[llvm-bugs] [Bug 43882] New: [SLPVectorizer] Failure to vectorize extract from two vectors

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43882

            Bug ID: 43882
           Summary: [SLPVectorizer] Failure to vectorize extract from two
                    vectors
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: lebedev.ri at gmail.com
                CC: llvm-bugs at lists.llvm.org

Stumbling into this while analyzing https://bugs.llvm.org/show_bug.cgi?id=43844

https://godbolt.org/z/a5HeXw

  %3 = getelementptr inbounds %"struct.std::array", %"struct.std::array"* %0,
i64 0, i32 0, i64 0
  %4 = bitcast %"struct.std::array.0"* %1 to <4 x i32>*
  %5 = load <4 x i32>, <4 x i32>* %4, align 8, !tbaa !2
  %6 = extractelement <4 x i32> %5, i32 0
  store i32 %6, i32* %3, align 4, !tbaa !2
  %7 = getelementptr inbounds %"struct.std::array", %"struct.std::array"* %0,
i64 0, i32 0, i64 2
  %8 = extractelement <4 x i32> %5, i32 1
  store i32 %8, i32* %7, align 4, !tbaa !2
  %9 = getelementptr inbounds %"struct.std::array", %"struct.std::array"* %0,
i64 0, i32 0, i64 4
  %10 = extractelement <4 x i32> %5, i32 2
  store i32 %10, i32* %9, align 4, !tbaa !2
  %11 = getelementptr inbounds %"struct.std::array", %"struct.std::array"* %0,
i64 0, i32 0, i64 6
  %12 = extractelement <4 x i32> %5, i32 3
  store i32 %12, i32* %11, align 4, !tbaa !2
  %13 = getelementptr inbounds %"struct.std::array.0", %"struct.std::array.0"*
%1, i64 0, i32 0, i64 4
  %14 = getelementptr inbounds %"struct.std::array", %"struct.std::array"* %0,
i64 0, i32 0, i64 1
  %15 = bitcast i32* %13 to <4 x i32>*
  %16 = load <4 x i32>, <4 x i32>* %15, align 8, !tbaa !2
  %17 = extractelement <4 x i32> %16, i32 0
  store i32 %17, i32* %14, align 4, !tbaa !2
  %18 = getelementptr inbounds %"struct.std::array", %"struct.std::array"* %0,
i64 0, i32 0, i64 3
  %19 = extractelement <4 x i32> %16, i32 1
  store i32 %19, i32* %18, align 4, !tbaa !2
  %20 = getelementptr inbounds %"struct.std::array", %"struct.std::array"* %0,
i64 0, i32 0, i64 5
  %21 = extractelement <4 x i32> %16, i32 2
  store i32 %21, i32* %20, align 4, !tbaa !2
  %22 = getelementptr inbounds %"struct.std::array", %"struct.std::array"* %0,
i64 0, i32 0, i64 7
  %23 = extractelement <4 x i32> %16, i32 3
  store i32 %23, i32* %22, align 4, !tbaa !2


I.e. e.g. the stores are fed by:
  [  %6 = extractelement <4 x i32> %5, i32 0;  %17 = extractelement <4 x i32>
%16, i32 0;  %8 = extractelement <4 x i32> %5, i32 1;  %19 = extractelement <4
x i32> %16, i32 1]
and so on.

This can be represented as a shuffle:  https://godbolt.org/z/KfWvCe

  %3 = bitcast %"struct.std::array.0"* %1 to <4 x i32>*
  %4 = load <4 x i32>, <4 x i32>* %3, align 8, !tbaa !2
  %5 = getelementptr inbounds %"struct.std::array", %"struct.std::array"* %0,
i64 0, i32 0, i64 4
  %6 = getelementptr inbounds %"struct.std::array.0", %"struct.std::array.0"*
%1, i64 0, i32 0, i64 4
  %7 = bitcast i32* %6 to <4 x i32>*
  %8 = load <4 x i32>, <4 x i32>* %7, align 8, !tbaa !2
  %9 = shufflevector <4 x i32> %4, <4 x i32> %8, <4 x i32> <i32 0, i32 4, i32
1, i32 5>
  %10 = bitcast %"struct.std::array"* %0 to <4 x i32>*
  store <4 x i32> %9, <4 x i32>* %10, align 4, !tbaa !2
  %11 = shufflevector <4 x i32> %4, <4 x i32> %8, <4 x i32> <i32 2, i32 6, i32
3, i32 7>
  %12 = bitcast i32* %5 to <4 x i32>*
  store <4 x i32> %11, <4 x i32>* %12, align 4, !tbaa !2

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