[llvm-dev] Aliasing Issue after vectorization

[Charith Mendis via llvm-dev]

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
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