kbuild: Use -fmin-function-alignment when available

GCC recently added option -fmin-function-alignment, which should appear
in GCC 14. Unlike -falign-functions, this option causes all functions to
be aligned at the specified value, including the cold ones.

In particular, when an arm64 kernel is built with
DYNAMIC_FTRACE_WITH_CALL_OPS=y, the 8-byte function alignment is
required for correct functionality. This was done by -falign-functions=8
and having workarounds in the kernel to force the compiler to follow
this alignment. The new -fmin-function-alignment option directly
guarantees it.

Detect availability of -fmin-function-alignment and use it instead of
-falign-functions when present. Introduce CC_HAS_SANE_FUNCTION_ALIGNMENT
and enable __cold to work as expected when it is set.

Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index bc54a6cc8dc5..d84c0fb215fd 100644
--- a/Makefile
+++ b/Makefile
@@ -974,8 +974,15 @@ export CC_FLAGS_CFI
 endif
 
 ifneq ($(CONFIG_FUNCTION_ALIGNMENT),0)
+# Set the minimal function alignment. Use the newer GCC option
+# -fmin-function-alignment if it is available, or fall back to -falign-funtions.
+# See also CONFIG_CC_HAS_SANE_FUNCTION_ALIGNMENT.
+ifdef CONFIG_CC_HAS_MIN_FUNCTION_ALIGNMENT
+KBUILD_CFLAGS += -fmin-function-alignment=$(CONFIG_FUNCTION_ALIGNMENT)
+else
 KBUILD_CFLAGS += -falign-functions=$(CONFIG_FUNCTION_ALIGNMENT)
 endif
+endif
 
 # arch Makefile may override CC so keep this after arch Makefile is included
 NOSTDINC_FLAGS += -nostdinc