[VectorCombine] foldShuffleOfShuffles - relax one-use of inner shuffles (#116062)

Allow multi-use of either of the inner shuffles and account for that in the cost comparison.

Author: RKSimon

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

@@ -1721,11 +1721,11 @@ bool VectorCombine::foldShuffleOfShuffles(Instruction &I) {
   Value *V0, *V1;
   UndefValue *U0, *U1;
   ArrayRef<int> OuterMask, InnerMask0, InnerMask1;
-  if (!match(&I, m_Shuffle(m_OneUse(m_Shuffle(m_Value(V0), m_UndefValue(U0),
-                                              m_Mask(InnerMask0))),
-                           m_OneUse(m_Shuffle(m_Value(V1), m_UndefValue(U1),
-                                              m_Mask(InnerMask1))),
-                           m_Mask(OuterMask))))
+  if (!match(&I,
+             m_Shuffle(
+                 m_Shuffle(m_Value(V0), m_UndefValue(U0), m_Mask(InnerMask0)),
+                 m_Shuffle(m_Value(V1), m_UndefValue(U1), m_Mask(InnerMask1)),
+                 m_Mask(OuterMask))))
     return false;
 
   auto *ShufI0 = dyn_cast<Instruction>(I.getOperand(0));
@@ -1769,17 +1769,24 @@ bool VectorCombine::foldShuffleOfShuffles(Instruction &I) {
   // Try to merge the shuffles if the new shuffle is not costly.
   TTI::TargetCostKind CostKind = TTI::TCK_RecipThroughput;
 
-  InstructionCost OldCost =
+  InstructionCost InnerCost0 =
       TTI.getShuffleCost(TargetTransformInfo::SK_PermuteSingleSrc, ShuffleSrcTy,
-                         InnerMask0, CostKind, 0, nullptr, {V0, U0}, ShufI0) +
+                         InnerMask0, CostKind, 0, nullptr, {V0, U0}, ShufI0);
+  InstructionCost InnerCost1 =
       TTI.getShuffleCost(TargetTransformInfo::SK_PermuteSingleSrc, ShuffleSrcTy,
-                         InnerMask1, CostKind, 0, nullptr, {V1, U1}, ShufI1) +
+                         InnerMask1, CostKind, 0, nullptr, {V1, U1}, ShufI1);
+  InstructionCost OuterCost =
       TTI.getShuffleCost(TargetTransformInfo::SK_PermuteTwoSrc, ShuffleImmTy,
                          OuterMask, CostKind, 0, nullptr, {ShufI0, ShufI1}, &I);
+  InstructionCost OldCost = InnerCost0 + InnerCost1 + OuterCost;
 
   InstructionCost NewCost =
       TTI.getShuffleCost(TargetTransformInfo::SK_PermuteTwoSrc, ShuffleSrcTy,
                          NewMask, CostKind, 0, nullptr, {V0, V1});
+  if (!ShufI0->hasOneUse())
+    NewCost += InnerCost0;
+  if (!ShufI1->hasOneUse())
+    NewCost += InnerCost1;
 
   LLVM_DEBUG(dbgs() << "Found a shuffle feeding two shuffles: " << I
                     << "\n  OldCost: " << OldCost << " vs NewCost: " << NewCost

llvm/test/Transforms/VectorCombine/AArch64/shuffletoidentity.ll

@@ -1080,11 +1080,9 @@ define <16 x i64> @operandbundles(<4 x i64> %a, <4 x i64> %b, <4 x i64> %c) {
 
 define <8 x i8> @operandbundles_first(<8 x i8> %a) {
 ; CHECK-LABEL: @operandbundles_first(
-; CHECK-NEXT:    [[AB:%.*]] = shufflevector <8 x i8> [[A:%.*]], <8 x i8> poison, <4 x i32> <i32 3, i32 2, i32 1, i32 0>
-; CHECK-NEXT:    [[AT:%.*]] = shufflevector <8 x i8> [[A]], <8 x i8> poison, <4 x i32> <i32 7, i32 6, i32 5, i32 4>
+; CHECK-NEXT:    [[AT:%.*]] = shufflevector <8 x i8> [[A:%.*]], <8 x i8> poison, <4 x i32> <i32 7, i32 6, i32 5, i32 4>
 ; CHECK-NEXT:    [[ABT:%.*]] = call <4 x i8> @llvm.abs.v4i8(<4 x i8> [[AT]], i1 false) [ "jl_roots"(ptr addrspace(10) null, ptr addrspace(10) null) ]
-; CHECK-NEXT:    [[TMP1:%.*]] = shufflevector <4 x i8> [[AT]], <4 x i8> [[AB]], <8 x i32> <i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0>
-; CHECK-NEXT:    [[R:%.*]] = call <8 x i8> @llvm.abs.v8i8(<8 x i8> [[TMP1]], i1 false)
+; CHECK-NEXT:    [[R:%.*]] = call <8 x i8> @llvm.abs.v8i8(<8 x i8> [[A]], i1 false)
 ; CHECK-NEXT:    ret <8 x i8> [[R]]
 ;
   %ab = shufflevector <8 x i8> %a, <8 x i8> poison, <4 x i32> <i32 3, i32 2, i32 1, i32 0>
@@ -1098,10 +1096,8 @@ define <8 x i8> @operandbundles_first(<8 x i8> %a) {
 define <8 x i8> @operandbundles_second(<8 x i8> %a) {
 ; CHECK-LABEL: @operandbundles_second(
 ; CHECK-NEXT:    [[AB:%.*]] = shufflevector <8 x i8> [[A:%.*]], <8 x i8> poison, <4 x i32> <i32 3, i32 2, i32 1, i32 0>
-; CHECK-NEXT:    [[AT:%.*]] = shufflevector <8 x i8> [[A]], <8 x i8> poison, <4 x i32> <i32 7, i32 6, i32 5, i32 4>
 ; CHECK-NEXT:    [[ABB:%.*]] = call <4 x i8> @llvm.abs.v4i8(<4 x i8> [[AB]], i1 false) [ "jl_roots"(ptr addrspace(10) null, ptr addrspace(10) null) ]
-; CHECK-NEXT:    [[TMP1:%.*]] = shufflevector <4 x i8> [[AT]], <4 x i8> [[AB]], <8 x i32> <i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0>
-; CHECK-NEXT:    [[R:%.*]] = call <8 x i8> @llvm.abs.v8i8(<8 x i8> [[TMP1]], i1 false)
+; CHECK-NEXT:    [[R:%.*]] = call <8 x i8> @llvm.abs.v8i8(<8 x i8> [[A]], i1 false)
 ; CHECK-NEXT:    ret <8 x i8> [[R]]
 ;
   %ab = shufflevector <8 x i8> %a, <8 x i8> poison, <4 x i32> <i32 3, i32 2, i32 1, i32 0>