From: Koakuma <koachan@xxxxxxxxxxxxxx> Remove flags not supported by clang and make sure that all the flags used are portable between clang and GCC. The reasoning for removing the -fcall-used* ones is as follows: In the (normal) 32-bit ABI, %g5 and %g7 is normally reserved, and in the 64-bit ABI, %g7 is the reserved one. Linux turns them into volatile registers by the way of -fcall-used-*, but on the other hand, omitting the flags shouldn't be harmful; compilers will now simply refuse to touch them, and any assembly code that happens to touch them would still work like usual (because Linux' conventions already treats them as volatile anyway). Signed-off-by: Koakuma <koachan@xxxxxxxxxxxxxx> --- Hello~ This changes the CFLAGS for building the SPARC kernel so that it can be built with clang, as a follow up from the discussion in this thread: https://lore.kernel.org/lkml/JAYB7uS-EdLABTR4iWZdtFOVa5MvlKosIrD_cKTzgeozCOGRM7lhxeLigFB1g3exX445I_W5VKB-tAzl2_G1zCVJRQjp67ODfsSqiZWOZ9o=@protonmail.com/T/#u The changes are removal of various `-fcall-used-*` flags, and changing `-mv8plus` to `-mcpu=v9`: - `-fcall-used-*` flags should be safe to remove; the compiler will stop using the registers specified as temporaries, but it is a safe change wrt. the ABI. Assembly code can still use those registers as needed. It does bring a theoretical possible slowdown due to the compiler having less registers to work with, but in practice - in my case, at least - it seems to not make any difference with daily usage. - More trivial is to change `-mv8plus` -> `-mcpu=v9`. This should be safe too since the kernel seems to require a V9 processor to run anyway, so I'm changing the flag to one that is portable between GCC and clang. Also, as stated in the thread, building with these changes still result in a working kernel, at least for Sun T5120 and qemu virtual machines. On the LLVM side, the effort for building Linux/SPARC is tracked here: https://github.com/llvm/llvm-project/issues/40792 --- arch/sparc/Makefile | 4 ++-- arch/sparc/vdso/Makefile | 4 ++-- 2 files changed, 4 insertions(+), 4 deletions(-) diff --git a/arch/sparc/Makefile b/arch/sparc/Makefile index 757451c3ea1d..7318a8b452c3 100644 --- a/arch/sparc/Makefile +++ b/arch/sparc/Makefile @@ -29,7 +29,7 @@ UTS_MACHINE := sparc # versions of gcc. Some gcc versions won't pass -Av8 to binutils when you # give -mcpu=v8. This silently worked with older bintutils versions but # does not any more. -KBUILD_CFLAGS += -m32 -mcpu=v8 -pipe -mno-fpu -fcall-used-g5 -fcall-used-g7 +KBUILD_CFLAGS += -m32 -mcpu=v8 -pipe -mno-fpu KBUILD_CFLAGS += -Wa,-Av8 KBUILD_AFLAGS += -m32 -Wa,-Av8 @@ -45,7 +45,7 @@ export BITS := 64 UTS_MACHINE := sparc64 KBUILD_CFLAGS += -m64 -pipe -mno-fpu -mcpu=ultrasparc -mcmodel=medlow -KBUILD_CFLAGS += -ffixed-g4 -ffixed-g5 -fcall-used-g7 -Wno-sign-compare +KBUILD_CFLAGS += -ffixed-g4 -ffixed-g5 -Wno-sign-compare KBUILD_CFLAGS += -Wa,--undeclared-regs KBUILD_CFLAGS += $(call cc-option,-mtune=ultrasparc3) KBUILD_AFLAGS += -m64 -mcpu=ultrasparc -Wa,--undeclared-regs diff --git a/arch/sparc/vdso/Makefile b/arch/sparc/vdso/Makefile index 243dbfc4609d..929140facabf 100644 --- a/arch/sparc/vdso/Makefile +++ b/arch/sparc/vdso/Makefile @@ -46,7 +46,7 @@ CFL := $(PROFILING) -mcmodel=medlow -fPIC -O2 -fasynchronous-unwind-tables -m64 -fno-omit-frame-pointer -foptimize-sibling-calls \ -DDISABLE_BRANCH_PROFILING -DBUILD_VDSO -SPARC_REG_CFLAGS = -ffixed-g4 -ffixed-g5 -fcall-used-g5 -fcall-used-g7 +SPARC_REG_CFLAGS = -ffixed-g4 -ffixed-g5 $(vobjs): KBUILD_CFLAGS := $(filter-out $(RANDSTRUCT_CFLAGS) $(GCC_PLUGINS_CFLAGS) $(SPARC_REG_CFLAGS),$(KBUILD_CFLAGS)) $(CFL) @@ -86,7 +86,7 @@ KBUILD_CFLAGS_32 += -fno-stack-protector KBUILD_CFLAGS_32 += $(call cc-option, -foptimize-sibling-calls) KBUILD_CFLAGS_32 += -fno-omit-frame-pointer KBUILD_CFLAGS_32 += -DDISABLE_BRANCH_PROFILING -KBUILD_CFLAGS_32 += -mv8plus +KBUILD_CFLAGS_32 += -mcpu=v9 $(obj)/vdso32.so.dbg: KBUILD_CFLAGS = $(KBUILD_CFLAGS_32) $(obj)/vdso32.so.dbg: FORCE \ --- base-commit: 92e5605a199efbaee59fb19e15d6cc2103a04ec2 change-id: 20240620-sparc-cflags-e7f2dbbd4b9d Best regards, -- Koakuma <koachan@xxxxxxxxxxxxxx>
