[LLVMdev] Limit loop vectorizer to SSE

[Nadav Rotem]

You are right!  Good catch. 

On Nov 15, 2013, at 2:05 PM, Arnold Schwaighofer <aschwaighofer at apple.com> wrote:

> 
> 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;
>   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).  
>> 
>> 
>