[DAGCombiner] Stop visitEXTRACT_SUBVECTOR creating illegal BITCASTs post legalisation.

[lebedev.ri]

Test?

[paulwalker-arm]

In D108086#2945565, @lebedev.ri wrote:

Test?

I'm working on a minimal reproducer, I just wanted to have a downloadable fix available just in case anybody is blocked by the issue (see rG01b846674d27f035becac4c980a91c1204b6587b).

[paulwalker-arm]

Add a test.

[lebedev.ri]

LG, thank you.

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

if (V.getOpcode() == ISD::INSERT_SUBVECTOR) {
 
     // Combine:
     //    (extract_subvec (insert_subvec V1, V2, InsIdx), ExtIdx)
     // Into:
     //    indices are equal or bit offsets are equal => V1
     //    otherwise => (extract_subvec V1, ExtIdx)
     uint64_t InsIdx = V.getConstantOperandVal(2);
     if (InsIdx * SmallVT.getScalarSizeInBits() ==
-        ExtIdx * NVT.getScalarSizeInBits())
+        ExtIdx * NVT.getScalarSizeInBits()) {
+      if (LegalOperations && !TLI.isOperationLegal(ISD::BITCAST, NVT))
+        return SDValue();
+
       return DAG.getBitcast(NVT, V.getOperand(1));
+    }
     return DAG.getNode(
         ISD::EXTRACT_SUBVECTOR, SDLoc(N), NVT,
         DAG.getBitcast(N->getOperand(0).getValueType(), V.getOperand(0)),
         N->getOperand(1));
   }
 
   if (SDValue NarrowBOp = narrowExtractedVectorBinOp(N, DAG, LegalOperations))
     return NarrowBOp;