gh-159437: Align release build functions to 64-byte boundaries

Author: mfalco-oai

Makefile.pre.in

@@ -102,7 +102,13 @@ CONFIGURE_LDFLAGS=	@LDFLAGS@
 # command line to append to these values without stomping the pre-set
 # values.
 PY_CFLAGS=	$(BASECFLAGS) $(OPT) $(CONFIGURE_CFLAGS) $(CFLAGS) $(EXTRA_CFLAGS)
-PY_CFLAGS_NODIST=$(CONFIGURE_CFLAGS_NODIST) $(CFLAGS_NODIST) -I$(srcdir)/Include/internal -I$(srcdir)/Include/internal/mimalloc
+# Configure validates function alignment once, but make-time CFLAGS can still
+# select a size-optimized build after configure has completed.
+PY_ALIGN_FUNCTIONS_CFLAGS=@FUNCTION_ALIGNMENT_CFLAGS@
+ifneq ($(filter -Os -Oz,$(PY_CFLAGS) $(CONFIGURE_CFLAGS_NODIST) $(CFLAGS_NODIST)),)
+PY_ALIGN_FUNCTIONS_CFLAGS=
+endif
+PY_CFLAGS_NODIST=$(CONFIGURE_CFLAGS_NODIST) $(CFLAGS_NODIST) $(PY_ALIGN_FUNCTIONS_CFLAGS) -I$(srcdir)/Include/internal -I$(srcdir)/Include/internal/mimalloc
 # Both CPPFLAGS and LDFLAGS need to contain the shell's value for setup.py to
 # be able to build extension modules using the directories specified in the
 # environment variables

Misc/NEWS.d/next/Build/2026-10-18-21-30-00.gh-issue-159437.align64.rst

@@ -0,0 +1 @@
+Request 64-byte function alignment for performance-oriented Linux and macOS builds. This can reduce performance variation caused by changes to the size of preceding functions.

configure

@@ -2430,6 +2430,21 @@ then
     esac
 fi
 
+# Keep performance-oriented release builds aligned on validated platforms
+# without changing size-optimized, mobile, or third-party extension builds.
+FUNCTION_ALIGNMENT_CFLAGS=
+AS_CASE([$host],
+  [*-*-linux-android*], [],
+  [*-*-linux*|*-apple-darwin*], [
+    AS_VAR_IF([Py_DEBUG], [true], [], [
+      AS_CASE([" $OPT $CFLAGS "],
+        [*" -Os "*|*" -Oz "*], [],
+        [AX_CHECK_COMPILE_FLAG([-falign-functions=64], [
+          FUNCTION_ALIGNMENT_CFLAGS="-falign-functions=64"
+        ], [], [-Werror])])
+    ])
+  ])
+
 # WASM flags
 AS_CASE([$ac_sys_system],
   [Emscripten], [
@@ -2558,6 +2573,7 @@ AS_CASE([$enable_wasm_dynamic_linking],
 
 AC_SUBST([BASECFLAGS])
 AC_SUBST([CFLAGS_NODIST])
+AC_SUBST([FUNCTION_ALIGNMENT_CFLAGS])
 AC_SUBST([LDFLAGS_NODIST])
 AC_SUBST([EXE_LDFLAGS])
 AC_SUBST([LDFLAGS_NOLTO])