[llvm] [RISCV] Strength reduce mul by 2^N - 2^M (PR #88983)

[Yingwei Zheng via llvm-commits]

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@@ -13474,6 +13476,19 @@ static SDValue expandMul(SDNode *N, SelectionDAG &DAG,
     }
   }
 
+  // 2^N - 2^M -> (sub (shl X, C1), (shl X, C2))
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dtcxzyw wrote:

Isn't it handled by `DAGCombiner::visitMUL`?
https://github.com/llvm/llvm-project/blob/e11b17a4ed90e74147594012207fc35a60515944/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp#L4325-L4371

I'd like to introduce ISD::SHXADD node and move all logic into DAGCombiner.


https://github.com/llvm/llvm-project/pull/88983