linux-kernel - Re: [PATCH v2 1/2] ARM: drop WASM to work around LLVM issue

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Date:   Tue, 19 Mar 2019 23:07:35 +0100
From:   Stefan Agner <stefan@...er.ch>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Russell King <linux@...linux.org.uk>,
        Robin Murphy <robin.murphy@....com>,
        Will Deacon <will.deacon@....com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        julien.thierry@....com, Mark Rutland <mark.rutland@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 1/2] ARM: drop WASM to work around LLVM issue

On 18.03.2019 19:09, Nick Desaulniers wrote:
> On Sun, Mar 17, 2019 at 4:05 PM Stefan Agner <stefan@...er.ch> wrote:
>>
>> Currently LLVM's integrated assembler does not recognize .w form
>> of the pld instructions (LLVM Bug 40972 [0]):
>>
>>   ./arch/arm/include/asm/processor.h:133:5: error: invalid instruction
>>                           "pldw.w\t%a0 \n"
>>                            ^
>>   <inline asm>:2:1: note: instantiated into assembly here
>>   pldw.w  [r0]
>>   ^
>>   1 error generated.
>>
>> The W macro for generating wide instructions when targeting Thumb-2
>> is not strictly required for the preload data instructions (pld, pldw)
>> since they are only available as wide instructions. The GNU assembler
>> works with or without the .w appended when compiling an Thumb-2 kernel.
>>
>> Drop the macro to work around LLVM Bug 40972 issue.
>>
>> [0] https://bugs.llvm.org/show_bug.cgi?id=40972
>>
>> Signed-off-by: Stefan Agner <stefan@...er.ch>
> 
> Thanks for the bug report and patch.
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> 
> Just curious, there are only 3 other expansion sites of this macro.
> Are any of those problematic?  Looks like nop.w, sev.w and b.w?
> 

All three sites are in inline assembly, and I did a bunch of successful
Thumb2 builds using the integrated assembler, so I think all those sites
have been assembled by LLVM successfully.

Also confirmed those three instructions with the reproducer example from
the LLVM bug above.

--
Stefan

>> ---
>> Changes in v2:
>> - Reword commit message to reflect the fact that this is a work around
>>   for LLVM.
>>
>>  arch/arm/include/asm/processor.h | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/processor.h b/arch/arm/include/asm/processor.h
>> index 57fe73ea0f72..5d06f75ffad4 100644
>> --- a/arch/arm/include/asm/processor.h
>> +++ b/arch/arm/include/asm/processor.h
>> @@ -135,8 +135,8 @@ static inline void prefetchw(const void *ptr)
>>         __asm__ __volatile__(
>>                 ".arch_extension        mp\n"
>>                 __ALT_SMP_ASM(
>> -                       WASM(pldw)              "\t%a0",
>> -                       WASM(pld)               "\t%a0"
>> +                       "pldw\t%a0",
>> +                       "pld\t%a0"
>>                 )
>>                 :: "p" (ptr));
>>  }
>> --
>> 2.21.0
>>