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From: Michael Ellerman <mpe@ellerman.id.au>
To: Nicholas Piggin <npiggin@gmail.com>, Daniel Axtens <dja@axtens.net>,
 linuxppc-dev@lists.ozlabs.org, llvmlinux@lists.linuxfoundation.org
Subject: Re: [RFC PATCH 2/8] powerpc: check for support for -Wa, -m{power4,
 any}
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References: <20210225031006.1204774-1-dja@axtens.net>
 <20210225031006.1204774-3-dja@axtens.net>
 <1616117194.vwe39qw3i4.astroid@bobo.none>
Date: Fri, 19 Mar 2021 21:53:47 +1100
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Nicholas Piggin <npiggin@gmail.com> writes:
> 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@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.

It uses $(CC) but it doesn't pass it $CFLAGS AFAIK. So it would use
whatever the compiler default is for -mcpu etc. I think.

> I assume it used to be useful for cross compiling when as was called
> directly but I'm not sure.

We still use it directly in vdso32/Makefile.

cheers