Re: [PATCH v3 1/2] sparc/build: Put usage of -fcall-used* flags behind cc-option

[Nathan Chancellor <nathan@kernel.org>]

On Tue, Oct 29, 2024 at 09:49:07PM +0700, Koakuma via B4 Relay wrote:
> From: Koakuma <koachan@protonmail.com>
> 
> Place -fcall-used* flags behind cc-option so that clang (which doesn't
> support them) can still compile the kernel.
> 
> This is a safe change, the reasoning 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@protonmail.com>

Clang builds now succeed with this series and builds with GCC 14.2.0
continue to pass and boot successfully.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>

One comment below, please carry these tags forward if there are future
revisions without substantial technical changes.

> ---
>  arch/sparc/Makefile      | 4 ++--
>  arch/sparc/vdso/Makefile | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/sparc/Makefile b/arch/sparc/Makefile
> index 757451c3ea1df63b948e68a45988c78f5974f9ea..0400078076e588be93a702d1c64eb9fd34466075 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 $(call cc-option,-fcall-used-g5) $(call cc-option,-fcall-used-g7)

Small nit, this (and the one in the vdso) could probably be one
cc-option call? Is it likely that one flag would be implemented in the
compiler without the other?

  $(call cc-option,-fcall-used-g5 -fcall-used-g7)

>  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 $(call cc-option,-fcall-used-g7) -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 243dbfc4609d804fc221c3591eebe891107ffdab..50ec2978cda5397841daad6ffdc9682811b9b38e 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 $(call cc-option,-fcall-used-g5) $(call cc-option,-fcall-used-g7)
>  
>  $(vobjs): KBUILD_CFLAGS := $(filter-out $(RANDSTRUCT_CFLAGS) $(GCC_PLUGINS_CFLAGS) $(SPARC_REG_CFLAGS),$(KBUILD_CFLAGS)) $(CFL)
>  
> 
> -- 
> 2.47.0
> 
> 
>