[llvm] [GlobalIsel] Combine extract vector element (PR #90339)

[Matt Arsenault via llvm-commits]

================
@@ -325,6 +325,113 @@ bool CombinerHelper::matchExtractVectorElementWithBuildVectorTrunc(
   return true;
 }
 
+bool CombinerHelper::matchExtractVectorElementWithShuffleVector(
+    const MachineOperand &MO, BuildFnTy &MatchInfo) {
+  GExtractVectorElement *Extract =
+      cast<GExtractVectorElement>(getDefIgnoringCopies(MO.getReg(), MRI));
+
+  //
+  //  %zero:_(s64) = G_CONSTANT i64 0
+  //  %sv:_(<4 x s32>) = G_SHUFFLE_SHUFFLE %arg1(<4 x s32>), %arg2(<4 x s32>),
+  //                     shufflemask(0, 0, 0, 0)
+  //  %extract:_(s32) = G_EXTRACT_VECTOR_ELT %sv(<4 x s32>), %zero(s64)
+  //
+  //  -->
+  //
+  //  %zero1:_(s64) = G_CONSTANT i64 0
+  //  %extract:_(s32) = G_EXTRACT_VECTOR_ELT %arg1(<4 x s32>), %zero1(s64)
+  //
+  //
+  //
+  //
+  //  %three:_(s64) = G_CONSTANT i64 3
+  //  %sv:_(<4 x s32>) = G_SHUFFLE_SHUFFLE %arg1(<4 x s32>), %arg2(<4 x s32>),
+  //                     shufflemask(0, 0, 0, -1)
+  //  %extract:_(s32) = G_EXTRACT_VECTOR_ELT %sv(<4 x s32>), %three(s64)
+  //
+  //  -->
+  //
+  //  %extract:_(s32) = G_IMPLICIT_DEF
+  //
+  //
+  //
+  //
+  //
+  //  %sv:_(<4 x s32>) = G_SHUFFLE_SHUFFLE %arg1(<4 x s32>), %arg2(<4 x s32>),
+  //                     shufflemask(0, 0, 0, -1)
+  //  %extract:_(s32) = G_EXTRACT_VECTOR_ELT %sv(<4 x s32>), %opaque(s64)
+  //
+  //  -->
+  //
+  //  %sv:_(<4 x s32>) = G_SHUFFLE_SHUFFLE %arg1(<4 x s32>), %arg2(<4 x s32>),
+  //                     shufflemask(0, 0, 0, -1)
+  //  %extract:_(s32) = G_EXTRACT_VECTOR_ELT %sv(<4 x s32>), %opaque(s64)
+  //
+  //
+
+  // We try to get the value of the Index register.
+  std::optional<ValueAndVReg> MaybeIndex =
+      getIConstantVRegValWithLookThrough(Extract->getIndexReg(), MRI);
+  if (!MaybeIndex)
+    return false;
+
+  GShuffleVector *Shuffle =
+      cast<GShuffleVector>(getDefIgnoringCopies(Extract->getVectorReg(), MRI));
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arsenm wrote:

It's unfortunate you need to re-find the def instruction when it was already matched 

https://github.com/llvm/llvm-project/pull/90339