[llvm] [SelectionDAG] Use Magic Algorithm for Splitting UDIV/UREM by Constant (PR #154968)

[Marius Kamp via llvm-commits]

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@@ -8352,6 +8325,157 @@ bool TargetLowering::expandDIVREMByConstant(SDNode *N,
   return true;
 }
 
+bool TargetLowering::expandUDIVREMByConstantViaUMulHiMagic(
+    SDNode *N, const APInt &Divisor, SmallVectorImpl<SDValue> &Result,
+    EVT HiLoVT, SelectionDAG &DAG, SDValue LL, SDValue LH) const {
+
+  SDValue N0 = N->getOperand(0);
+  EVT VT = N0->getValueType(0);
+  SDLoc DL{N};
+
+  assert(!Divisor.isOne() && "Magic algorithm does not work for division by 1");
+
+  // This helper creates a MUL_LOHI of the pair (LL, LH) by a constant.
+  auto MakeMUL_LOHIByConst = [&](unsigned Opc, SDValue LL, SDValue LH,
+                                 const APInt &Const,
+                                 SmallVectorImpl<SDValue> &Result) {
+    SDValue LHS = DAG.getNode(ISD::BUILD_PAIR, DL, VT, LL, LH);
+    SDValue RHS = DAG.getConstant(Const, DL, VT);
+    auto [RL, RH] = DAG.SplitScalar(RHS, DL, HiLoVT, HiLoVT);
+    return expandMUL_LOHI(Opc, VT, DL, LHS, RHS, Result, HiLoVT, DAG,
+                          TargetLowering::MulExpansionKind::OnlyLegalOrCustom,
+                          LL, LH, RL, RH);
+  };
+
+  // This helper creates an ADD/SUB of the pairs (LL, LH) and (RL, RH).
+  auto MakeAddSubLong = [&](unsigned Opc, SDValue LL, SDValue LH, SDValue RL,
+                            SDValue RH) {
+    SDValue AddSubNode =
+        DAG.getNode(Opc == ISD::ADD ? ISD::UADDO : ISD::USUBO, DL,
+                    DAG.getVTList(HiLoVT, MVT::i1), LL, RL);
+    SDValue OutL, OutH, Overflow;
+    expandUADDSUBO(AddSubNode.getNode(), OutL, Overflow, DAG);
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mskamp wrote:

Right, this is not necessary. While at it, I've improved codegen a bit as well.

https://github.com/llvm/llvm-project/pull/154968