InstCombine: Fold more shuffles of shuffles.
Always fold a shuffle-of-shuffle into a single shuffle when there's only one
input vector in the first place. Continue to be more conservative when there's
multiple inputs.
rdar://13402653
PR15866
llvm-svn: 180802
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index de8a3ac..5624305 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -614,11 +614,16 @@
// we are absolutely afraid of producing a shuffle mask not in the input
// program, because the code gen may not be smart enough to turn a merged
// shuffle into two specific shuffles: it may produce worse code. As such,
- // we only merge two shuffles if the result is either a splat or one of the
- // input shuffle masks. In this case, merging the shuffles just removes
- // one instruction, which we know is safe. This is good for things like
+ // we only merge two shuffles if the result is a splat, one of the input
+ // input shuffle masks, or if there's only one input to the shuffle.
+ // In this case, merging the shuffles just removes one instruction, which
+ // we know is safe. This is good for things like
// turning: (splat(splat)) -> splat, or
// merge(V[0..n], V[n+1..2n]) -> V[0..2n]
+ //
+ // FIXME: This is almost certainly far, far too conservative. We should
+ // have a better model. Perhaps a TargetTransformInfo hook to ask whether
+ // a shuffle is considered OK?
ShuffleVectorInst* LHSShuffle = dyn_cast<ShuffleVectorInst>(LHS);
ShuffleVectorInst* RHSShuffle = dyn_cast<ShuffleVectorInst>(RHS);
if (LHSShuffle)
@@ -743,8 +748,10 @@
}
// If the result mask is equal to one of the original shuffle masks,
- // or is a splat, do the replacement.
- if (isSplat || newMask == LHSMask || newMask == RHSMask || newMask == Mask) {
+ // or is a splat, do the replacement. Similarly, if there is only one
+ // input vector, go ahead and do the folding.
+ if (isSplat || newMask == LHSMask || newMask == RHSMask || newMask == Mask ||
+ isa<UndefValue>(RHS)) {
SmallVector<Constant*, 16> Elts;
Type *Int32Ty = Type::getInt32Ty(SVI.getContext());
for (unsigned i = 0, e = newMask.size(); i != e; ++i) {