cpython/Tools/cases_generator/README.md

# Tooling to generate interpreters

Documentation for the instruction definitions in `Python/bytecodes.c`
("the DSL") is [here](interpreter_definition.md).

What's currently here:

- `analyzer.py`: code for converting `AST` generated by `Parser`
  to more high-level structure for easier interaction
- `lexer.py`: lexer for C, originally written by Mark Shannon
- `plexer.py`: OO interface on top of lexer.py; main class: `PLexer`
- `parsing.py`: Parser for instruction definition DSL; main class: `Parser`
- `parser.py` helper for interactions with `parsing.py`
- `tierN_generator.py`: a couple of driver scripts to read `Python/bytecodes.c` and
  write `Python/generated_cases.c.h` (and several other files)
- `optimizer_generator.py`: reads `Python/bytecodes.c` and
  `Python/optimizer_bytecodes.c` and writes
  `Python/optimizer_cases.c.h`
- `stack.py`: code to handle generalized stack effects
- `cwriter.py`: code which understands tokens and how to format C code;
  main class: `CWriter`
- `generators_common.py`: helpers for generators
- `opcode_id_generator.py`: generate a list of opcodes and write them to
  `Include/opcode_ids.h`
- `opcode_metadata_generator.py`: reads the instruction definitions and
  write the metadata to `Include/internal/pycore_opcode_metadata.h`
- `py_metadata_generator.py`: reads the instruction definitions and
  write the metadata to `Lib/_opcode_metadata.py`
- `target_generator.py`: generate targets for computed goto dispatch and
  write them to `Python/opcode_targets.h`
- `uop_id_generator.py`: generate a list of uop IDs and write them to
  `Include/internal/pycore_uop_ids.h`
- `uop_metadata_generator.py`: reads the instruction definitions and
  write the metadata to `Include/internal/pycore_uop_metadata.h`

Note that there is some dummy C code at the top and bottom of
`Python/bytecodes.c`
to fool text editors like VS Code into believing this is valid C code.

## A bit about the parser

The parser class uses a pretty standard recursive descent scheme,
but with unlimited backtracking.
The `PLexer` class tokenizes the entire input before parsing starts.
We do not run the C preprocessor.
Each parsing method returns either an AST node (a `Node` instance)
or `None`, or raises `SyntaxError` (showing the error in the C source).

Most parsing methods are decorated with `@contextual`, which automatically
resets the tokenizer input position when `None` is returned.
Parsing methods may also raise `SyntaxError`, which is irrecoverable.
When a parsing method returns `None`, it is possible that after backtracking
a different parsing method returns a valid AST.

Neither the lexer nor the parsers are complete or fully correct.
Most known issues are tersely indicated by `# TODO:` comments.
We plan to fix issues as they become relevant.