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

[Benjamin Kramer]

================
Comment at: lib/Transforms/InstCombine/InstCombineSelect.cpp:734
@@ -733,1 +733,3 @@
 
+static bool CanFoldSelectOfShuffleVectors(SelectInst &SI, Value *CondVal,
+                                          Value *TrueVal, Value *FalseVal) {
----------------
Argument "SI" is now unused :)

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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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  s/&&/||/g

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Comment at: lib/Transforms/InstCombine/InstCombineSelect.cpp:750
@@ +749,3 @@
+  // shufflevector(undef, v) into shufflevector(v, undef)
+  if (!(isa<UndefValue>(TrueV2) && isa<UndefValue>(FalseV2)))
+    return false;
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I'd prefer
  if (!isa<UndefValue>(TrueV2) || !isa<UndefValue>(FalseV2))

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Comment at: test/Transforms/InstCombine/select.ll:1037
@@ +1036,3 @@
+; CHECK-LABEL: @add2f_0
+; CHECK-NOT: select
+; CHECK: ret
----------------
Can we have a more positive test case? The mask generation code isn't entirely trivial, would be good to have it verified by a test.

http://reviews.llvm.org/D3561