[InstCombine] Teach InstCombine how to handle an obfuscated splat.

An obfuscated splat is where the frontend poorly generates code for a splat
using several different shuffles to create the splat, i.e.,

  %A = load <4 x float>* %in_ptr, align 16
  %B = shufflevector <4 x float> %A, <4 x float> undef, <4 x i32> <i32 0, i32 0, i32 undef, i32 undef>
  %C = shufflevector <4 x float> %B, <4 x float> %A, <4 x i32> <i32 0, i32 1, i32 4, i32 undef>
  %D = shufflevector <4 x float> %C, <4 x float> %A, <4 x i32> <i32 0, i32 1, i32 2, i32 4>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166061 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 5 insertions, 2 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index cf60f0f..dd7ea14 100644
--- a/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -636,8 +636,11 @@ Instruction *InstCombiner::visitShuffleVectorInst(ShuffleVectorInst &SVI) {
 
       // If LHS's width is changed, shift the mask value accordingly.
       // If newRHS == NULL, i.e. LHSOp0 == RHSOp0, we want to remap any
-      // references to RHSOp0 to LHSOp0, so we don't need to shift the mask.
-      if (eltMask >= 0 && newRHS != NULL)
+      // references from RHSOp0 to LHSOp0, so we don't need to shift the mask.
+      // If newRHS == newLHS, we want to remap any references from newRHS to
+      // newLHS so that we can properly identify splats that may occur due to
+      // obfuscation accross the two vectors.
+      if (eltMask >= 0 && newRHS != NULL && newLHS != newRHS)
         eltMask += newLHSWidth;
     }