[llvm] [VectorCombine] foldShuffleOfShuffles - fold "shuffle (shuffle x, undef), (shuffle y, undef)" -> "shuffle x, y" (PR #88743)

[Simon Pilgrim via llvm-commits]

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@@ -1547,6 +1548,73 @@ bool VectorCombine::foldShuffleOfCastops(Instruction &I) {
   return true;
 }
 
+/// Try to convert "shuffle (shuffle x, undef), (shuffle y, undef)"
+/// into "shuffle x, y".
+bool VectorCombine::foldShuffleOfShuffles(Instruction &I) {
+  Value *V0, *V1;
+  ArrayRef<int> OuterMask, InnerMask0, InnerMask1;
+  if (!match(&I, m_Shuffle(m_OneUse(m_Shuffle(m_Value(V0), m_Undef(),
+                                              m_Mask(InnerMask0))),
+                           m_OneUse(m_Shuffle(m_Value(V1), m_Undef(),
+                                              m_Mask(InnerMask1))),
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RKSimon wrote:

I've added a bailout (+ tests)

https://github.com/llvm/llvm-project/pull/88743