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

[Max Graey via llvm-commits]

================
@@ -15436,6 +15439,140 @@ 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}
+  uint64_t E = MulAmt;
+  uint64_t I = 0;
+  while (E > 0) {
+    if (E & 1) {
+      if (I >= BitWidth)
+        break;
+      int8_t Z = ((E & 3) == 1) ? 1 : -1;
+      Sequence.push_back({(Z == 1), I});
+      E -= Z;
+    }
+    E >>= 1;
+    I++;
+  }
----------------
MaxGraey wrote:

How about to simplify this a little:
```suggestion
llvm::SmallVector<std::pair<bool, uint64_t>> Sequence;
for (auto 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;
  }
}
```

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