[PATCH] Fold selects when the non-selected elements are undef from shufflevectors

[Benjamin Kramer]

================
Comment at: lib/Transforms/InstCombine/InstCombineSelect.cpp:739
@@ +738,3 @@
+  ShuffleVectorInst *FalseSV = dyn_cast<ShuffleVectorInst>(FalseVal);
+  if (!(VecTy && TrueSV && FalseSV))
+    return false;
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IMO DeMorgan's law makes this kind of condition more readable
  if (!VecTy || !TrueSV || !FalseSV)

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Comment at: lib/Transforms/InstCombine/InstCombineSelect.cpp:743
@@ +742,3 @@
+  ConstantVector *CondV = dyn_cast<ConstantVector>(CondVal);
+  if (!(CondV && SI.hasOneUse()))
+    return false;
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Why is it important that SI has only one use?

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Comment at: lib/Transforms/InstCombine/InstCombineSelect.cpp:756-758
@@ +755,5 @@
+  // shufflevector <4 x i32> <2 x i32>
+  VectorType *TrueShuffleSrc = cast<VectorType>(TrueV2->getType());
+  VectorType *FalseShuffleSrc = cast<VectorType>(FalseV2->getType());
+  if (TrueShuffleSrc != FalseShuffleSrc)
+    return false;
----------------
Since you never use any VectorType methods you can skip the cast<>s here.

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Comment at: lib/Transforms/InstCombine/InstCombineSelect.cpp:778
@@ +777,3 @@
+    if (!Element) {
+      ShuffleMask.push_back(-1);
+      continue;
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If it's not a ConstantInt it could also be a ConstantExpr instead of an Undef. You probably want to bail out in that case.

http://reviews.llvm.org/D3561