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

The left-recursion driver now keeps a direct reference to its memo
entry and updates it in place on every iteration.

Author: pablogsal

Misc/NEWS.d/next/Core_and_Builtins/2026-07-11-14-59-17.gh-issue-153568.leftrec.rst

@@ -0,0 +1,2 @@
+Speed up parsing of left-recursive rules by updating their memoization
+entries in place.

Parser/parser.c

@@ -102,6 +102,17 @@ _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)

Parser/pegen.c

@@ -148,6 +148,7 @@ 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 *);

Parser/pegen.h

@@ -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) {")