[llvm] [DirectX] Add atan2 intrinsic and expand for DXIL backend (p1) (PR #108865)

[Farzon Lotfi via llvm-commits]

================
@@ -305,6 +306,48 @@ static Value *expandNormalizeIntrinsic(CallInst *Orig) {
   return Builder.CreateFMul(X, MultiplicandVec);
 }
 
+static Value *expandAtan2Intrinsic(CallInst *Orig) {
+  Value *Y = Orig->getOperand(0);
+  Value *X = Orig->getOperand(1);
+  Type *Ty = X->getType();
+  IRBuilder<> Builder(Orig);
+
+  Value *Tan = Builder.CreateFDiv(Y, X);
+
+  Value *Atan =
+      Builder.CreateIntrinsic(Ty, Intrinsic::atan, {Tan}, nullptr, "Elt.Atan");
+
+  Constant *Pi = ConstantFP::get(Ty, llvm::numbers::pi);
+  Constant *HalfPi = ConstantFP::get(Ty, llvm::numbers::pi / 2);
+  Constant *NegHalfPi = ConstantFP::get(Ty, -llvm::numbers::pi / 2);
+  Constant *Zero = ConstantFP::get(Ty, 0);
+
+  Value *AtanAddPi = Builder.CreateFAdd(Atan, Pi);
+  Value *AtanSubPi = Builder.CreateFSub(Atan, Pi);
+
+  Value *Result = Atan;
+
+  Value *XLt0 = Builder.CreateFCmpOLT(X, Zero);
----------------
farzonl wrote:

can you add some comments and group the `atan`,  `atan + pi`,  `atan - pi`,   `atan + pi/2`,  and `atan - pi/2` cases.

https://github.com/llvm/llvm-project/pull/108865