[llvm-dev] Aliasing Issue after vectorization (original) (raw)

Charith Mendis via llvm-dev llvm-dev at lists.llvm.org
Fri Sep 8 10:16:37 PDT 2017

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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170908/33ae7cfe/attachment.html>

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