[llvm] [X86][SelectionDAG] Fix the Gather's base and index by modifying the Scale value (PR #134979)

[Simon Pilgrim via llvm-commits]

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@@ -56520,13 +56520,49 @@ 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());
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RKSimon wrote:

we should assert that Scale->getAsZExtVal() is pow2 

https://github.com/llvm/llvm-project/pull/134979