[PATCH v4 4/4] ARM: pass -march= only to compiler

Nick Desaulniers
2022-10-14T20:15:57Z

When both -march= and -Wa,-march= are specified for assembler or
assembler-with-cpp sources, GCC and Clang will prefer the -Wa,-march=
value but Clang will warn that -march= is unused.

warning: argument unused during compilation: '-march=armv6k'
[-Wunused-command-line-argument]

This is the top group of warnings we observe when using clang to
assemble the kernel via `ARCH=arm make LLVM=1`.

Split the arch-y make variable into two, so that -march= flags only get
passed to the compiler, not the assembler. -D flags are added to
KBUILD_CPPFLAGS which is used for both C and assembler-with-cpp sources.

Clang is trying to warn that it doesn't support different values for
-march= and -Wa,-march= (like GCC does, but the kernel doesn't need this)
though the value of the preprocessor define __thumb2__ is based on
-march=. Make sure to re-set __thumb2__ via -D flag for assembler
sources now that we're no longer passing -march= to the assembler. Set
it to a different value than the preprocessor would for -march= in case
-march= gets accidentally re-added to KBUILD_AFLAGS in the future.
Thanks to Ard and Nathan for this suggestion.

Link: https://github.com/ClangBuiltLinux/linux/issues/1315
Link: https://github.com/ClangBuiltLinux/linux/issues/1587
Link: https://github.com/llvm/llvm-project/issues/55656
Suggested-by: Ard Biesheuvel <ardb@kernel.org>
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Changes v3 -> v4:
* Add -D__thumb2__=2 to KBUILD_AFLAGS as per in-person discussion with
Ard and Nathan, and their SB tags.
* Reword commit message.

arch/arm/Makefile | 36 +++++++++++++++++++++++++-----------
1 file changed, 25 insertions(+), 11 deletions(-)

diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index ee888070b2ff..b58998749ead 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -60,21 +60,34 @@ endif
KBUILD_CFLAGS += $(call cc-option,-fno-ipa-sra)

# This selects which instruction set is used.
+arch-$(CONFIG_CPU_32v7M) :=-march=armv7-m
+arch-$(CONFIG_CPU_32v7) :=-march=armv7-a
+arch-$(CONFIG_CPU_32v6) :=-march=armv6
+# Only override the compiler option if ARMv6. The ARMv6K extensions are
+# always available in ARMv7
+ifeq ($(CONFIG_CPU_32v6),y)
+arch-$(CONFIG_CPU_32v6K) :=-march=armv6k
+endif
+arch-$(CONFIG_CPU_32v5) :=-march=armv5te
+arch-$(CONFIG_CPU_32v4T) :=-march=armv4t
+arch-$(CONFIG_CPU_32v4) :=-march=armv4
+arch-$(CONFIG_CPU_32v3) :=-march=armv3m
+
# Note that GCC does not numerically define an architecture version
# macro, but instead defines a whole series of macros which makes
# testing for a specific architecture or later rather impossible.
-arch-$(CONFIG_CPU_32v7M) :=-D__LINUX_ARM_ARCH__=7 -march=armv7-m
-arch-$(CONFIG_CPU_32v7) :=-D__LINUX_ARM_ARCH__=7 -march=armv7-a
-arch-$(CONFIG_CPU_32v6) :=-D__LINUX_ARM_ARCH__=6 -march=armv6
-# Only override the compiler opt:ion if ARMv6. The ARMv6K extensions are
+cpp-$(CONFIG_CPU_32v7M) :=-D__LINUX_ARM_ARCH__=7
+cpp-$(CONFIG_CPU_32v7) :=-D__LINUX_ARM_ARCH__=7
+cpp-$(CONFIG_CPU_32v6) :=-D__LINUX_ARM_ARCH__=6
+# Only override the compiler option if ARMv6. The ARMv6K extensions are
# always available in ARMv7
ifeq ($(CONFIG_CPU_32v6),y)
-arch-$(CONFIG_CPU_32v6K) :=-D__LINUX_ARM_ARCH__=6 -march=armv6k
+cpp-$(CONFIG_CPU_32v6K) :=-D__LINUX_ARM_ARCH__=6
endif
-arch-$(CONFIG_CPU_32v5) :=-D__LINUX_ARM_ARCH__=5 -march=armv5te
-arch-$(CONFIG_CPU_32v4T) :=-D__LINUX_ARM_ARCH__=4 -march=armv4t
-arch-$(CONFIG_CPU_32v4) :=-D__LINUX_ARM_ARCH__=4 -march=armv4
-arch-$(CONFIG_CPU_32v3) :=-D__LINUX_ARM_ARCH__=3 -march=armv3m
+cpp-$(CONFIG_CPU_32v5) :=-D__LINUX_ARM_ARCH__=5
+cpp-$(CONFIG_CPU_32v4T) :=-D__LINUX_ARM_ARCH__=4
+cpp-$(CONFIG_CPU_32v4) :=-D__LINUX_ARM_ARCH__=4
+cpp-$(CONFIG_CPU_32v3) :=-D__LINUX_ARM_ARCH__=3

# This selects how we optimise for the processor.
tune-$(CONFIG_CPU_ARM7TDMI) :=-mtune=arm7tdmi
@@ -119,15 +132,16 @@ AFLAGS_NOWARN :=$(call as-option,-Wa$(comma)-mno-warn-deprecated,-Wa$(comma)-W)

ifeq ($(CONFIG_THUMB2_KERNEL),y)
CFLAGS_ISA :=-mthumb -Wa,-mimplicit-it=always $(AFLAGS_NOWARN)
-AFLAGS_ISA :=$(CFLAGS_ISA) -Wa$(comma)-mthumb
+AFLAGS_ISA :=$(CFLAGS_ISA) -Wa$(comma)-mthumb -D__thumb2__=2
else
CFLAGS_ISA :=$(call cc-option,-marm,) $(AFLAGS_NOWARN)
AFLAGS_ISA :=$(CFLAGS_ISA)
endif

# Need -Uarm for gcc < 3.x
+KBUILD_CPPFLAGS +=$(cpp-y)
KBUILD_CFLAGS +=$(CFLAGS_ABI) $(CFLAGS_ISA) $(arch-y) $(tune-y) $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,)) -msoft-float -Uarm
-KBUILD_AFLAGS +=$(CFLAGS_ABI) $(AFLAGS_ISA) $(arch-y) $(tune-y) -include asm/unified.h -msoft-float
+KBUILD_AFLAGS +=$(CFLAGS_ABI) $(AFLAGS_ISA) -Wa,$(arch-y) $(tune-y) -include asm/unified.h -msoft-float

CHECKFLAGS += -D__arm__

--
2.38.0.413.g74048e4d9e-goog