[llvm] [RISCV] Add combine for shadd family of instructions. (PR #130829)

[Nemanja Ivanovic via llvm-commits]

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@@ -14306,6 +14312,79 @@ static SDValue transformAddShlImm(SDNode *N, SelectionDAG &DAG,
   return DAG.getNode(ISD::SHL, DL, VT, SHADD, DAG.getConstant(Bits, DL, VT));
 }
 
+// Check if this SDValue is an add immediate that is fed by a shift of 1, 2, or 3.
+static bool checkAddiForShift(SDValue AddI, int64_t &AddConst, int64_t &ShlConst) {
+  // Based on testing it seems that performance degrades if the ADDI has
+  // more than 2 uses.
+  if (AddI->use_size() > 2)
+    return false;
+
+  auto *AddConstNode = dyn_cast<ConstantSDNode>(AddI->getOperand(1));
+  if (!AddConstNode)
+    return false;
+  AddConst = AddConstNode->getSExtValue();
+
+  SDValue SHLVal = AddI->getOperand(0);
+  if (SHLVal->getOpcode() != ISD::SHL)
+    return false;
+
+  auto *ShiftNode = dyn_cast<ConstantSDNode>(SHLVal->getOperand(1));
+  if (!ShiftNode)
+    return false;
+
+  ShlConst = ShiftNode->getSExtValue();
+  if (ShlConst < 1 || ShlConst > 3)
+    return false;
+
+  return true;
+}
+
+// Optimize (add (add (shl x, c0),  c1), y) ->
+//          (ADDI (SH*ADD y, x), c1), if c0 equals to [1|2|3].
+static SDValue combineShlAddIAdd(SDNode *N, SelectionDAG &DAG,
+                                 const RISCVSubtarget &Subtarget) {
+  // Perform this optimization only in the zba extension.
+  if (!ReassocShlAddiAdd || !Subtarget.hasStdExtZba())
+    return SDValue();
+
+  // Skip for vector types and larger types.
+  EVT VT = N->getValueType(0);
+  if (VT.isVector() || VT.getSizeInBits() > Subtarget.getXLen())
+    return SDValue();
+
+  // Looking for a reg-reg add and not an addi.
+  if (isa<ConstantSDNode>(N->getOperand(1)))
+    return SDValue();
+
+  SDValue AddI = N->getOperand(0);
+  SDValue Other = N->getOperand(1);
+  bool LHSIsAdd = AddI.getOpcode() == ISD::ADD;
+  bool RHSIsAdd = Other.getOpcode() == ISD::ADD;
+  int64_t AddConst;
+  int64_t ShlConst;
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nemanjai wrote:

Minor nit: I suspect that at least for some compilers, this may produce "Variable used before it was set" warnings. Maybe initialize them to safe values (presumably zeros).

https://github.com/llvm/llvm-project/pull/130829