learning_c/2025/making_a_c_compiler/TODO.md

write a compiler driver - ✅

Options

• --lex -> run the lexer, don't parse
• --parse -> lex and parse, don't generate assembly
• --codegen -> lex, parse, generate assembly, but don't emit code

Steps

Preprocess

> gcc -E -P <input-file> -o <preprocessed-file>.i

Compile (stub for now)

Assemble and Link

> gcc <assembly-file>.s -o <output-file>

write a lexer - ✅

WORD_BOUNDARY = '\b'
IDENTIFIER = "IDENTIFIER"
CONSTANT = "CONSTANT"
KW_INT = "KW_INT"
KW_VOID = "KW_VOID"
KW_RETURN = "KW_RETURN"
PAREN_OPEN = "PAREN_OPEN"
PAREN_CLOSE = "PARENT_CLOSE"
BRACE_OPEN = "BRACE_OPEN"
BRACE_CLOSE = "BRACE_CLOSE"
SEMICOLON = "SEMICOLON"

REGEX_TABLE = (
    (r"[a-zA-Z_]\w*\b", IDENTIFIER),
    (r"[0-9]+\b", CONSTANT),
    (r"int\b", KW_INT),
    (r"void\b", KW_VOID),
    (r"return\b", KW_RETURN),
    (r"\(", PAREN_OPEN),
    (r"\)", PAREN_CLOSE),
    (r"{", BRACE_OPEN),
    (r"}", BRACE_CLOSE),
    (r";", SEMICOLON),
)

while input isn't empty:
  if input starts with whitespace:
    trim whitespace from start of input
  else:
    find longest match at start of input for any regex in REGEX_TABLE
    if no match is found, raise an error
    convert matching substring into a token
    remove matching substring from start of input

We must return a list of tokens. Some have values (constants, keywords); others do not (PAREN_OPEN, SEMICOLON).

Write a parser

• program: Program nodes can have only one child, a function_definition
• function_definition has one child, body: Statement, and one attribute name: Identifier
• body has one child, return_value: Expression
• we only have one Express, which is a Constant