From f6792a7d80e3135ee3da64398366d520a914f7b2 Mon Sep 17 00:00:00 2001
From: Pieter Eendebak <pieter.eendebak@gmail.com>
Date: Fri, 18 Sep 2026 09:17:09 +0200
Subject: [PATCH] Make PyErr_CheckSignals() cheap when no signal is pending

PyErr_CheckSignals() is called from long-running C loops (int
multiplication and division) and from every PyObject_Repr() and
PyObject_Str().  Before looking at whether any signal had arrived, it
fetched the interpreter config for the remote debugger and called
pthread_self() for the main-thread check.

Test the cheap flags first: _PyErr_CheckSignalsTstate() now does the
main-thread check itself, after the is_tripped test, and runs the handlers
in an out-of-line helper; _PyRunRemoteDebugger() tests its pending flag
before fetching the config.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
---
 Modules/signalmodule.c | 31 +++++++++++++++++++------------
 Python/ceval_gil.c     | 10 ++++------
 2 files changed, 23 insertions(+), 18 deletions(-)

diff --git a/Modules/signalmodule.c b/Modules/signalmodule.c
index bc5aef55648e07..c749f3ee0c2b19 100644
--- a/Modules/signalmodule.c
+++ b/Modules/signalmodule.c
@@ -1796,25 +1796,18 @@ PyErr_CheckSignals(void)
     _PyRunRemoteDebugger(tstate);
 #endif
 
-    if (_Py_ThreadCanHandleSignals(tstate->interp)) {
-        if (_PyErr_CheckSignalsTstate(tstate) < 0) {
-            return -1;
-        }
+    if (_PyErr_CheckSignalsTstate(tstate) < 0) {
+        return -1;
     }
 
     return 0;
 }
 
 
-/* Declared in cpython/pyerrors.h */
-int
-_PyErr_CheckSignalsTstate(PyThreadState *tstate)
+// Out of line, to keep the common case of _PyErr_CheckSignalsTstate() cheap.
+Py_NO_INLINE static int
+run_tripped_handlers(PyThreadState *tstate)
 {
-    _Py_CHECK_EMSCRIPTEN_SIGNALS();
-    if (!_Py_atomic_load_int(&is_tripped)) {
-        return 0;
-    }
-
     /*
      * The is_tripped variable is meant to speed up the calls to
      * PyErr_CheckSignals (both directly or via pending calls) when no
@@ -1895,6 +1888,20 @@ _PyErr_CheckSignalsTstate(PyThreadState *tstate)
 }
 
 
+/* Declared in cpython/pyerrors.h */
+int
+_PyErr_CheckSignalsTstate(PyThreadState *tstate)
+{
+    _Py_CHECK_EMSCRIPTEN_SIGNALS();
+    if (!_Py_atomic_load_int(&is_tripped)) {
+        return 0;
+    }
+    if (!_Py_ThreadCanHandleSignals(tstate->interp)) {
+        return 0;
+    }
+    return run_tripped_handlers(tstate);
+}
+
 
 int
 _PyErr_CheckSignals(void)
diff --git a/Python/ceval_gil.c b/Python/ceval_gil.c
index 2425bc1b39f0dc..346c731d7bac72 100644
--- a/Python/ceval_gil.c
+++ b/Python/ceval_gil.c
@@ -829,9 +829,6 @@ handle_signals(PyThreadState *tstate)
 {
     assert(_PyThreadState_CheckConsistency(tstate));
     _Py_unset_eval_breaker_bit(tstate, _PY_SIGNALS_PENDING_BIT);
-    if (!_Py_ThreadCanHandleSignals(tstate->interp)) {
-        return 0;
-    }
     if (_PyErr_CheckSignalsTstate(tstate) < 0) {
         /* On failure, re-schedule a call to handle_signals(). */
         _Py_set_eval_breaker_bit(tstate, _PY_SIGNALS_PENDING_BIT);
@@ -1257,10 +1254,11 @@ static inline void run_remote_debugger_script(PyObject *path)
 
 int _PyRunRemoteDebugger(PyThreadState *tstate)
 {
+    if (tstate->remote_debugger_support.debugger_pending_call != 1) {
+        return 0;
+    }
     const PyConfig *config = _PyInterpreterState_GetConfig(tstate->interp);
-    if (config->remote_debug == 1
-         && tstate->remote_debugger_support.debugger_pending_call == 1)
-    {
+    if (config->remote_debug == 1) {
         tstate->remote_debugger_support.debugger_pending_call = 0;
 
         // Immediately make a copy in case of a race with another debugger