From: T. Wouters <thomas@python.org>
Date: Tue, 15 Apr 2025 09:39:32 +0000 (+0200)
Subject: gh-129987: Selectively re-enable SLP autovectorization of _PyEval_EvalFrameDefault... 
X-Git-Tag: v3.14.0b1~474
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=c66ffcf8e3ab889a30aae43520aa29c167344bd3;p=thirdparty%2FPython%2Fcpython.git

gh-129987: Selectively re-enable SLP autovectorization of _PyEval_EvalFrameDefault (#132530)

Only disable SLP autovectorization of `_PyEval_EvalFrameDefault` on newer
GCCs, as the optimization bug seems to exist only on GCC 12 and later, and
before GCC 9 disabling the optimization has a dramatic performance impact.
---

diff --git a/Python/ceval.c b/Python/ceval.c
index 278d9e36045d..e534c7e2b883 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -948,11 +948,15 @@ _PyObjectArray_Free(PyObject **array, PyObject **scratch)
 #include "generated_cases.c.h"
 #endif
 
-#if (defined(__GNUC__) && !defined(__clang__)) && defined(__x86_64__)
+#if (defined(__GNUC__) && __GNUC__ >= 10 && !defined(__clang__)) && defined(__x86_64__)
 /*
- * gh-129987: The SLP autovectorizer can cause poor code generation for opcode
- * dispatch, negating any benefit we get from vectorization elsewhere in the
- * interpreter loop.
+ * gh-129987: The SLP autovectorizer can cause poor code generation for
+ * opcode dispatch in some GCC versions (observed in GCCs 12 through 15,
+ * probably caused by https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115777),
+ * negating any benefit we get from vectorization elsewhere in the
+ * interpreter loop. Disabling it significantly affected older GCC versions
+ * (prior to GCC 9, 40% performance drop), so we have to selectively disable
+ * it.
  */
 #define DONT_SLP_VECTORIZE __attribute__((optimize ("no-tree-slp-vectorize")))
 #else