DAGCombine: Let truncates negate extension through extract-subvector

Summary:
Fold cases such as:
(v8i8 truncate (v8i32 extract_subvector (v16i32 sext (v16i8 V), Idx)))
->
(v8i8 extract_subvector (v16i8 V), Idx)

This can be generalized to cases where the truncate and extend do not
fully cancel each other out, but it may require querying the target
about profitability.

Reviewers: RKSimon, craig.topper, spatel, efriedma

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41927

llvm-svn: 322300

1 files changed, 16 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 1820d5f944f..4ae22d69eaf 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -8779,6 +8779,22 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
     return DAG.getNode(N0.getOpcode(), SL, VTs, X, Y, N0.getOperand(2));
   }
 
+  // fold (truncate (extract_subvector(ext x))) ->
+  //      (extract_subvector x)
+  // TODO: This can be generalized to cover cases where the truncate and extract
+  // do not fully cancel each other out.
+  if (!LegalTypes && N0.getOpcode() == ISD::EXTRACT_SUBVECTOR) {
+    SDValue N00 = N0.getOperand(0);
+    if (N00.getOpcode() == ISD::SIGN_EXTEND ||
+        N00.getOpcode() == ISD::ZERO_EXTEND ||
+        N00.getOpcode() == ISD::ANY_EXTEND) {
+      if (N00.getOperand(0)->getValueType(0).getVectorElementType() ==
+          VT.getVectorElementType())
+        return DAG.getNode(ISD::EXTRACT_SUBVECTOR, SDLoc(N0->getOperand(0)), VT,
+                           N00.getOperand(0), N0.getOperand(1));
+    }
+  }
+
   if (SDValue NewVSel = matchVSelectOpSizesWithSetCC(N))
     return NewVSel;