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gh-153568: Stop re-walking memo lists when growing left-recursive rules #153574

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gh-153568: Stop re-walking memo lists when growing left-recursive rules #153574

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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-07-11-14-59-17.gh-issue-153568.leftrec.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Speed up parsing of left-recursive rules by updating their memoization
entries in place.
120 changes: 70 additions & 50 deletions Parser/parser.c
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11 changes: 11 additions & 0 deletions Parser/pegen.c
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Expand Up @@ -93,6 +93,17 @@ _PyPegen_insert_memo(Parser *p, int mark, int type, void *node)
return 0;
}

// Like _PyPegen_insert_memo(), but returns the inserted Memo so callers
// can update it in place without re-walking the token's memo list.
Memo *
_PyPegen_insert_memo_direct(Parser *p, int mark, int type)
{
if (_PyPegen_insert_memo(p, mark, type, NULL) < 0) {
return NULL;
}
return p->tokens[mark]->memo;
}

// Like _PyPegen_insert_memo(), but updates an existing node if found.
int
_PyPegen_update_memo(Parser *p, int mark, int type, void *node)
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1 change: 1 addition & 0 deletions Parser/pegen.h
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Expand Up @@ -143,6 +143,7 @@ PyObject *_PyPegen_get_memo_statistics(void);

int _PyPegen_insert_memo(Parser *p, int mark, int type, void *node);
int _PyPegen_update_memo(Parser *p, int mark, int type, void *node);
Memo *_PyPegen_insert_memo_direct(Parser *p, int mark, int type);
int _PyPegen_is_memoized(Parser *p, int type, void *pres);

int _PyPegen_lookahead(int, void *(func)(Parser *), Parser *);
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10 changes: 7 additions & 3 deletions Tools/peg_generator/pegen/c_generator.py
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Expand Up @@ -572,11 +572,15 @@ def _set_up_rule_memoization(self, node: Rule, result_type: str) -> None:
self.print("}")
self.print("int _mark = p->mark;")
self.print("int _resmark = p->mark;")
self.print(f"Memo *_memo = _PyPegen_insert_memo_direct(p, _mark, {node.name}_type);")
self.print("if (_memo == NULL) {")
with self.indent():
self.add_return("NULL")
self.print("}")
self.print("while (1) {")
with self.indent():
self.call_with_errorcheck_return(
f"_PyPegen_update_memo(p, _mark, {node.name}_type, _res)", "_res"
)
self.print("_memo->node = _res;")
self.print("_memo->mark = p->mark;")
self.print("p->mark = _mark;")
self.print(f"void *_raw = {node.name}_raw(p);")
self.print("if (p->error_indicator) {")
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