[CostModel] Treat Identity shuffle masks as zero cost

As discussed on D47985, identity shuffle masks should probably be free.

I've limited this to the case where the input and output types all match - but we could probably accept all cases.

Differential Revision: https://reviews.llvm.org/D47986

llvm-svn: 334506

1 files changed, 20 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/TargetTransformInfo.cpp b/llvm/lib/Analysis/TargetTransformInfo.cpp
index 193e5931993..14ec5ee5107 100644
--- a/llvm/lib/Analysis/TargetTransformInfo.cpp
+++ b/llvm/lib/Analysis/TargetTransformInfo.cpp
@@ -658,6 +658,22 @@ static bool isZeroEltBroadcastVectorMask(ArrayRef<int> Mask) {
   return true;
 }
 
+static bool isIdentityVectorMask(ArrayRef<int> Mask) {
+  bool IdentityLHS = true;
+  bool IdentityRHS = true;
+  unsigned MaskSize = Mask.size();
+
+  // Example: shufflevector A, B, <0,1,u,3>
+  // Example: shufflevector A, B, <4,u,6,u>
+  for (unsigned i = 0; i < MaskSize && (IdentityLHS || IdentityRHS); ++i) {
+    if (Mask[i] < 0)
+      continue;
+    IdentityLHS &= (Mask[i] == i);
+    IdentityRHS &= (Mask[i] == (i + MaskSize));
+  }
+  return IdentityLHS || IdentityRHS;
+}
+
 static bool isAlternateVectorMask(ArrayRef<int> Mask) {
   bool isAlternate = true;
   unsigned MaskSize = Mask.size();
@@ -1213,9 +1229,13 @@ int TargetTransformInfo::getInstructionThroughput(const Instruction *I) const {
     SmallVector<int, 16> Mask = Shuffle->getShuffleMask();
 
     if (NumVecElems == Mask.size()) {
+      if (isIdentityVectorMask(Mask))
+        return 0;
+
       if (isReverseVectorMask(Mask))
         return TTIImpl->getShuffleCost(TargetTransformInfo::SK_Reverse,
                                        VecTypOp0, 0, nullptr);
+
       if (isAlternateVectorMask(Mask))
         return TTIImpl->getShuffleCost(TargetTransformInfo::SK_Alternate,
                                        VecTypOp0, 0, nullptr);