[VectorCombine] foldInsExtVectorToShuffle - canonicalize new shuffle(undef,x) -> shuffle(x,undef).

foldInsExtVectorToShuffle is likely to be inserting into an undef value, so make sure we've canonicalized this to the RHS in the folded shuffle to help further VectorCombine folds.

Minor tweak to help #34072

Author: RKSimon

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

@@ -2848,6 +2848,12 @@ bool VectorCombine::foldInsExtVectorToShuffle(Instruction &I) {
   if (OldCost < NewCost)
     return false;
 
+  // Canonicalize undef param to RHS to help further folds.
+  if (isa<UndefValue>(DstVec) && !isa<UndefValue>(SrcVec)) {
+    ShuffleVectorInst::commuteShuffleMask(Mask, NumElts);
+    std::swap(DstVec, SrcVec);
+  }
+
   Value *Shuf = Builder.CreateShuffleVector(DstVec, SrcVec, Mask);
   replaceValue(I, *Shuf);
 

llvm/test/Transforms/VectorCombine/X86/extract-binop-inseltpoison.ll

@@ -474,8 +474,7 @@ define <4 x float> @PR34724(<4 x float> %a, <4 x float> %b) {
 ; SSE-NEXT:    [[TMP2:%.*]] = fadd <4 x float> [[B]], [[SHIFT1]]
 ; SSE-NEXT:    [[SHIFT2:%.*]] = shufflevector <4 x float> [[B]], <4 x float> poison, <4 x i32> <i32 poison, i32 poison, i32 poison, i32 2>
 ; SSE-NEXT:    [[TMP3:%.*]] = fadd <4 x float> [[SHIFT2]], [[B]]
-; SSE-NEXT:    [[V1:%.*]] = shufflevector <4 x float> poison, <4 x float> [[TMP1]], <4 x i32> <i32 0, i32 6, i32 2, i32 3>
-; SSE-NEXT:    [[V2:%.*]] = shufflevector <4 x float> [[V1]], <4 x float> [[TMP2]], <4 x i32> <i32 0, i32 1, i32 4, i32 3>
+; SSE-NEXT:    [[V2:%.*]] = shufflevector <4 x float> [[TMP1]], <4 x float> [[TMP2]], <4 x i32> <i32 poison, i32 2, i32 4, i32 poison>
 ; SSE-NEXT:    [[V3:%.*]] = shufflevector <4 x float> [[V2]], <4 x float> [[TMP3]], <4 x i32> <i32 0, i32 1, i32 2, i32 7>
 ; SSE-NEXT:    ret <4 x float> [[V3]]
 ;
@@ -489,7 +488,7 @@ define <4 x float> @PR34724(<4 x float> %a, <4 x float> %b) {
 ; AVX-NEXT:    [[B01:%.*]] = extractelement <4 x float> [[TMP2]], i32 0
 ; AVX-NEXT:    [[SHIFT2:%.*]] = shufflevector <4 x float> [[B]], <4 x float> poison, <4 x i32> <i32 poison, i32 poison, i32 poison, i32 2>
 ; AVX-NEXT:    [[TMP3:%.*]] = fadd <4 x float> [[SHIFT2]], [[B]]
-; AVX-NEXT:    [[V1:%.*]] = shufflevector <4 x float> poison, <4 x float> [[TMP1]], <4 x i32> <i32 0, i32 6, i32 2, i32 3>
+; AVX-NEXT:    [[V1:%.*]] = shufflevector <4 x float> [[TMP1]], <4 x float> poison, <4 x i32> <i32 4, i32 2, i32 6, i32 7>
 ; AVX-NEXT:    [[V2:%.*]] = insertelement <4 x float> [[V1]], float [[B01]], i32 2
 ; AVX-NEXT:    [[V3:%.*]] = shufflevector <4 x float> [[V2]], <4 x float> [[TMP3]], <4 x i32> <i32 0, i32 1, i32 2, i32 7>
 ; AVX-NEXT:    ret <4 x float> [[V3]]

llvm/test/Transforms/VectorCombine/X86/extract-binop.ll

@@ -474,7 +474,7 @@ define <4 x float> @PR34724(<4 x float> %a, <4 x float> %b) {
 ; SSE-NEXT:    [[TMP2:%.*]] = fadd <4 x float> [[B]], [[SHIFT1]]
 ; SSE-NEXT:    [[SHIFT2:%.*]] = shufflevector <4 x float> [[B]], <4 x float> poison, <4 x i32> <i32 poison, i32 poison, i32 poison, i32 2>
 ; SSE-NEXT:    [[TMP3:%.*]] = fadd <4 x float> [[SHIFT2]], [[B]]
-; SSE-NEXT:    [[V1:%.*]] = shufflevector <4 x float> undef, <4 x float> [[TMP1]], <4 x i32> <i32 0, i32 6, i32 2, i32 3>
+; SSE-NEXT:    [[V1:%.*]] = shufflevector <4 x float> [[TMP1]], <4 x float> undef, <4 x i32> <i32 4, i32 2, i32 6, i32 7>
 ; SSE-NEXT:    [[V2:%.*]] = shufflevector <4 x float> [[V1]], <4 x float> [[TMP2]], <4 x i32> <i32 0, i32 1, i32 4, i32 3>
 ; SSE-NEXT:    [[V3:%.*]] = shufflevector <4 x float> [[V2]], <4 x float> [[TMP3]], <4 x i32> <i32 0, i32 1, i32 2, i32 7>
 ; SSE-NEXT:    ret <4 x float> [[V3]]
@@ -489,7 +489,7 @@ define <4 x float> @PR34724(<4 x float> %a, <4 x float> %b) {
 ; AVX-NEXT:    [[B01:%.*]] = extractelement <4 x float> [[TMP2]], i32 0
 ; AVX-NEXT:    [[SHIFT2:%.*]] = shufflevector <4 x float> [[B]], <4 x float> poison, <4 x i32> <i32 poison, i32 poison, i32 poison, i32 2>
 ; AVX-NEXT:    [[TMP3:%.*]] = fadd <4 x float> [[SHIFT2]], [[B]]
-; AVX-NEXT:    [[V1:%.*]] = shufflevector <4 x float> undef, <4 x float> [[TMP1]], <4 x i32> <i32 0, i32 6, i32 2, i32 3>
+; AVX-NEXT:    [[V1:%.*]] = shufflevector <4 x float> [[TMP1]], <4 x float> undef, <4 x i32> <i32 4, i32 2, i32 6, i32 7>
 ; AVX-NEXT:    [[V2:%.*]] = insertelement <4 x float> [[V1]], float [[B01]], i32 2
 ; AVX-NEXT:    [[V3:%.*]] = shufflevector <4 x float> [[V2]], <4 x float> [[TMP3]], <4 x i32> <i32 0, i32 1, i32 2, i32 7>
 ; AVX-NEXT:    ret <4 x float> [[V3]]