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, 12 insertions, 0 deletions

diff --git a/arch/Kconfig b/arch/Kconfig
index a5af0edd3eb8..bd6c6335efac 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1507,4 +1507,16 @@ config FUNCTION_ALIGNMENT
 	default 4 if FUNCTION_ALIGNMENT_4B
 	default 0
 
+config CC_HAS_MIN_FUNCTION_ALIGNMENT
+	# Detect availability of the GCC option -fmin-function-alignment which
+	# guarantees minimal alignment for all functions, unlike
+	# -falign-functions which the compiler ignores for cold functions.
+	def_bool $(cc-option, -fmin-function-alignment=8)
+
+config CC_HAS_SANE_FUNCTION_ALIGNMENT
+	# Set if the guaranteed alignment with -fmin-function-alignment is
+	# available or extra care is required in the kernel. Clang provides
+	# strict alignment always, even with -falign-functions.
+	def_bool CC_HAS_MIN_FUNCTION_ALIGNMENT || CC_IS_CLANG
+
 endmenu