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

[Craig Topper via llvm-commits]

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@@ -15452,6 +15453,36 @@ static SDValue performXORCombine(SDNode *N, SelectionDAG &DAG,
   return combineSelectAndUseCommutative(N, DAG, /*AllOnes*/ false, Subtarget);
 }
 
+// Try to expand a multiply to a sequence of shifts and add/subs,
+// for a machine without native mul instruction.
+static SDValue expandMulToNAFSequence(SDNode *N, SelectionDAG &DAG,
+                                      uint64_t MulAmt) {
+  SDLoc DL(N);
+  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) {
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topperc wrote:

Why do we need 2 loops?

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