[DAGCombine] Avoid INSERT_SUBVECTOR reinsertions (PR28678)

If the input vector to INSERT_SUBVECTOR is another INSERT_SUBVECTOR, and this inserted subvector replaces the last insertion, then insert into the common source vector.

i.e. 
INSERT_SUBVECTOR( INSERT_SUBVECTOR( Vec, SubOld, Idx ), SubNew, Idx ) --> INSERT_SUBVECTOR( Vec, SubNew, Idx )

Differential Revision: https://reviews.llvm.org/D23330

llvm-svn: 278211

1 files changed, 10 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index b32737f7aa0..7fc71994e36 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -13802,10 +13802,20 @@ SDValue DAGCombiner::visitSCALAR_TO_VECTOR(SDNode *N) {
 }
 
 SDValue DAGCombiner::visitINSERT_SUBVECTOR(SDNode *N) {
+  EVT VT = N->getValueType(0);
   SDValue N0 = N->getOperand(0);
   SDValue N1 = N->getOperand(1);
   SDValue N2 = N->getOperand(2);
 
+  // Combine INSERT_SUBVECTORs where we are inserting to the same index.
+  // INSERT_SUBVECTOR( INSERT_SUBVECTOR( Vec, SubOld, Idx ), SubNew, Idx )
+  // --> INSERT_SUBVECTOR( Vec, SubNew, Idx )
+  if (N0.getOpcode() == ISD::INSERT_SUBVECTOR &&
+      N0.getOperand(1).getValueType() == N1.getValueType() &&
+      N0.getOperand(2) == N2)
+    return DAG.getNode(ISD::INSERT_SUBVECTOR, SDLoc(N), VT, N0.getOperand(0),
+                       N1, N2);
+
   if (N0.getValueType() != N1.getValueType())
     return SDValue();
 
@@ -13814,7 +13824,6 @@ SDValue DAGCombiner::visitINSERT_SUBVECTOR(SDNode *N) {
   if (N0.getOpcode() == ISD::CONCAT_VECTORS && N0->getNumOperands() == 2 &&
       N2.getOpcode() == ISD::Constant) {
     APInt InsIdx = cast<ConstantSDNode>(N2)->getAPIntValue();
-    EVT VT = N->getValueType(0);
 
     // Lower half: fold (insert_subvector (concat_vectors X, Y), Z) ->
     // (concat_vectors Z, Y)