[llvm] [NVPTX] Fold (add (select 0, (mul a, b)), c) -> (select c, (mad a, b, c)) (PR #96352)

[Alex MacLean via llvm-commits]

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@@ -5215,103 +5215,129 @@ bool NVPTXTargetLowering::allowUnsafeFPMath(MachineFunction &MF) const {
   return F.getFnAttribute("unsafe-fp-math").getValueAsBool();
 }
 
+static bool isConstZero(const SDValue &Operand) {
+  const auto *Const = dyn_cast<ConstantSDNode>(Operand);
+  return Const && Const->getZExtValue() == 0;
+}
+
 /// PerformADDCombineWithOperands - Try DAG combinations for an ADD with
 /// operands N0 and N1.  This is a helper for PerformADDCombine that is
 /// called with the default operands, and if that fails, with commuted
 /// operands.
-static SDValue PerformADDCombineWithOperands(
-    SDNode *N, SDValue N0, SDValue N1, TargetLowering::DAGCombinerInfo &DCI,
-    const NVPTXSubtarget &Subtarget, CodeGenOptLevel OptLevel) {
-  SelectionDAG  &DAG = DCI.DAG;
-  // Skip non-integer, non-scalar case
-  EVT VT=N0.getValueType();
-  if (VT.isVector())
+static SDValue
+PerformADDCombineWithOperands(SDNode *N, SDValue N0, SDValue N1,
+                              TargetLowering::DAGCombinerInfo &DCI) {
+  EVT VT = N0.getValueType();
+
+  // Since integer multiply-add costs the same as integer multiply
+  // but is more costly than integer add, do the fusion only when
+  // the mul is only used in the add.
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AlexMaclean wrote:

Sounds good, added a TODO

https://github.com/llvm/llvm-project/pull/96352