[llvm] [SelectionDAG] Use SimplifyDemandedBits from SimplifyDemandedVectorElts Bitcast. (PR #133717)

[Simon Pilgrim via llvm-commits]

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@@ -3163,10 +3163,22 @@ bool TargetLowering::SimplifyDemandedVectorElts(
     SDValue Src = Op.getOperand(0);
     EVT SrcVT = Src.getValueType();
 
-    // We only handle vectors here.
-    // TODO - investigate calling SimplifyDemandedBits/ComputeKnownBits?
-    if (!SrcVT.isVector())
+    if (!SrcVT.isVector()) {
+      // TODO - bigendian once we have test coverage.
+      if (IsLE) {
+        APInt DemandedSrcBits = APInt::getZero(SrcVT.getSizeInBits());
+        for (unsigned i = 0; i != NumElts; ++i)
+          if (DemandedElts[i]) {
+            unsigned Offset = i * VT.getScalarSizeInBits();
+            DemandedSrcBits.insertBits(
+                APInt::getAllOnes(VT.getScalarSizeInBits()), Offset);
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RKSimon wrote:

Use DemandedSrcBits.setBits() ?

https://github.com/llvm/llvm-project/pull/133717