passes/pmgen/README.md


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Pattern Matcher Generator
=========================

The program `pmgen.py` reads a `.pmg` (Pattern Matcher Generator) file and
writes a header-only C++ library that implements that pattern matcher.

The "patterns" in this context are subgraphs in a Yosys RTLIL netlist.

The algorithm used in the generated pattern matcher is a simple recursive
search with backtracking. It is left to the author of the `.pmg` file to
determine an efficient cell order for the search that allows for maximum
use of indices and early backtracking.


API of Generated Matcher
========================

When `pmgen.py` reads a `foobar.pmg` file, it writes `foobar_pm.h` containing
a class `foobar_pm`. That class is instantiated with an RTLIL module and a
list of cells from that module:

    foobar_pm pm(module, module->selected_cells());

The caller must make sure that none of the cells in the 2nd argument are
deleted for as long as the patter matcher instance is used.

At any time it is possible to disable cells, preventing them from showing
up in any future matches:

    pm.blacklist(some_cell);

The `.run_<pattern_name>(callback_function)` method searches for all matches
for the pattern`<pattern_name>` and calls the callback function for each found
match:

    pm.run_foobar([&](){
        log("found matching 'foo' cell: %s\n", log_id(pm.st.foo));
        log("          with 'bar' cell: %s\n", log_id(pm.st.bar));
    });

The `.pmg` file declares matcher state variables that are accessible via the
`.st_<pattern_name>.<state_name>` members. (The `.st_<pattern_name>` member is
of type `foobar_pm::state_<pattern_name>_t`.)

Similarly the `.pmg` file declares user data variables that become members of
`.ud_<pattern_name>`, a struct of type `foobar_pm::udata_<pattern_name>_t`.

There are three versions of the `run_<pattern_name>()` method: Without callback,
callback without arguments, and callback with reference to `pm`. All versions
of the `run_<pattern_name>()` method return the number of found matches.


The .pmg File Format
====================

The `.pmg` file format is a simple line-based file format. For the most part