gh-153568: Make the parser's per-rule stack check an inline comparison - #153573
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pablogsal wants to merge 2 commits into
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gh-153568: Make the parser's per-rule stack check an inline comparison#153573pablogsal wants to merge 2 commits into
pablogsal wants to merge 2 commits into
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…parison The parser now caches the thread state and stack limit up front and only falls back to the full check when the stack is nearly exhausted.
maurycy
reviewed
Jul 17, 2026
eendebakpt
reviewed
Sep 14, 2026
| p->known_err_token = NULL; | ||
| p->tstate = PyThreadState_Get(); | ||
| // Force the stack limits to be initialized before caching them. | ||
| (void)_Py_ReachedRecursionLimitWithMargin(p->tstate, 1); |
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According to Claude the initialization has either been done already, or the soft limit is still UINTPTR_MAX and we have a wrap around.
| _PyPegen_stack_exhausted(Parser *p) | ||
| { | ||
| #if _Py_STACK_GROWS_DOWN | ||
| if (_Py_get_machine_stack_pointer() > p->stack_soft_limit) { |
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Add an assert so in debug builds we check this corresponds to _Py_ReachedRecursionLimitWithMargin(p->tstate, 1)?
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Every rule entry paid a thread-state fetch plus an out-of-line call just to ask whether the C stack is about to overflow. Caching the thread state and soft stack limit in the parser turns that into one inline pointer comparison, and the overflow guard still fires exactly as before.
Benchmark (parsing 8 of the largest stdlib files, 1.3 MB, 20 times per run with
_PyParser_ASTFromString— parser only, no AST-to-Python conversion; pyperf, interleaved runs):