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

diegorusso · tekknolagi · web-flow · commit 4c20f46fa011 · 2025-04-26T02:57:42.000+08:00
Reduce unnecessary guards whenever `len()` is called and used
after.

Co-authored-by: Max Bernstein &lt;tekknolagi@gmail.com&gt;
diff --git a/Lib/test/test_capi/test_opt.py b/Lib/test/test_capi/test_opt.py
@@ -1911,6 +1911,18 @@ def testfunc(n):
         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
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-04-25-14-56-45.gh-issue-131798.NpcKub.rst b/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
diff --git a/Python/optimizer_bytecodes.c b/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 //
 
 }
diff --git a/Python/optimizer_cases.c.h b/Python/optimizer_cases.c.h

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	+Allow the JIT to remove int guards after ``_CALL_LEN`` by setting the return type to int. Patch by Diego Russo
Original file line number	Diff line number	Diff line change
`@@ -1055,6 +1055,10 @@ dummy_func(void) {`
`1055`	`1055`	`sym_set_const(callable, (PyObject *)&PyUnicode_Type);`
`1056`	`1056`	`}`
`1057`	`1057`
	`1058`	`+ op(_CALL_LEN, (callable[1], self_or_null[1], args[oparg] -- res)) {`
	`1059`	`+ res = sym_new_type(ctx, &PyLong_Type);`
	`1060`	`+ }`
	`1061`	`+`
`1058`	`1062`	`// END BYTECODES //`
`1059`	`1063`
`1060`	`1064`	`}`