[DirectX] Add atan2 intrinsic and expand for DXIL backend

This change is part of this proposal: https://discourse.llvm.org/t/rfc-all-the-math-intrinsics/78294

This preliminary work adds the intrinsic to llvm and expands using atan intrinsic for DXIL backend, since DXIL has no atan2 op.

llvm/docs/LangRef.rst

@@ -15568,6 +15568,43 @@ trapping or setting ``errno``.
 When specified with the fast-math-flag 'afn', the result may be approximated
 using a less accurate calculation.
 
+'``llvm.atan2.*``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+This is an overloaded intrinsic. You can use ``llvm.atan2`` on any
+floating-point or vector of floating-point type. Not all targets support
+all types however.
+
+::
+
+      declare float     @llvm.atan2.f32(float  %X, float %Y)
+      declare double    @llvm.atan2.f64(double %X, double %Y)
+      declare x86_fp80  @llvm.atan2.f80(x86_fp80  %X, x86_fp80 %Y)
+      declare fp128     @llvm.atan2.f128(fp128 %X, fp128 %Y)
+      declare ppc_fp128 @llvm.atan2.ppcf128(ppc_fp128  %X, ppc_fp128 %Y)
+
+Overview:
+"""""""""
+
+The '``llvm.atan2.*``' intrinsics return the arctangent of the operand.
+
+Arguments:
+""""""""""
+
+The arguments and return value are floating-point numbers of the same type.
+
+Semantics:
+""""""""""
+
+Return the same value as a corresponding libm '``atan2``' function but without
+trapping or setting ``errno``.
+
+When specified with the fast-math-flag 'afn', the result may be approximated
+using a less accurate calculation.
+
 '``llvm.sinh.*``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

llvm/include/llvm/IR/Intrinsics.td

@@ -1016,6 +1016,7 @@ let IntrProperties = [IntrNoMem, IntrSpeculatable, IntrWillReturn] in {
   def int_asin : DefaultAttrsIntrinsic<[llvm_anyfloat_ty], [LLVMMatchType<0>]>;
   def int_acos : DefaultAttrsIntrinsic<[llvm_anyfloat_ty], [LLVMMatchType<0>]>;
   def int_atan : DefaultAttrsIntrinsic<[llvm_anyfloat_ty], [LLVMMatchType<0>]>;
+  def int_atan2 : DefaultAttrsIntrinsic<[llvm_anyfloat_ty], [LLVMMatchType<0>, LLVMMatchType<0>]>;
   def int_sin  : DefaultAttrsIntrinsic<[llvm_anyfloat_ty], [LLVMMatchType<0>]>;
   def int_cos  : DefaultAttrsIntrinsic<[llvm_anyfloat_ty], [LLVMMatchType<0>]>;
   def int_tan  : DefaultAttrsIntrinsic<[llvm_anyfloat_ty], [LLVMMatchType<0>]>;

llvm/lib/Target/DirectX/DXILIntrinsicExpansion.cpp

@@ -35,6 +35,7 @@ using namespace llvm;
 static bool isIntrinsicExpansion(Function &F) {
   switch (F.getIntrinsicID()) {
   case Intrinsic::abs:
+  case Intrinsic::atan2:
   case Intrinsic::exp:
   case Intrinsic::log:
   case Intrinsic::log10:
@@ -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);
+  Value *XEq0 = Builder.CreateFCmpOEQ(X, Zero);
+
+  Value *YGe0 = Builder.CreateFCmpOGE(Y, Zero);
+  Value *YLt0 = Builder.CreateFCmpOLT(Y, Zero);
+
+  Value *XLt0AndYGe0 = Builder.CreateAnd(XLt0, YGe0);
+  Result = Builder.CreateSelect(XLt0AndYGe0, AtanAddPi, Result);
+
+  Value *XLt0AndYLt0 = Builder.CreateAnd(XLt0, YLt0);
+  Result = Builder.CreateSelect(XLt0AndYLt0, AtanSubPi, Result);
+
+  Value *XEq0AndYLt0 = Builder.CreateAnd(XEq0, YLt0);
+  Result = Builder.CreateSelect(XEq0AndYLt0, NegHalfPi, Result);
+
+  Value *XEq0AndYGe0 = Builder.CreateAnd(XEq0, YGe0);
+  Result = Builder.CreateSelect(XEq0AndYGe0, HalfPi, Result);
+
+  return Result;
+}
+
 static Value *expandPowIntrinsic(CallInst *Orig) {
 
   Value *X = Orig->getOperand(0);
@@ -394,6 +437,9 @@ static bool expandIntrinsic(Function &F, CallInst *Orig) {
   case Intrinsic::abs:
     Result = expandAbs(Orig);
     break;
+  case Intrinsic::atan2:
+    Result = expandAtan2Intrinsic(Orig);
+    break;
   case Intrinsic::exp:
     Result = expandExpIntrinsic(Orig);
     break;

llvm/test/CodeGen/DirectX/atan2.ll

@@ -0,0 +1,52 @@
+; RUN: opt -S -dxil-op-lower -mtriple=dxil-pc-shadermodel6.3-library %s | FileCheck %s
+
+; Make sure correct dxil expansions for atan2 are generated for float and half.
+
+define noundef float @atan2_float(float noundef %y, float noundef %x) {
+entry:
+; CHECK: [[DIV:%.+]] = fdiv float %y, %x
+; CHECK: [[TAN:%.+]] = call float @dx.op.unary.f32(i32 17, float [[DIV]])
+; CHECK-DAG: [[ADD_PI:%.+]] = fadd float [[TAN]], 0x400921FB60000000
+; CHECK-DAG: [[SUB_PI:%.+]] = fsub float [[TAN]], 0x400921FB60000000
+; CHECK-DAG: [[X_LT_0:%.+]] = fcmp olt float %x, 0.000000e+00
+; CHECK-DAG: [[X_EQ_0:%.+]] = fcmp oeq float %x, 0.000000e+00 
+; CHECK-DAG: [[Y_GE_0:%.+]] = fcmp oge float %y, 0.000000e+00 
+; CHECK-DAG: [[Y_LT_0:%.+]] = fcmp olt float %y, 0.000000e+00
+; CHECK: [[XLT0_AND_YGE0:%.+]] = and i1 [[X_LT_0]], [[Y_GE_0]]
+; CHECK: [[SELECT_ADD_PI:%.+]] = select i1 [[XLT0_AND_YGE0]], float [[ADD_PI]], float [[TAN]]
+; CHECK: [[XLT0_AND_YLT0:%.+]] = and i1 [[X_LT_0]], [[Y_LT_0]]
+; CHECK: [[SELECT_SUB_PI:%.+]] = select i1 [[XLT0_AND_YLT0]], float [[SUB_PI]], float [[SELECT_ADD_PI]]
+; CHECK: [[XEQ0_AND_YLT0:%.+]] = and i1 [[X_EQ_0]], [[Y_LT_0]]
+; CHECK: [[SELECT_NEGHPI:%.+]] = select i1 [[XEQ0_AND_YLT0]], float 0xBFF921FB60000000, float [[SELECT_SUB_PI]]
+; CHECK: [[XEQ0_AND_YGE0:%.+]] = and i1 [[X_EQ_0]], [[Y_GE_0]]
+; CHECK: [[SELECT_HPI:%.+]] = select i1 [[XEQ0_AND_YGE0]], float 0x3FF921FB60000000, float [[SELECT_NEGHPI]]
+; CHECK: ret float [[SELECT_HPI]]
+  %elt.atan2 = call float @llvm.atan2.f32(float %y, float %x)
+  ret float %elt.atan2
+}
+
+define noundef half @atan2_half(half noundef %y, half noundef %x) {
+entry:
+; CHECK: [[DIV:%.+]] = fdiv half %y, %x
+; CHECK: [[TAN:%.+]] = call half @dx.op.unary.f16(i32 17, half [[DIV]])
+; CHECK-DAG: [[ADD_PI:%.+]] = fadd half [[TAN]], 0xH4248
+; CHECK-DAG: [[SUB_PI:%.+]] = fsub half [[TAN]], 0xH4248
+; CHECK-DAG: [[X_LT_0:%.+]] = fcmp olt half %x, 0xH0000
+; CHECK-DAG: [[X_EQ_0:%.+]] = fcmp oeq half %x, 0xH0000 
+; CHECK-DAG: [[Y_GE_0:%.+]] = fcmp oge half %y, 0xH0000 
+; CHECK-DAG: [[Y_LT_0:%.+]] = fcmp olt half %y, 0xH0000
+; CHECK: [[XLT0_AND_YGE0:%.+]] = and i1 [[X_LT_0]], [[Y_GE_0]]
+; CHECK: [[SELECT_ADD_PI:%.+]] = select i1 [[XLT0_AND_YGE0]], half [[ADD_PI]], half [[TAN]]
+; CHECK: [[XLT0_AND_YLT0:%.+]] = and i1 [[X_LT_0]], [[Y_LT_0]]
+; CHECK: [[SELECT_SUB_PI:%.+]] = select i1 [[XLT0_AND_YLT0]], half [[SUB_PI]], half [[SELECT_ADD_PI]]
+; CHECK: [[XEQ0_AND_YLT0:%.+]] = and i1 [[X_EQ_0]], [[Y_LT_0]]
+; CHECK: [[SELECT_NEGHPI:%.+]] = select i1 [[XEQ0_AND_YLT0]], half 0xHBE48, half [[SELECT_SUB_PI]]
+; CHECK: [[XEQ0_AND_YGE0:%.+]] = and i1 [[X_EQ_0]], [[Y_GE_0]]
+; CHECK: [[SELECT_HPI:%.+]] = select i1 [[XEQ0_AND_YGE0]], half 0xH3E48, half [[SELECT_NEGHPI]]
+; CHECK: ret half [[SELECT_HPI]]
+  %elt.atan2 = call half @llvm.atan2.f16(half %y, half %x)
+  ret half %elt.atan2
+}
+
+declare half @llvm.atan2.f16(half, half)
+declare float @llvm.atan2.f32(float, float)

llvm/test/CodeGen/DirectX/atan2_error.ll

@@ -0,0 +1,11 @@
+; RUN: not opt -S -dxil-op-lower -mtriple=dxil-pc-shadermodel6.3-library %s 2>&1 | FileCheck %s
+
+; DXIL operation atan does not support double overload type
+; CHECK: in function atan2_double
+; CHECK-SAME: Cannot create ATan operation: Invalid overload type
+
+define noundef double @atan2_double(double noundef %a, double noundef %b) #0 {
+entry:
+  %1 = call double @llvm.atan2.f64(double %a, double %b)
+  ret double %1
+}