[llvm] [RISCV] Add 2^N + 2^M expanding pattern for mul (PR #137954)

[Philip Reames via llvm-commits]

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@@ -15653,22 +15677,7 @@ static SDValue expandMul(SDNode *N, SelectionDAG &DAG,
         return DAG.getNode(ISD::SUB, DL, VT, Shift1, Mul359);
       }
     }
-  }
 
-  // 2^N - 2^M -> (sub (shl X, C1), (shl X, C2))
-  uint64_t MulAmtLowBit = MulAmt & (-MulAmt);
-  if (isPowerOf2_64(MulAmt + MulAmtLowBit)) {
-    uint64_t ShiftAmt1 = MulAmt + MulAmtLowBit;
-    SDLoc DL(N);
-    SDValue Shift1 = DAG.getNode(ISD::SHL, DL, VT, N->getOperand(0),
-                                 DAG.getConstant(Log2_64(ShiftAmt1), DL, VT));
-    SDValue Shift2 =
-        DAG.getNode(ISD::SHL, DL, VT, N->getOperand(0),
-                    DAG.getConstant(Log2_64(MulAmtLowBit), DL, VT));
-    return DAG.getNode(ISD::SUB, DL, VT, Shift1, Shift2);
-  }
-
-  if (HasShlAdd) {
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preames wrote:

It combines with the if-clause above, which uses the same test.

https://github.com/llvm/llvm-project/pull/137954