Revert "InstCombine: Fold more shuffles of shuffles."

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

3 files changed, 16 insertions, 22 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index 56243059a6..de8a3acdbd 100644
--- a/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -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) {
diff --git a/test/Transforms/BBVectorize/simple.ll b/test/Transforms/BBVectorize/simple.ll
index a22ad15940..a447908d16 100644
--- a/test/Transforms/BBVectorize/simple.ll
+++ b/test/Transforms/BBVectorize/simple.ll
@@ -139,10 +139,11 @@ define <8 x i8> @test6(<8 x i8> %A1, <8 x i8> %A2, <8 x i8> %B1, <8 x i8> %B2) {
 ; CHECK: %Z1 = add <16 x i8> %Y1, %X1.v.i1
         %Q1 = shufflevector <8 x i8> %Z1, <8 x i8> %Z2, <8 x i32> <i32 15, i32 8, i32 6, i32 1, i32 13, i32 10, i32 4, i32 3>
         %Q2 = shufflevector <8 x i8> %Z2, <8 x i8> %Z2, <8 x i32> <i32 6, i32 7, i32 0, i32 1, i32 2, i32 4, i32 4, i32 1>
-        %R  = mul <8 x i8> %Q1, %Q2
-; CHECK:  %Q1.v.i1 = shufflevector <16 x i8> %Z1, <16 x i8> undef, <16 x i32> <i32 8, i32 undef, i32 10, i32 undef, i32 undef, i32 13, i32 undef, i32 15, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef>
-; CHECK:  %Q1.v.r1 = shufflevector <16 x i8> %Z1, <16 x i8> %Q1.v.i1, <8 x i32> <i32 23, i32 16, i32 6, i32 1, i32 21, i32 18, i32 4, i32 3>
-; CHECK:  %Q1.v.r2 = shufflevector <16 x i8> %Z1, <16 x i8> undef, <8 x i32> <i32 14, i32 15, i32 8, i32 9, i32 10, i32 12, i32 12, i32 9>
+; CHECK: %Q1.v.i1 = shufflevector <16 x i8> %Z1, <16 x i8> undef, <16 x i32> <i32 8, i32 undef, i32 10, i32 undef, i32 undef, i32 13, i32 undef, i32 15, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef>
+; CHECK: %Q1 = shufflevector <16 x i8> %Z1, <16 x i8> %Q1.v.i1, <16 x i32> <i32 23, i32 16, i32 6, i32 1, i32 21, i32 18, i32 4, i32 3, i32 14, i32 15, i32 8, i32 9, i32 10, i32 12, i32 12, i32 9>
+	%R  = mul <8 x i8> %Q1, %Q2
+; CHECK: %Q1.v.r1 = shufflevector <16 x i8> %Q1, <16 x i8> undef, <8 x i32> <i32 0, i32 1, i32 2, i32 3, i32 4, i32 5, i32 6, i32 7>
+; CHECK: %Q1.v.r2 = shufflevector <16 x i8> %Q1, <16 x i8> undef, <8 x i32> <i32 8, i32 9, i32 10, i32 11, i32 12, i32 13, i32 14, i32 15>
 ; CHECK: %R = mul <8 x i8> %Q1.v.r1, %Q1.v.r2
 	ret <8 x i8> %R
 ; CHECK: ret <8 x i8> %R
diff --git a/test/Transforms/InstCombine/vec_shuffle.ll b/test/Transforms/InstCombine/vec_shuffle.ll
index 37c19f1558..37d4d56e91 100644
--- a/test/Transforms/InstCombine/vec_shuffle.ll
+++ b/test/Transforms/InstCombine/vec_shuffle.ll
@@ -86,14 +86,14 @@ define <4 x i8> @test9(<16 x i8> %tmp6) nounwind {
 }
 
 ; Same as test9, but make sure that "undef" mask values are not confused with
-; mask values of 2*N, where N is the mask length of the result.  Make sure when
-; folding these shuffles that 'undef' mask values stay that way in the result
-; instead of getting mapped to the 2*N'th entry of the source.
-define <4 x i8> @test9a(<16 x i8> %in, <16 x i8> %in2) nounwind {
+; mask values of 2*N, where N is the mask length.  These shuffles should not
+; be folded (because [8,9,4,8] may not be a mask supported by the target).
+define <4 x i8> @test9a(<16 x i8> %tmp6) nounwind {
 ; CHECK: @test9a
-; CHECK-NEXT: shufflevector <16 x i8> %in, <16 x i8> %in2, <4 x i32> <i32 16, i32 9, i32 4, i32 undef>
+; CHECK-NEXT: shufflevector
+; CHECK-NEXT: shufflevector
 ; CHECK-NEXT: ret
-	%tmp7 = shufflevector <16 x i8> %in, <16 x i8> %in2, <4 x i32> < i32 undef, i32 9, i32 4, i32 16 >		; <<4 x i8>> [#uses=1]
+	%tmp7 = shufflevector <16 x i8> %tmp6, <16 x i8> undef, <4 x i32> < i32 undef, i32 9, i32 4, i32 8 >		; <<4 x i8>> [#uses=1]
 	%tmp9 = shufflevector <4 x i8> %tmp7, <4 x i8> undef, <4 x i32> < i32 3, i32 1, i32 2, i32 0 >		; <<4 x i8>> [#uses=1]
 	ret <4 x i8> %tmp9
 }