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docs/rosette: Reword
Acknowledge the minimal functional backend, highlighting what's new/different for our SExpr targets. Add and use the reference `minimal backend`. Use `note` directives to point out missing code sections (highlighting that the included diffs are not complete). Racket *is* still strongly-typed, it's just dynamic instead of static. Adjust phrasing to reflect that. Adjust some of the literal includes, adding a new section from the `Functional::AbstractVisitor`, splitting the `Module::write_eval()` in twain and adding a `smtlib.cc` literal include for the node iteration, as well as for the `FunctionalSmtBackend` to compare against the minimal backend. Move `Backend` description up to minimal functional backend section.
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Krystine Sherwin
parent
e01a413722
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3c493d2bef
1 changed files with 102 additions and 64 deletions
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@ -144,6 +144,8 @@ S-expressions can be constructed with ``SExpr::list``, for example ``SExpr expr
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parentheses.
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- The destructor calls ``flush`` but also closes all unclosed parentheses.
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.. _minimal backend:
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Example: A minimal functional backend
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-------------------------------------
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@ -158,6 +160,13 @@ Example: A minimal functional backend
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+ handle outputs and next state
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- backend pass boiler plate gives us ``write_functional_dummy`` command
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The final part is the ``Backend`` itself, which registers the command in Yosys.
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The ``execute`` method is the part that runs when the user calls the command,
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handling any options, preparing the output file for writing, and iterating over
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selected modules in the design. For more on adding new commands to Yosys and
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how they work, refer to :doc:`/yosys_internals/extending_yosys/extensions`.
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- pointer ``f`` is a ``std::ostream`` we can write to, being either a file or
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stdout
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- FunctionalIR conversion done by ``Functional::IR::from_module()``
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@ -233,18 +242,23 @@ The major changes from the SMT-LIB backend are as follows:
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Scope
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~~~~~
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As described above, the ``Functional::Scope`` class is derived in order to avoid
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collisions between identifiers in the generated output. In the SMT-LIB version
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the ``SmtScope`` class implements ``Scope<int>``; provides a constructor that
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iterates over a list of reserved keywords, calling ``reserve`` on each; and
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defines the ``is_character_legal`` method to reject any characters which are not
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allowed in SMT-LIB variable names to then be replaced with underscores in the
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output.
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Our first addition to the `minimal backend`_ above is that for both SMT-LIB and
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Rosette backends, we are now targetting real languages which bring with them
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their own sets of constraints with what we can use as identifiers. This is
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where the ``Functional::Scope`` class described above comes in; by using this
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class we can safely rename our identifiers in the generated output without
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worrying about collisions or illegal names/characters.
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In the Rosette version we switch out ``Smt`` in the class name for ``Smtr`` to
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mean ``smtlib_rosette``; this will happen through the rest of the code too. We
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also update list of legal ascii characters in the ``is_character_legal`` method
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to only those allowed in Racket variable names.
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In the SMT-LIB version, the ``SmtScope`` class implements ``Scope<int>``;
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provides a constructor that iterates over a list of reserved keywords, calling
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``reserve`` on each; and defines the ``is_character_legal`` method to reject any
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characters which are not allowed in SMT-LIB variable names to then be replaced
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with underscores in the output. To use this scope we create an instance of it,
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and call the ``Scope::unique_name()`` method to generate a unique and legal name
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for each of our identifiers.
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In the Rosette version we update the list of legal ascii characters in the
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``is_character_legal`` method to only those allowed in Racket variable names.
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.. literalinclude:: /generated/functional/rosette.diff
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:language: diff
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@ -276,8 +290,8 @@ provides this functionality natively with ``cons``, which we will see later.
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nullptr
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};
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Note that we skip over the actual list of reserved keywords from both the smtlib
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and racket specifications to save on space in this document.
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.. note:: We skip over the actual list of reserved keywords from both the smtlib
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and racket specifications to save on space in this document.
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Sort
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~~~~
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@ -297,16 +311,17 @@ signals represented as ``bitvector``\ s, and memories as ``list``\ s of signals.
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Struct
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~~~~~~
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The ``Functional::IR`` class tracks the set of inputs, the set of outputs, and
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the set of "state" variables. The SMT-LIB backend maps each of these sets into
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its own ``SmtStruct``, with each variable getting a corresponding field in the
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struct and a specified `Sort`_. `write_functional_smt2` then defines each of
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these structs as a new ``datatype``, with each element being strongly-typed.
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As we saw in the `minimal backend`_ above, the ``Functional::IR`` class tracks
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the set of inputs, the set of outputs, and the set of "state" variables. The
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SMT-LIB backend maps each of these sets into its own ``SmtStruct``, with each
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variable getting a corresponding field in the struct and a specified `Sort`_.
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`write_functional_smt2` then defines each of these structs as a new
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``datatype``, with each element being strongly-typed.
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In Rosette, rather than defining new datatypes for our structs, we use the
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native ``struct``. We also only declare each field by name because Racket is
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not as strongly-typed. For ease of use, we provide the expected type for each
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field as comments.
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native ``struct``. We also only declare each field by name because Racket
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provides less static typing. For ease of use, we provide the expected type for
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each field as comments.
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.. literalinclude:: /generated/functional/rosette.diff
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:language: diff
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@ -355,18 +370,19 @@ Rosette ``struct``\ s.
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PrintVisitor
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~~~~~~~~~~~~
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The ``PrintVisitor`` implements the abstract ``Functional::AbstractVisitor``
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class, described above in `What is FunctionalIR`_, with a return type of
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``SExpr``. This class converts FunctionalIR functions into s-expressions,
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including reading inputs/current state with the ``access`` method from the
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`Struct`_. For most functions, the Rosette output is very similar to the
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corresponding SMT-LIB function with minor adjustments for syntax.
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Remember in the `minimal backend`_ we converted nodes into strings for writing
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using the ``node.to_string()`` method, which wrapped ``node.visit()`` with a
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private visitor. We now want a custom visitor which can convert nodes into
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s-expressions. This is where the ``PrintVisitor`` comes in, implementing the
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abstract ``Functional::AbstractVisitor`` class with a return type of ``SExpr``.
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For most functions, the Rosette output is very similar to the corresponding
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SMT-LIB function with minor adjustments for syntax.
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.. literalinclude:: /generated/functional/rosette.diff
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:language: diff
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:caption: portion of ``Functional::AbstractVisitor`` implementation diff showing similarities
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:start-at: SExpr logical_shift_left
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:end-before: SExpr input
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:end-at: "list-set-bv"
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However there are some differences in the two formats with regards to how
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booleans are handled, with Rosette providing built-in functions for conversion.
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@ -377,13 +393,30 @@ booleans are handled, with Rosette providing built-in functions for conversion.
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:start-at: SExpr from_bool
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:end-before: SExpr extract
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Of note here is the rare instance of the Rosette implementation *gaining* static
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typing rather than losing it. Where SMT_LIB calls zero/sign extension with the
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number of extra bits needed (given by ``out_width - a.width()``), Rosette
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instead specifies the type of the output (given by ``list("bitvector",
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out_width)``).
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.. literalinclude:: /generated/functional/rosette.diff
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:language: diff
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:caption: zero/sign extension implementation diff
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:start-after: SExpr buf(
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:end-before: SExpr concat(
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:lines: 2-3, 5-6
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.. note:: Be sure to check the source code for the full list of differences here.
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Module
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~~~~~~
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The ``Functional::IR`` is wrapped in the ``SmtModule`` class, with the mapping
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from RTLIL module to FunctionalIR happening in the constructor. Each of the
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three ``SmtStruct``\ s; inputs, outputs, and state; are created, with each value
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in the corresponding lists in the IR being ``insert``\ ed.
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With most of the supporting classes out of the way, we now reach our three main
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steps from the `minimal backend`_. These are all handled by the ``SmtModule``
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class, with the mapping from RTLIL module to FunctionalIR happening in the
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constructor. Each of the three ``SmtStruct``\ s; inputs, outputs, and state;
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are also created in the constructor, with each value in the corresponding lists
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in the IR being ``insert``\ ed.
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.. literalinclude:: /generated/functional/smtlib.cc
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:language: c++
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@ -403,7 +436,7 @@ uses an underscore for the name of the initial state.
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The ``write`` method is then responsible for writing the FunctionalIR to the
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output file, formatted for the corresponding backend. ``SmtModule::write()``
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breaks the output file down into four parts: defining the three structs,
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declaring the ``pair`` datatype, defining the mapping function ``(inputs,
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declaring the ``pair`` datatype, defining the transfer function ``(inputs,
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current_state) -> (outputs, next_state)`` with ``write_eval``, and declaring the
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initial state with ``write_initial``. The only change for the ``SmtrModule`` is
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that the ``pair`` declaration isn't needed.
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@ -414,33 +447,35 @@ that the ``pair`` declaration isn't needed.
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:start-at: void write(std::ostream &out)
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:end-at: }
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For the ``write_eval`` method, the main differences are syntactical. First we
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change the function declaration line for the Rosette style which drops the
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explicit output typing and uses the ``define`` keyword instead of
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``define-fun``. And then we change the final result from a ``pair`` to the
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native ``cons`` which acts in much the same way, returning both the ``outputs``
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and the ``next_state`` in a single variable. Iteration over all of the
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``Functional::Node``\ s in the IR is the same in both.
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The ``write_eval`` method is where the FunctionalIR nodes, outputs, and next
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state are handled. Just as with the `minimal backend`_, we iterate over the
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nodes with ``for(auto n : ir)``, and then use the ``Struct::write_value()``
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method for the ``output_struct`` and ``state_struct`` to iterate over the
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outputs and next state respectively.
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.. inlined diff for showing the whole function while skipping the middle part
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.. code-block:: diff
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:caption: diff of ``Module::write_eval()`` method
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.. literalinclude:: /generated/functional/smtlib.cc
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:language: c++
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:caption: iterating over FunctionalIR nodes in ``SmtModule::write_eval()``
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:start-at: for(auto n : ir)
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:end-at: }
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void write_eval(SExprWriter &w)
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{
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w.push();
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- w.open(list("define-fun", name,
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- list(list("inputs", input_struct.name),
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- list("state", state_struct.name)),
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- list("Pair", output_struct.name, state_struct.name)));
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+ w.open(list("define", list(name, "inputs", "state")));
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...
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- w.open(list("pair"));
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+ w.open(list("cons"));
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output_struct.write_value(w, [&](IdString name) { return node_to_sexpr(ir.output(name).value()); });
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state_struct.write_value(w, [&](IdString name) { return node_to_sexpr(ir.state(name).next_value()); });
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w.pop();
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}
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The main differences between our two backends here are syntactical. First we
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change the ``define-fun`` for the Racket style ``define`` which drops the
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explicitly typed inputs/outputs. And then we change the final result from a
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``pair`` to the native ``cons`` which acts in much the same way, returning both
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the ``outputs`` and the ``next_state`` in a single variable.
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.. literalinclude:: /generated/functional/rosette.diff
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:language: diff
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:caption: diff of ``Module::write_eval()`` transfer function declaration
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:start-at: w.open(list("define-fun"
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:end-at: w.open(list("define"
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.. literalinclude:: /generated/functional/rosette.diff
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:language: diff
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:caption: diff of output/next state handling ``Module::write_eval()``
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:start-at: w.open(list("pair"
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:end-at: w.pop();
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For the ``write_initial`` method, the SMT-LIB backend uses ``declare-const`` and
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``assert``\ s which must always hold true. For Rosette we instead define the
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@ -458,12 +493,15 @@ whereas the SMT-LIB code can only verify that a given ``next_state`` is correct.
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Backend
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~~~~~~~
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The final part is the ``Backend`` itself, which registers the command in Yosys.
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The ``execute`` method is the part that runs when the user calls
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`write_functional_rosette`, handling any options, preparing the output file for
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writing, and iterating over selected modules in the design. For more on adding
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new commands to Yosys and how they work, refer to
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:doc:`/yosys_internals/extending_yosys/extensions`.
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The final part is the ``Backend`` itself, with much of the same boiler plate as
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the `minimal backend`_. The main difference is that we use the `Module`_ to
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perform the actual processing.
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.. literalinclude:: /generated/functional/smtlib.cc
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:language: c++
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:caption: The ``FunctionalSmtBackend``
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:start-at: struct FunctionalSmtBackend
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:end-at: } FunctionalSmtBackend;
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There are two additions here for Rosette. The first is that the output file
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needs to start with the ``#lang`` definition which tells the
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