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.

Author: mpage

Diff for: 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 for: 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 for: 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 for: 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>