[llvm] [X86][SelectionDAG] Fix the Gather's base and index by modifying the Scale value (PR #137813)

[Simon Pilgrim via llvm-commits]

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@@ -56578,13 +56578,49 @@ static SDValue combineGatherScatter(SDNode *N, SelectionDAG &DAG,
   const TargetLowering &TLI = DAG.getTargetLoweringInfo();
 
   if (DCI.isBeforeLegalize()) {
+    // Attempt to move shifted index into the address scale, allows further
+    // index truncation below.
+    if (Index.getOpcode() == ISD::SHL && isa<ConstantSDNode>(Scale)) {
+      unsigned BitWidth = Index.getScalarValueSizeInBits();
+      unsigned MaskBits = BitWidth - Log2_32(Scale->getAsZExtVal());
+      APInt DemandedBits = APInt::getLowBitsSet(BitWidth, MaskBits);
+      if (TLI.SimplifyDemandedBits(Index, DemandedBits, DCI)) {
+        if (N->getOpcode() != ISD::DELETED_NODE)
+          DCI.AddToWorklist(N);
+        return SDValue(N, 0);
+      }
+      uint64_t ScaleAmt = Scale->getAsZExtVal();
+      if (auto MinShAmt = DAG.getValidMinimumShiftAmount(Index)) {
+        if (*MinShAmt >= 1 && (*MinShAmt + Log2_64(ScaleAmt)) < 4 &&
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RKSimon wrote:

merge Log2_64 call with the existing Log2_32 call

https://github.com/llvm/llvm-project/pull/137813