[llvm] [RISCV] Expand constant multiplication for targets without M extension (PR #137195)

[Max Graey via llvm-commits]

================
@@ -15436,6 +15439,134 @@ static SDValue performXORCombine(SDNode *N, SelectionDAG &DAG,
   return combineSelectAndUseCommutative(N, DAG, /*AllOnes*/ false, Subtarget);
 }
 
+static SDValue expandMulToNAFSequence(SDNode *N, SelectionDAG &DAG,
+                                      const SDLoc &DL, uint64_t MulAmt) {
+  EVT VT = N->getValueType(0);
+  const uint64_t BitWidth = VT.getFixedSizeInBits();
+
+  // Find the Non-adjacent form of the multiplier.
+  llvm::SmallVector<std::pair<bool, uint64_t>> Sequence; // {isAdd, shamt}
+  for (uint64_t E = MulAmt, I = 0; E && I < BitWidth; ++I, E >>= 1) {
+    if (E & 1) {
+      bool isAdd = (E & 3) == 1;
+      Sequence.push_back({isAdd, I});
+      E -= isAdd ? 1 : -1;
+    }
+  }
+
+  SDValue Result = DAG.getConstant(0, DL, N->getValueType(0));
+  SDValue N0 = N->getOperand(0);
+
+  for (const auto &Op : Sequence) {
+    SDValue ShiftVal;
+    if (Op.second > 0)
+      ShiftVal =
+          DAG.getNode(ISD::SHL, DL, VT, N0, DAG.getConstant(Op.second, DL, VT));
+    else
+      ShiftVal = N0;
+
+    ISD::NodeType AddSubOp = Op.first ? ISD::ADD : ISD::SUB;
+    Result = DAG.getNode(AddSubOp, DL, VT, Result, ShiftVal);
+  }
+  return Result;
+}
+// Try to expand a multiply to a sequence of shifts and add/subs,
+// for a machine without native mul instruction.
+static SDValue expandMulToBasicOps(SDNode *N, SelectionDAG &DAG,
+                                   uint64_t MulAmt) {
+  EVT VT = N->getValueType(0);
+  const uint64_t BitWidth = VT.getFixedSizeInBits();
+  SDLoc DL(N);
+
+  if (MulAmt == 0)
+    return DAG.getConstant(0, DL, N->getValueType(0));
+
+  // Try to factorize into (2^N) * (2^M_1 +/- 1) * (2^M_2 +/- 1) * ...
+  uint64_t E = MulAmt;
+  uint64_t TrailingZeros = 0;
+
+  while (E > 0 && (E & 1) == 0) {
+    E >>= 1;
+    TrailingZeros++;
+  }
----------------
MaxGraey wrote:

```suggestion
  uint64_t TrailingZeros = llvm::countTrailingZeros(E);
```

https://github.com/llvm/llvm-project/pull/137195