[LLVMdev] Limit loop vectorizer to SSE

[Hal Finkel]

----- 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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