function(str, "$result"); // enables recursion
 		decl->expand_genblock(prefix);
@@ -3610,24 +3637,9 @@ replace_fcall_later:;
 		case AST_TERNARY:
 			if (children[0]->isConst())
 			{
-				bool found_sure_true = false;
+				auto pair = get_tern_choice();
-				bool found_maybe_true = false;
+				AstNode *choice = pair.first;
-
+				AstNode *not_choice = pair.second;
-				if (children[0]->type == AST_CONSTANT)
-					for (auto &bit : children[0]->bits) {
-						if (bit == RTLIL::State::S1)
-							found_sure_true = true;
-						if (bit > RTLIL::State::S1)
-							found_maybe_true = true;
-					}
-				else
-					found_sure_true = children[0]->asReal(sign_hint) != 0;
-
-				AstNode *choice = NULL, *not_choice = NULL;
-				if (found_sure_true)
-					choice = children[1], not_choice = children[2];
-				else if (!found_maybe_true)
-					choice = children[2], not_choice = children[1];
 
 				if (choice != NULL) {
 					if (choice->type == AST_CONSTANT) {
@@ -4845,4 +4857,54 @@ void AstNode::allocateDefaultEnumValues()
 	}
 }
 
+bool AstNode::is_recursive_function() const
+{
+	std::set<const AstNode *> visited;
+	std::function<bool(const AstNode *node)> visit = [&](const AstNode *node) {
+		if (visited.count(node))
+			return node == this;
+		visited.insert(node);
+		if (node->type == AST_FCALL) {
+			auto it = current_scope.find(node->str);
+			if (it != current_scope.end() && visit(it->second))
+				return true;
+		}
+		for (const AstNode *child : node->children) {
+			if (visit(child))
+				return true;
+		}
+		return false;
+	};
+
+	log_assert(type == AST_FUNCTION);
+	return visit(this);
+}
+
+std::pair<AstNode*, AstNode*> AstNode::get_tern_choice()
+{
+	if (!children[0]->isConst())
+		return {};
+
+	bool found_sure_true = false;
+	bool found_maybe_true = false;
+
+	if (children[0]->type == AST_CONSTANT)
+		for (auto &bit : children[0]->bits) {
+			if (bit == RTLIL::State::S1)
+				found_sure_true = true;
+			if (bit > RTLIL::State::S1)
+				found_maybe_true = true;
+		}
+	else
+		found_sure_true = children[0]->asReal(true) != 0;
+
+	AstNode *choice = nullptr, *not_choice = nullptr;
+	if (found_sure_true)
+		choice = children[1], not_choice = children[2];
+	else if (!found_maybe_true)
+		choice = children[2], not_choice = children[1];
+
+	return {choice, not_choice};
+}
+
 YOSYS_NAMESPACE_END

tests/various/fib_tern.v

@@ -0,0 +1,70 @@
+module gate(
+    off, fib0, fib1, fib2, fib3, fib4, fib5, fib6, fib7, fib8, fib9
+);
+    input wire signed [31:0] off;
+
+    function automatic blah(
+        input x
+    );
+        blah = x;
+    endfunction
+
+    function automatic integer fib(
+        input integer k
+    );
+        fib = k == 0
+                ? 0
+                : k == 1
+                    ? 1
+                    : fib(k - 1) + fib(k - 2);
+    endfunction
+
+    function automatic integer fib_wrap(
+        input integer k,
+        output integer o
+    );
+        o = off + fib(k);
+    endfunction
+
+    output integer fib0;
+    output integer fib1;
+    output integer fib2;
+    output integer fib3;
+    output integer fib4;
+    output integer fib5;
+    output integer fib6;
+    output integer fib7;
+    output integer fib8;
+    output integer fib9;
+
+    initial begin : blk
+        integer unused;
+        unused = fib_wrap(0, fib0);
+        unused = fib_wrap(1, fib1);
+        unused = fib_wrap(2, fib2);
+        unused = fib_wrap(3, fib3);
+        unused = fib_wrap(4, fib4);
+        unused = fib_wrap(5, fib5);
+        unused = fib_wrap(6, fib6);
+        unused = fib_wrap(7, fib7);
+        unused = fib_wrap(8, fib8);
+        unused = fib_wrap(9, fib9);
+    end
+endmodule
+
+module gold(
+    off, fib0, fib1, fib2, fib3, fib4, fib5, fib6, fib7, fib8, fib9
+);
+    input wire signed [31:0] off;
+
+    output integer fib0 = off + 0;
+    output integer fib1 = off + 1;
+    output integer fib2 = off + 1;
+    output integer fib3 = off + 2;
+    output integer fib4 = off + 3;
+    output integer fib5 = off + 5;
+    output integer fib6 = off + 8;
+    output integer fib7 = off + 13;
+    output integer fib8 = off + 21;
+    output integer fib9 = off + 34;
+endmodule

tests/various/fib_tern.ys

@@ -0,0 +1,6 @@
+read_verilog fib_tern.v
+hierarchy
+proc
+equiv_make gold gate equiv
+equiv_simple
+equiv_status -assert