[llvm] cfbb924 - [SDAG] Fix pow2 assumption when splitting vectors

[Carl Ritson via llvm-commits]

Author: Carl Ritson
Date: 2021-06-11T08:58:16+09:00
New Revision: cfbb92441f17d1f5a9d9c3e195646df4117cb0ca

URL: https://github.com/llvm/llvm-project/commit/cfbb92441f17d1f5a9d9c3e195646df4117cb0ca
DIFF: https://github.com/llvm/llvm-project/commit/cfbb92441f17d1f5a9d9c3e195646df4117cb0ca.diff

LOG: [SDAG] Fix pow2 assumption when splitting vectors

When reducing vector builds to shuffles it possible that
the DAG combiner may try to extract invalid subvectors.

This happens as the existing code assumes vectors will be power
of 2 sizes, which is already untrue, but becomes more noticable
with v6 and v7 types.
Specifically the existing code assumes that half PowerOf2Ceil of
a given vector index will fit twice into a given vector.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D103880

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 f6f0549b1462c..b470a21b9ec24 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -19370,7 +19370,9 @@ SDValue DAGCombiner::reduceBuildVecToShuffle(SDNode *N) {
       unsigned SplitSize = NearestPow2 / 2;
       EVT SplitVT = EVT::getVectorVT(*DAG.getContext(),
                                      InVT.getVectorElementType(), SplitSize);
-      if (TLI.isTypeLegal(SplitVT)) {
+      if (TLI.isTypeLegal(SplitVT) &&
+          SplitSize + SplitVT.getVectorNumElements() <=
+              InVT.getVectorNumElements()) {
         SDValue VecIn2 = DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, SplitVT, Vec,
                                      DAG.getVectorIdxConstant(SplitSize, DL));
         SDValue VecIn1 = DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, SplitVT, Vec,