[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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@@ -1552,6 +1553,86 @@ 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, *U0, *U1;
+  ArrayRef<int> OuterMask, InnerMask0, InnerMask1;
+  if (!match(&I, m_Shuffle(m_OneUse(m_Shuffle(m_Value(V0), m_Value(U0),
+                                              m_Mask(InnerMask0))),
+                           m_OneUse(m_Shuffle(m_Value(V1), m_Value(U1),
+                                              m_Mask(InnerMask1))),
+                           m_Mask(OuterMask))) ||
+      !isa<UndefValue>(U0) || !isa<UndefValue>(U1))
+    return false;
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RKSimon wrote:

Tricky as we'll have to match it the inner shuffles anyhow - I'll see if I can create a m_UndefValue() class matcher that we can capture the operand from instead - that should give us the early out.

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