[X86] Add a combine to recognize when we have two insert subvectors that together write the whole vector, but the starting vector isn't undef.

In this case we should replace the starting vector with undef.

llvm-svn: 312462

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index f7fe3e8add4..80f1afe310f 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -35750,6 +35750,18 @@ static SDValue combineInsertSubvector(SDNode *N, SelectionDAG &DAG,
         return DAG.getNode(ISD::INSERT_SUBVECTOR, dl, OpVT,
                            getZeroVector(OpVT, Subtarget, DAG, dl), SubVec2,
                            Vec.getOperand(2));
+
+      // If we are inserting into both halves of the vector, the starting
+      // vector should be undef. If it isn't, make it so. Only do this if the
+      // the early insert has no other uses.
+      // TODO: Should this be a generic DAG combine?
+      if (!Vec.getOperand(0).isUndef() && Vec.hasOneUse()) {
+        Vec = DAG.getNode(ISD::INSERT_SUBVECTOR, dl, OpVT, DAG.getUNDEF(OpVT),
+                          SubVec2, Vec.getOperand(2));
+        DCI.AddToWorklist(Vec.getNode());
+        return DAG.getNode(ISD::INSERT_SUBVECTOR, dl, OpVT, Vec, SubVec, Idx);
+
+      }
     }
   }