Cameron McInally
2012-Mar-01 21:18 UTC
[LLVMdev] Stack alignment on X86 AVX seems incorrect
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> wrote:
Message: 2
Date: Thu, 1 Mar 2012 18:16:46 +0000
From: "Demikhovsky, Elena" <elena.demikhovsky at intel.com>
Subject: [LLVMdev] Stack alignment on X86 AVX seems incorrect
To: "llvmdev at cs.uiuc.edu" <llvmdev at cs.uiuc.edu>
Message-ID:
<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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Evandro Menezes
2012-Mar-01 21:29 UTC
[LLVMdev] Stack alignment on X86 AVX seems incorrect
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 > > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
On Thu, Mar 1, 2012 at 1:18 PM, Cameron McInally <cameron.mcinally at nyu.edu> 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.Really? There's supposed to be code to realign the stack when necessary... it's possible that code is broken, though. -Eli
Cameron McInally
2012-Mar-01 22:30 UTC
[LLVMdev] Stack alignment on X86 AVX seems incorrect
On Thu, Mar 1, 2012 at 4:29 PM, Evandro Menezes <emenezes at codeaurora.org>wrote: ...> 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. >Sorry, I confused myself. This was worked out about a year ago, so it's not in my cache. You're right. In my last email I wrote "align the stack", when I should have written "align the frame when variable sized objects are in play". Take main(...) for example, with a few alloca's. If one would like to spill AVX regs with aligned moves, one must align the frame to 32 bytes to ensure that the spill slots are aligned correctly, since spill slots are based off of the frame pointer. The x86-64 ABI lays out the stack frame as... ... 16(%rbp) mem arg[0] 8(%rbp) return address 0(%rbp) previous %rbp -8(%rbp) stack My patch breaks the ABI since the ABI requires the return address to be found at 8(%rbp) and mem arg[0] at 16(%rbp). Unfortunately, to align the frame at runtime, it's sometimes required to insert padding in between the two. I'll ask for my company's permission to share my implementation. Until then, I'll have to bite my tongue. -Cameron -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20120301/e9a0be25/attachment.html>