Fix for wrong instcombine on vector insert/extract

When trying to collapse sequences of insertelement/extractelement
instructions into single shuffle instructions, there is one specific
case where the Instruction Combiner wrongly updates the resulting
Mask of shuffle indexes.

The problem is in function CollectShuffleElments.

If we have a sequence of insert/extract element instructions
like the one below:

  %tmp1 = extractelement <4 x float> %LHS, i32 0
  %tmp2 = insertelement <4 x float> %RHS, float %tmp1, i32 1
  %tmp3 = extractelement <4 x float> %RHS, i32 2
  %tmp4 = insertelement <4 x float> %tmp2, float %tmp3, i32 3

Where:
  . %RHS will have a mask of [4,5,6,7]
  . %LHS will have a mask of [0,1,2,3]

The Mask of shuffle indexes is wrongly computed to [4,1,6,7]
instead of [4,0,6,7].
When analyzing %tmp2 in order to compute the Mask for the
resulting shuffle instruction, the algorithm forgets to update
the mask index at position 1 with the index associated to the
element extracted from %LHS by instruction %tmp1.

Patch by Andrea DiBiagio!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179291 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 4 insertions, 0 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index 4f71db1..bbfad86 100644
--- a/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -336,6 +336,10 @@ static Value *CollectShuffleElements(Value *V, SmallVectorImpl<Constant*> &Mask,
 
         if (VecOp == RHS) {
           Value *V = CollectShuffleElements(EI->getOperand(0), Mask, RHS);
+          // Update Mask to reflect that `ScalarOp' has been inserted at
+          // position `InsertedIdx' within the vector returned by IEI.
+          Mask[InsertedIdx % NumElts] = Mask[ExtractedIdx];
+
           // Everything but the extracted element is replaced with the RHS.
           for (unsigned i = 0; i != NumElts; ++i) {
             if (i != InsertedIdx)