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

[Brandon Wu 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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4vtomat wrote:

One stupid question: Why does it can't be signed int 12 in this line?
https://github.com/llvm/llvm-project/blob/5d314353fbec1a15cd8900f466dcdcf2af40e8c9/llvm/lib/Target/RISCV/RISCVISelLowering.cpp#L20626

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