[LLVMdev] Limit loop vectorizer to SSE

[Hal Finkel]

----- Original Message -----
> From: "Arnold Schwaighofer" <aschwaighofer at apple.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "LLVM Dev" <llvmdev at cs.uiuc.edu>, "Joshua Klontz" <josh.klontz at gmail.com>
> Sent: Friday, November 15, 2013 4:15:03 PM
> Subject: Re: [LLVMdev] Limit loop vectorizer to SSE
> 
> Yes,
> 
> I was just about to send out:
> 
>  DL->getABITypeAlignment(ScalarDataTy);
> 
> The question is:
> 
>  “… ABI alignment for the target …"
> 
> is that
> 
> getPrefTypeAlignment
>  or
> getABITypeAlignment
> 
> I would have thought the latter.

Yep; I think you're right (or call both and take the minimum).

 -Hal

> 
> 
> On Nov 15, 2013, at 4:12 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> > ----- Original Message -----
> >> From: "Arnold Schwaighofer" <aschwaighofer at apple.com>
> >> To: "Joshua Klontz" <josh.klontz at gmail.com>
> >> Cc: "LLVM Dev" <llvmdev at cs.uiuc.edu>
> >> Sent: Friday, November 15, 2013 4:05:53 PM
> >> Subject: Re: [LLVMdev] Limit loop vectorizer to SSE
> >> 
> >> 
> >> Something like:
> >> 
> >> index 6db7f68..68564cb 100644
> >> --- a/lib/Transforms/Vectorize/LoopVectorize.cpp
> >> +++ b/lib/Transforms/Vectorize/LoopVectorize.cpp
> >> @@ -1208,6 +1208,8 @@ void
> >> InnerLoopVectorizer::vectorizeMemoryInstruction(Instr
> >>   Type *DataTy = VectorType::get(ScalarDataTy, VF);
> >>   Value *Ptr = LI ? LI->getPointerOperand() :
> >>   SI->getPointerOperand();
> >>   unsigned Alignment = LI ? LI->getAlignment() :
> >>   SI->getAlignment();
> >> +  if (Alignment == 0)
> >> +    Alignment = 1;
> > 
> > That may be conservatively correct, but I don't think it is really
> > right. An alignment of 0 is supposed to mean the platform default
> > alignment (IIRC, DataLayout::getPrefTypeAlignment tells you what
> > it is).
> > 
> > -Hal
> > 
> >>   unsigned AddressSpace =
> >>   Ptr->getType()->getPointerAddressSpace();
> >>   unsigned ScalarAllocatedSize =
> >>   DL->getTypeAllocSize(ScalarDataTy);
> >>   unsigned VectorElementSize = DL->getTypeStoreSize(DataTy)/VF;
> >> 
> >> Should fix this.
> >> 
> >> On Nov 15, 2013, at 3:49 PM, Joshua Klontz <josh.klontz at gmail.com>
> >> wrote:
> >> 
> >>> Nadav,
> >>> 
> >>> I believe aligned accesses to unaligned pointers is precisely the
> >>> issue. Consider the function `add_u8S` before[1] and after[2] the
> >>> loop vectorizer pass. There is no alignment assumption associated
> >>> with %kernel_data prior to vectorization. I can't tell if it's
> >>> the
> >>> loop vectorizer or the codegen at fault, but the alignment
> >>> assumption seems to sneak in somewhere.
> >>> 
> >>> v/r,
> >>> Josh
> >>> 
> >>> [1] http://pastebin.com/kc95WtGG
> >>> [2] http://pastebin.com/VY3ZLVJK
> >>> 
> >>> 
> >>> On Fri, Nov 15, 2013 at 3:58 PM, Nadav Rotem <nrotem at apple.com>
> >>> wrote:
> >>> 
> >>> On Nov 15, 2013, at 12:36 PM, Renato Golin
> >>> <renato.golin at linaro.org> wrote:
> >>> 
> >>>> On 15 November 2013 20:24, Joshua Klontz <josh.klontz at gmail.com>
> >>>> wrote:
> >>>> Agreed, is there a pass that will insert a runtime alignment
> >>>> check? Also, what's the easiest way to get at
> >>>> TargetTransformInfo::getRegisterBitWidth() so I don't have to
> >>>> hard code 32? Thanks!
> >>>> 
> >>>> I think that's a fair question, and it's about safety. If you're
> >>>> getting this on the JIT, means we may be generating unsafe
> >>>> transformations on the vectorizer.
> >>>> 
> >>>> Arnold, Nadav, I don't remember seeing code to generate any
> >>>> run-time alignment checks on the incoming pointer, is there such
> >>>> a thing? If not, shouldn't we add one?
> >>> 
> >>> 
> >>> If the the vectorizer generates aligned memory accesses to
> >>> unaligned addresses then this is a serious bug.  But I don’t
> >>> think
> >>> that Josh said that the vectorizer generated aligned accesses to
> >>> unaligned pointers.
> >>> 
> >>> There is no point in LLVM checking for alignment because if the
> >>> memory is unaligned then the program will crash.  Users who want
> >>> to crash with a readable error message can simply write code that
> >>> checks the pointer (by masking the high bits and comparing to
> >>> zero).
> >>> 
> >>> 
> >> 
> >> 
> >> _______________________________________________
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> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory