[PATCH] powerpc/32: Avoid unsupported flags with clang

[Michael Ellerman]

Joel Stanley <joel at jms.id.au> writes:

> When building for ppc32 with clang these flags are unsupported:
>
>   -ffixed-r2 and -mmultiple
>
> llvm's lib/Target/PowerPC/PPCRegisterInfo.cpp marks r2 as reserved on
> when building for SVR4ABI and !ppc64:
>
>   // The SVR4 ABI reserves r2 and r13
>   if (Subtarget.isSVR4ABI()) {
>     // We only reserve r2 if we need to use the TOC pointer. If we have no
>     // explicit uses of the TOC pointer (meaning we're a leaf function with
>     // no constant-pool loads, etc.) and we have no potential uses inside an
>     // inline asm block, then we can treat r2 has an ordinary callee-saved
>     // register.
>     const PPCFunctionInfo *FuncInfo = MF.getInfo<PPCFunctionInfo>();
>     if (!TM.isPPC64() || FuncInfo->usesTOCBasePtr() || MF.hasInlineAsm())
>       markSuperRegs(Reserved, PPC::R2);  // System-reserved register
>     markSuperRegs(Reserved, PPC::R13); // Small Data Area pointer register
>   }
>
> This means we can safely omit -ffixed-r2 when building for 32-bit
> targets.
>
> The -mmultiple/-mno-multiple flags are not supported by clang, so
> platforms that might support multiple miss out on using multiple word
> instructions.
>
> Clang 8 can then build a ppc44x_defconfig which boots in Qemu:
>
>   make CC=clang-8 ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnu-  ppc44x_defconfig
>   make CC=clang-8 ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnu-
>
>   qemu-system-ppc -M bamboo \
>    -kernel arch/powerpc/boot/zImage \
>    -dtb arch/powerpc/boot/dts/bamboo.dtb \
>    -initrd ~/ppc32-440-rootfs.cpio \
>    -nographic -serial stdio -monitor pty -append "console=ttyS0"
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/261
> Signed-off-by: Joel Stanley <joel at jms.id.au>
> ---
> I considered putting these behind a cc-option test. If someone has an
> opinion on that I'm happy to change.
>
>  arch/powerpc/Makefile | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
> index 17be664dafa2..5fa678eee747 100644
> --- a/arch/powerpc/Makefile
> +++ b/arch/powerpc/Makefile
> @@ -152,7 +152,9 @@ endif
>  CFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mcmodel=medium,$(call cc-option,-mminimal-toc))
>  CFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mno-pointers-to-nested-functions)
>  
> -CFLAGS-$(CONFIG_PPC32)	:= -ffixed-r2 $(MULTIPLEWORD)
> +ifndef CONFIG_CC_IS_CLANG
> +#CFLAGS-$(CONFIG_PPC32)	:= -ffixed-r2 $(MULTIPLEWORD)
> +endif

I think using cc-option with a comment would be nicer, you could avoid
the ifdef and it would also future-proof it against the option either
appearing in a future clang or disappearing in a future gcc.

eg ~=:

# Clang doesn't need or support -ffixed-r2
CFLAGS-$(CONFIG_PPC32)	+= $(cc-option,-ffixed-r2)


cheers