[llvm] [VectorCombine][AMDGPU] Narrow Phi of Shuffles. (PR #140188)

[Simon Pilgrim via llvm-commits]

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@@ -3483,6 +3484,109 @@ bool VectorCombine::foldInterleaveIntrinsics(Instruction &I) {
   return true;
 }
 
+// Attempt to narrow a phi of shufflevector instructions where the two incoming
+// values have the same operands but different masks. If the two shuffle masks
+// are offsets of one another we can use one branch to rotate the incoming
+// vector and perform one larger shuffle after the phi.
+bool VectorCombine::shrinkPhiOfShuffles(Instruction &I) {
+  auto *Phi = dyn_cast<PHINode>(&I);
+  if (!Phi || Phi->getNumIncomingValues() != 2u)
+    return false;
+
+  auto *Shuf0 = dyn_cast<ShuffleVectorInst>(Phi->getOperand(0u));
+  auto *Shuf1 = dyn_cast<ShuffleVectorInst>(Phi->getOperand(1u));
+  if (!Shuf0 || !Shuf1)
+    return false;
+
+  if (!Shuf0->hasOneUse() && !Shuf1->hasOneUse())
+    return false;
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RKSimon wrote:

you can almost certainly make this clearer with PatternMatch

https://github.com/llvm/llvm-project/pull/140188