[llvm] 556c94e - [DAG] visitINSERT_VECTOR_ELT - use mergeEltWithShuffle to merge inserted vector element chain into base shuffle node

[Simon Pilgrim via llvm-commits]

Author: Simon Pilgrim
Date: 2023-01-22T17:19:48Z
New Revision: 556c94e73ed001f6d1381e28ca44c2b8b5c8e92f

URL: https://github.com/llvm/llvm-project/commit/556c94e73ed001f6d1381e28ca44c2b8b5c8e92f
DIFF: https://github.com/llvm/llvm-project/commit/556c94e73ed001f6d1381e28ca44c2b8b5c8e92f.diff

LOG: [DAG] visitINSERT_VECTOR_ELT - use mergeEltWithShuffle to merge inserted vector element chain into base shuffle node

This allows us to merge insert_elt(insert_elt(shuffle(x,y),extract_elt(x,c1),c2),extract_elt(y,c3),c4) style insertion chains into a new shuffle node.

I had hoped to remove mergeInsertEltWithShuffle entirely, but that case doesn't have the one use limits so we would regress in a few other cases.

Fixes the vector-shuffle-combining.ll regressions in D127115

Added: 
    

Modified: 
    llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

Removed: 
    


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diff  --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 72674a3b0453d..120515fd0a2c3 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -20263,6 +20263,32 @@ SDValue DAGCombiner::visitINSERT_VECTOR_ELT(SDNode *N) {
             continue;
           }
 
+      // VECTOR_SHUFFLE - if all the operands match the shuffle's sources,
+      // update the shuffle mask (and second operand if we started with unary
+      // shuffle) and create a new legal shuffle.
+      if (CurVec.getOpcode() == ISD::VECTOR_SHUFFLE && CurVec.hasOneUse()) {
+        auto *SVN = cast<ShuffleVectorSDNode>(CurVec);
+        SDValue LHS = SVN->getOperand(0);
+        SDValue RHS = SVN->getOperand(1);
+        SmallVector<int, 16> Mask(SVN->getMask());
+        bool Merged = true;
+        for (auto I : enumerate(Ops)) {
+          SDValue &Op = I.value();
+          if (Op) {
+            SmallVector<int, 16> NewMask;
+            if (!mergeEltWithShuffle(LHS, RHS, Mask, NewMask, Op, I.index())) {
+              Merged = false;
+              break;
+            }
+            Mask = std::move(NewMask);
+          }
+        }
+        if (Merged)
+          if (SDValue NewShuffle =
+                  TLI.buildLegalVectorShuffle(VT, DL, LHS, RHS, Mask, DAG))
+            return NewShuffle;
+      }
+
       // Failed to find a match in the chain - bail.
       break;
     }