From a95b1a56bbba76a382a5c676b71db025915e8695 Mon Sep 17 00:00:00 2001
From: mpage <mpage@meta.com>
Date: Wed, 14 Feb 2024 12:15:05 -0800
Subject: [PATCH] gh-115041: Add wrappers that are atomic only in free-threaded
 builds (#115046)

These are intended to be used in places where atomics are required in
free-threaded builds but not in the default build. We don't want to
introduce the potential performance overhead of an atomic operation in the
default build.
---
 .../internal/pycore_pyatomic_ft_wrappers.h    | 35 +++++++++++++++++++
 Makefile.pre.in                               |  1 +
 PCbuild/pythoncore.vcxproj                    |  1 +
 PCbuild/pythoncore.vcxproj.filters            |  3 ++
 4 files changed, 40 insertions(+)
 create mode 100644 Include/internal/pycore_pyatomic_ft_wrappers.h

diff --git a/Include/internal/pycore_pyatomic_ft_wrappers.h b/Include/internal/pycore_pyatomic_ft_wrappers.h
new file mode 100644
index 00000000000000..d1313976da1cfc
--- /dev/null
+++ b/Include/internal/pycore_pyatomic_ft_wrappers.h
@@ -0,0 +1,35 @@
+// This header file provides wrappers around the atomic operations found in
+// `pyatomic.h` that are only atomic in free-threaded builds.
+//
+// These are intended to be used in places where atomics are required in
+// free-threaded builds, but not in the default build, and we don't want to
+// introduce the potential performance overhead of an atomic operation in the
+// default build.
+//
+// All usages of these macros should be replaced with unconditionally atomic or
+// non-atomic versions, and this file should be removed, once the dust settles
+// on free threading.
+#ifndef Py_ATOMIC_FT_WRAPPERS_H
+#define Py_ATOMIC_FT_WRAPPERS_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#error "this header requires Py_BUILD_CORE define"
+#endif
+
+#ifdef Py_GIL_DISABLED
+#define FT_ATOMIC_LOAD_SSIZE_RELAXED(value) \
+    _Py_atomic_load_ssize_relaxed(&value)
+#define FT_ATOMIC_STORE_SSIZE_RELAXED(value, new_value) \
+    _Py_atomic_store_ssize_relaxed(&value, new_value)
+#else
+#define FT_ATOMIC_LOAD_SSIZE_RELAXED(value) value
+#define FT_ATOMIC_STORE_SSIZE_RELAXED(value, new_value) value = new_value
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_ATOMIC_FT_WRAPPERS_H */
diff --git a/Makefile.pre.in b/Makefile.pre.in
index 96886adf309d81..8252e6631c5af5 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -1150,6 +1150,7 @@ PYTHON_HEADERS= \
 		$(srcdir)/Include/internal/pycore_parser.h \
 		$(srcdir)/Include/internal/pycore_pathconfig.h \
 		$(srcdir)/Include/internal/pycore_pyarena.h \
+		$(srcdir)/Include/internal/pycore_pyatomic_ft_wrappers.h \
 		$(srcdir)/Include/internal/pycore_pybuffer.h \
 		$(srcdir)/Include/internal/pycore_pyerrors.h \
 		$(srcdir)/Include/internal/pycore_pyhash.h \
diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index 4cc0ca4b9af8de..abfafbb2a32f45 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -266,6 +266,7 @@
     <ClInclude Include="..\Include\internal\pycore_parking_lot.h" />
     <ClInclude Include="..\Include\internal\pycore_pathconfig.h" />
     <ClInclude Include="..\Include\internal\pycore_pyarena.h" />
+    <ClInclude Include="..\Include\internal\pycore_pyatomic_ft_wrappers.h" />
     <ClInclude Include="..\Include\internal\pycore_pyerrors.h" />
     <ClInclude Include="..\Include\internal\pycore_pyhash.h" />
     <ClInclude Include="..\Include\internal\pycore_pylifecycle.h" />
diff --git a/PCbuild/pythoncore.vcxproj.filters b/PCbuild/pythoncore.vcxproj.filters
index ceaa21217267cf..d14f5a6d7fb0fc 100644
--- a/PCbuild/pythoncore.vcxproj.filters
+++ b/PCbuild/pythoncore.vcxproj.filters
@@ -723,6 +723,9 @@
     <ClInclude Include="..\Include\internal\pycore_pyarena.h">
       <Filter>Include\internal</Filter>
     </ClInclude>
+    <ClInclude Include="..\Include\internal\pycore_pyatomic_ft_wrappers.h">
+      <Filter>Include\internal</Filter>
+    </ClInclude>
     <ClInclude Include="..\Include\internal\pycore_pyerrors.h">
       <Filter>Include\internal</Filter>
     </ClInclude>