[llvm-dev] Aliasing Issue after vectorization

Hal Finkel via llvm-dev llvm-dev at lists.llvm.org
Fri Sep 8 16:23:05 PDT 2017 

Hi, Charith,

I don't think that we can figure this out from only what you've 
presented. Could you file a bug report with the full IR for the 
different cases at bugs.llvm.org? We'd need to at least see how the 
%"class.internal::BlockVectorIterators::Iterator" type is defined and 
how the TBAA metadata is defined in both cases, but we might need to 
look at what the AA implementation is doing in more detail.

  -Hal

On 09/08/2017 12:16 PM, Charith Mendis via llvm-dev wrote:
> Hi all,
>
> I used LLVM's SLP vectorization pass and set the vector register width 
> to 64. LLVM vectorizes A,B into E.
>
> The question is regarding aliasing, where before vectorization LLVM 
> returns that both A and B do not alias with C individually. But after 
> vectorization LLVM returns that E which is the vectorized value of A 
> and B does alias with D (same as C).
>
> Can this happen and why is LLVM's alias analysis returning they are 
> infact aliased?
>
> Thank You!
>
> *Before vectorization*
>
> entry:
>   %0 = bitcast %"class.internal::BlockVectorIterators::Iterator"* %c 
> to i64*
>   %1 = load i64, i64* %0, align 8, !tbaa !2
>   %2 = bitcast %"class.internal::BlockVectorIterators::Iterator"* 
> %this to i64*
>   store i64 %1, i64* %2, align 8, !tbaa !2
>   %global_index = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %c, i64 0, i32 1
>  A:%3 = load i32, i32* %global_index, align 8, !tbaa !8
>   %global_index3 = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %this, i64 0, i32 1
>   store i32 %3, i32* %global_index3, align 8, !tbaa !8
>   %index_within_block = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %c, i64 0, i32 3
>   %4 = load i32, i32* %index_within_block, align 8, !tbaa !9
>   %index_within_block4 = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %this, i64 0, i32 3
>  C: store i32 %4, i32* %index_within_block4, align 8, !tbaa !9
>   %current_block = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %c, i64 0, i32 2
>  B: %5 = load i32, i32* %current_block, align 4, !tbaa !10
>   %current_block5 = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %this, i64 0, i32 2
>   store i32 %5, i32* %current_block5, align 4, !tbaa !10
>   %next_break_forward = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %c, i64 0, i32 4
>   %6 = load i32, i32* %next_break_forward, align 4, !tbaa !11
>   %next_break_forward6 = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %this, i64 0, i32 4
>   store i32 %6, i32* %next_break_forward6, align 4, !tbaa !11
>   %next_break_backward = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %c, i64 0, i32 5
>   %7 = load i32, i32* %next_break_backward, align 8, !tbaa !12
>   %next_break_backward7 = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %this, i64 0, i32 5
>   store i32 %7, i32* %next_break_backward7, align 8, !tbaa !12
>   ret %"class.internal::BlockVectorIterators::Iterator"* %this
>
>
> *After vectorization*
>
> entry:
>   %0 = bitcast %"class.internal::BlockVectorIterators::Iterator"* %c 
> to i64*
>   %1 = load i64, i64* %0, align 8, !tbaa !2
>   %2 = bitcast %"class.internal::BlockVectorIterators::Iterator"* 
> %this to i64*
>   store i64 %1, i64* %2, align 8, !tbaa !2
>   %global_index = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %c, i64 0, i32 1
>   %global_index3 = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %this, i64 0, i32 1
>   %index_within_block = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %c, i64 0, i32 3
>   %3 = load i32, i32* %index_within_block, align 8, !tbaa !8
>   %index_within_block4 = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %this, i64 0, i32 3
> D: store i32 %3, i32* %index_within_block4, align 8, !tbaa !8
>   %current_block = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %c, i64 0, i32 2
>   %4 = bitcast i32* %global_index to <2 x i32>*
> E:  %5 = load <2 x i32>, <2 x i32>* %4, align 8, !tbaa !9
>   %current_block5 = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %this, i64 0, i32 2
>   %6 = bitcast i32* %global_index3 to <2 x i32>*
>   store <2 x i32> %5, <2 x i32>* %6, align 8, !tbaa !9
>   %next_break_forward = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %c, i64 0, i32 4
>   %7 = load i32, i32* %next_break_forward, align 4, !tbaa !10
>   %next_break_forward6 = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %this, i64 0, i32 4
>   store i32 %7, i32* %next_break_forward6, align 4, !tbaa !10
>   %next_break_backward = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %c, i64 0, i32 5
>   %8 = load i32, i32* %next_break_backward, align 8, !tbaa !11
>   %next_break_backward7 = getelementptr inbounds 
> %"class.internal::BlockVectorIterators::Iterator", 
> %"class.internal::BlockVectorIterators::Iterator"* %this, i64 0, i32 5
>   store i32 %8, i32* %next_break_backward7, align 8, !tbaa !11
>   ret %"class.internal::BlockVectorIterators::Iterator"* %this
>
>
> -- 
> Kind regards,
> Charith Mendis
>
> Graduate Student,
> CSAIL,
> Massachusetts Institute of Technology
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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