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Revert "InstCombine: Fold more shuffles of shuffles."

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This reverts commit r180802

There's ongoing discussion about whether this is the right place to make
this transformation. Reverting for now while we figure it out.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180834 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Jim Grosbach
2013-05-01 00:25:27 +00:00
parent c147c1b994
commit 10cc563bfe
3 changed files with 16 additions and 22 deletions
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17
lib/Transforms/InstCombine/InstCombineVectorOps.cpp
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@@ -614,16 +614,11 @@ Instruction *InstCombiner::visitShuffleVectorInst(ShuffleVectorInst &SVI) {
// 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 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
// 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
// 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)
@@ -748,10 +743,8 @@ Instruction *InstCombiner::visitShuffleVectorInst(ShuffleVectorInst &SVI) {
}
// If the result mask is equal to one of the original shuffle masks,
// 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)) {
// or is a splat, do the replacement.
if (isSplat || newMask == LHSMask || newMask == RHSMask || newMask == Mask) {
SmallVector<Constant*, 16> Elts;
Type *Int32Ty = Type::getInt32Ty(SVI.getContext());
for (unsigned i = 0, e = newMask.size(); i != e; ++i) {