[PATCH] D127595: [DAG] visitINSERT_VECTOR_ELT - attempt to reconstruct BUILD_VECTOR before other fold interfere

[Amaury SECHET via Phabricator via llvm-commits]

deadalnix accepted this revision.
deadalnix added a comment.
This revision is now accepted and ready to land.

The test for which this triggers seems overly restrictive, but this is good.



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Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:19433
+    // chain.
+    if (Elt == (NumElts - 1)) {
+      SmallVector<SDValue> ReverseInsertions;
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That seems overly restrictive:
 - The remaining elements could be undef - this is a relatively common occurrence, as far as i can tell.
 - The element could be inserted in pretty much any order (unless there is some canonicalization?)


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Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:19444
+        auto *CurIdx = dyn_cast<ConstantSDNode>(CurVec.getOperand(2));
+        if (!CurIdx || CurIdx->getAPIntValue() != ((NumElts - 1) - I))
+          break;
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this assumes proper order, do we make sure this is cannonical in some way?


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