gh-131798: JIT: Narrow the return type of _CALL_LEN to int (#132940)

Reduce unnecessary guards whenever `len()` is called and used
after.

Co-authored-by: Max Bernstein <tekknolagi@gmail.com>

Lib/test/test_capi/test_opt.py

@@ -1911,6 +1911,18 @@ class TestUopsOptimization(unittest.TestCase):
         self.assertNotIn("_COMPARE_OP_INT", uops)
         self.assertNotIn("_GUARD_IS_TRUE_POP", uops)
 
+    def test_call_len(self):
+        def testfunc(n):
+            a = [1, 2, 3, 4]
+            for _ in range(n):
+                _ = len(a) - 1
+
+        _, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+        uops = get_opnames(ex)
+        self.assertNotIn("_GUARD_NOS_INT", uops)
+        self.assertNotIn("_GUARD_TOS_INT", uops)
+        self.assertIn("_CALL_LEN", uops)
+
 
 def global_identity(x):
     return x

Misc/NEWS.d/next/Core_and_Builtins/2025-04-25-14-56-45.gh-issue-131798.NpcKub.rst

@@ -0,0 +1 @@
+Allow the JIT to remove int guards after ``_CALL_LEN`` by setting the return type to int. Patch by Diego Russo

Python/optimizer_bytecodes.c

@@ -1055,6 +1055,10 @@ dummy_func(void) {
         sym_set_const(callable, (PyObject *)&PyUnicode_Type);
     }
 
+    op(_CALL_LEN, (callable[1], self_or_null[1], args[oparg] -- res)) {
+        res = sym_new_type(ctx, &PyLong_Type);
+    }
+
 // END BYTECODES //
 
 }

Python/optimizer_cases.c.h

@@ -2014,7 +2014,7 @@
 
         case _CALL_LEN: {
             JitOptSymbol *res;
-            res = sym_new_not_null(ctx);
+            res = sym_new_type(ctx, &PyLong_Type);
             stack_pointer[-2 - oparg] = res;
             stack_pointer += -1 - oparg;
             assert(WITHIN_STACK_BOUNDS());