[LLVMdev] Stack alignment on X86 AVX seems incorrect

[Evandro Menezes]

Cameron,

Aligning the stack to 32 bytes when there are auto AVX vector variables 
present shouldn't necessarily break the x86-64 ABI, as long as smaller 
auto variables remain properly aligned.  A similar approach was taken 
for i386 in GCC in order to support SSE vectors.

Perhaps you could elaborate where the ABI was violated when your patch 
is applied.

HTH

-- 
Evandro Menezes          Austin, TX          emenezes at codeaurora.org
Qualcomm Innovation Center, Inc is a member of the Code Aurora Forum


On 03/01/12 15:18, Cameron McInally wrote:
> Hi Elena,
>
> You're correct. LLVM does not align the stack to 32-bytes for AVX and
> unaligned moves should be used for YMM spills.
>
> I wrote some code to align the stack to 32-bytes when AVX spills are
> present; it does break the x86-64 ABI though. If upstream would be
> interested in this code, I can arrange with my employer to send a patch
> to the mailing list.
>
> -Cameron
>
> On Mar 1, 2012, at 4:09 PM, <llvmdev-request at cs.uiuc.edu
> <mailto:llvmdev-request at cs.uiuc.edu>> wrote:
>
>> Message: 2
>> Date: Thu, 1 Mar 2012 18:16:46 +0000
>> From: "Demikhovsky, Elena" <elena.demikhovsky at intel.com
>> <mailto:elena.demikhovsky at intel.com>>
>> Subject: [LLVMdev] Stack alignment on X86 AVX seems incorrect
>> To: "llvmdev at cs.uiuc.edu <mailto:llvmdev at cs.uiuc.edu>"
>> <llvmdev at cs.uiuc.edu <mailto:llvmdev at cs.uiuc.edu>>
>> Message-ID:
>> <A0DC88CEB3010344830D52D66533DA8E0C2E7A at HASMSX103.ger.corp.intel.com
>> <mailto:A0DC88CEB3010344830D52D66533DA8E0C2E7A at HASMSX103.ger.corp.intel.com>>
>> Content-Type: text/plain; charset="windows-1252"
>>
>> ./llc -mattr=+avx -stack-alignment=16 < basic.ll | grep movaps | grep
>> ymm | grep rbp
>>        vmovaps -176(%rbp), %ymm14
>>        vmovaps -144(%rbp), %ymm11
>>        vmovaps -240(%rbp), %ymm13
>>        vmovaps -208(%rbp), %ymm9
>>        vmovaps -272(%rbp), %ymm7
>>        vmovaps -304(%rbp), %ymm0
>>        vmovaps -112(%rbp), %ymm0
>>        vmovaps -80(%rbp), %ymm1
>>        vmovaps -112(%rbp), %ymm0
>>        vmovaps -80(%rbp), %ymm0
>>        vmovaps -176(%rbp), %ymm15
>>        vmovaps -144(%rbp), %ymm0
>>        vmovaps -240(%rbp), %ymm0
>>        vmovaps -208(%rbp), %ymm0
>>        vmovaps -272(%rbp), %ymm0
>>        vmovaps -304(%rbp), %ymm0
>>
>> vmovaps should not access stack if it is not aligned to 32
>>
>> - Elena
>
>
>
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