[AArch64] Lower multiplication by a constant (NFC)

[Allen]

Summary

Refactor according https://reviews.llvm.org/D134706#inline-1298952

[efriedma]

LGTM

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

if (TrailingZeroes) {
     // folded into madd or msub.
     if (N->hasOneUse() && (N->use_begin()->getOpcode() == ISD::ADD ||
                            N->use_begin()->getOpcode() == ISD::SUB))
       return SDValue();
   }
   // Use ShiftedConstValue instead of ConstValue to support both shift+add/sub
   // and shift+add+shift.
   APInt ShiftedConstValue = ConstValue.ashr(TrailingZeroes);
-
   unsigned ShiftAmt;
-  // Is the shifted value the LHS operand of the add/sub?
-  bool ShiftValUseIsN0 = true;
-  // Do we need to negate the result?
-  bool NegateResult = false;
 
   auto Shl = [&](SDValue N0, unsigned N1) {
     SDValue RHS = DAG.getConstant(N1, DL, MVT::i64);
     return DAG.getNode(ISD::SHL, DL, VT, N0, RHS);
   };
+  auto Add = [&](SDValue N0, SDValue N1) {
+    return DAG.getNode(ISD::ADD, DL, VT, N0, N1);
+  };
   auto Sub = [&](SDValue N0, SDValue N1) {
     return DAG.getNode(ISD::SUB, DL, VT, N0, N1);
   };
+  auto Negate = [&](SDValue N) {
+    SDValue Zero = DAG.getConstant(0, DL, VT);
+    return DAG.getNode(ISD::SUB, DL, VT, Zero, N);
+  };
 
   if (ConstValue.isNonNegative()) {
-    // (mul x, 2^N + 1) => (add (shl x, N), x)
-    // (mul x, 2^N - 1) => (sub (shl x, N), x)
     // (mul x, (2^N + 1) * 2^M) => (shl (add (shl x, N), x), M)
+    // (mul x, 2^N - 1) => (sub (shl x, N), x)
     // (mul x, (2^(N-M) - 1) * 2^M) => (sub (shl x, N), (shl x, M))
     APInt SCVMinus1 = ShiftedConstValue - 1;
     APInt SCVPlus1 = ShiftedConstValue + 1;
     APInt CVPlus1 = ConstValue + 1;
     if (SCVMinus1.isPowerOf2()) {
       ShiftAmt = SCVMinus1.logBase2();
-      AddSubOpc = ISD::ADD;
+      return Shl(Add(Shl(N0, ShiftAmt), N0), TrailingZeroes);
     } else if (CVPlus1.isPowerOf2()) {
       ShiftAmt = CVPlus1.logBase2();
-      AddSubOpc = ISD::SUB;
+      return Sub(Shl(N0, ShiftAmt), N0);
     } else if (SCVPlus1.isPowerOf2()) {
       ShiftAmt = SCVPlus1.logBase2() + TrailingZeroes;
       return Sub(Shl(N0, ShiftAmt), Shl(N0, TrailingZeroes));
-    } else
-      return SDValue();
+    }
   } else {
     // (mul x, -(2^N - 1)) => (sub x, (shl x, N))
     // (mul x, -(2^N + 1)) => - (add (shl x, N), x)
     APInt CVNegPlus1 = -ConstValue + 1;
     APInt CVNegMinus1 = -ConstValue - 1;
     if (CVNegPlus1.isPowerOf2()) {
       ShiftAmt = CVNegPlus1.logBase2();
-      AddSubOpc = ISD::SUB;
-      ShiftValUseIsN0 = false;
+      return Sub(N0, Shl(N0, ShiftAmt));
     } else if (CVNegMinus1.isPowerOf2()) {
       ShiftAmt = CVNegMinus1.logBase2();
-      AddSubOpc = ISD::ADD;
-      NegateResult = true;
-    } else
-      return SDValue();
+      return Negate(Add(Shl(N0, ShiftAmt), N0));
+    }
   }
 
-  SDValue ShiftedVal0 = Shl(N0, ShiftAmt);
-  SDValue AddSubN0 = ShiftValUseIsN0 ? ShiftedVal0 : N0;
-  SDValue AddSubN1 = ShiftValUseIsN0 ? N0 : ShiftedVal0;
-  SDValue Res = DAG.getNode(AddSubOpc, DL, VT, AddSubN0, AddSubN1);
-  assert(!(NegateResult && TrailingZeroes) &&
-         "NegateResult and TrailingZeroes cannot both be true for now.");
-  // Negate the result.
-  if (NegateResult)
-    return Sub(DAG.getConstant(0, DL, VT), Res);
-  // Shift the result.
-  if (TrailingZeroes)
-    return Shl(Res, TrailingZeroes);
-  return Res;
+  return SDValue();
 }
 
 static SDValue performVectorCompareAndMaskUnaryOpCombine(SDNode *N,
                                                          SelectionDAG &DAG) {
   // Take advantage of vector comparisons producing 0 or -1 in each lane to
   // optimize away operation when it's from a constant.
   //
   // The general transformation is: