[RFC PATCH 2/8] powerpc: check for support for -Wa, -m{power4, any}

[Nicholas Piggin]

Excerpts from Daniel Axtens's message of February 25, 2021 1:10 pm:
> LLVM's integrated assembler does not like either -Wa,-mpower4
> or -Wa,-many. So just don't pass them if they're not supported.
> 
> Signed-off-by: Daniel Axtens <dja at axtens.net>
> ---
>  arch/powerpc/Makefile | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
> index 08cf0eade56a..3e2c72d20bb8 100644
> --- a/arch/powerpc/Makefile
> +++ b/arch/powerpc/Makefile
> @@ -252,7 +252,9 @@ cpu-as-$(CONFIG_E500)		+= -Wa,-me500
>  # When using '-many -mpower4' gas will first try and find a matching power4
>  # mnemonic and failing that it will allow any valid mnemonic that GAS knows
>  # about. GCC will pass -many to GAS when assembling, clang does not.
> -cpu-as-$(CONFIG_PPC_BOOK3S_64)	+= -Wa,-mpower4 -Wa,-many
> +# LLVM IAS doesn't understand either flag: https://github.com/ClangBuiltLinux/linux/issues/675
> +# but LLVM IAS only supports ISA >= 2.06 for Book3S 64 anyway...
> +cpu-as-$(CONFIG_PPC_BOOK3S_64)	+= $(call as-option,-Wa$(comma)-mpower4) $(call as-option,-Wa$(comma)-many)
>  cpu-as-$(CONFIG_PPC_E500MC)	+= $(call as-option,-Wa$(comma)-me500mc)
>  
>  KBUILD_AFLAGS += $(cpu-as-y)

I'm wondering why we even have this now. Kbuild's "AS" command goes 
through the C compiler now with relevant options like -mcpu. I assume it 
used to be useful for cross compiling when as was called directly but
I'm not sure.

Thanks,
Nick