[flang] Lower 2-argument F2008 atan intrinsic

Implement lowering and testing of the atan(y,x) form of the atan
intrinsic. Since the standard specifies that the behavior of this form
is identical to the atan2 intrinsic, everything is lowered to atan2.

In addition to tests for the atan(y,x) intrinsic, tests for the atan(x)
intrinsic were also added/updated.

flang/lib/Optimizer/Builder/IntrinsicCall.cpp

@@ -1059,6 +1059,12 @@ static constexpr MathOperation mathOperations[] = {
     {"atan", "catan", genFuncType<Ty::Complex<8>, Ty::Complex<8>>, genLibCall},
     {"atan", RTNAME_STRING(CAtanF128), FuncTypeComplex16Complex16,
      genLibF128Call},
+    {"atan", "atan2f", genFuncType<Ty::Real<4>, Ty::Real<4>, Ty::Real<4>>,
+     genMathOp<mlir::math::Atan2Op>},
+    {"atan", "atan2", genFuncType<Ty::Real<8>, Ty::Real<8>, Ty::Real<8>>,
+     genMathOp<mlir::math::Atan2Op>},
+    {"atan", RTNAME_STRING(Atan2F128), FuncTypeReal16Real16Real16,
+     genLibF128Call},
     {"atan2", "atan2f", genFuncType<Ty::Real<4>, Ty::Real<4>, Ty::Real<4>>,
      genMathOp<mlir::math::Atan2Op>},
     {"atan2", "atan2", genFuncType<Ty::Real<8>, Ty::Real<8>, Ty::Real<8>>,

flang/test/Lower/Intrinsics/atan.f90

@@ -0,0 +1,55 @@
+! Check that correct runtime calls are used.
+! RUN: bbc --math-runtime=precise -emit-fir %s -o - | FileCheck --check-prefixes=CHECK-RUNTIME %s
+! RUN: %flang_fc1 -mllvm -math-runtime=precise -emit-fir %s -o - | FileCheck --check-prefixes=CHECK-RUNTIME %s
+
+! Check that the correct math dialect operations are used.
+! RUN: bbc -emit-fir %s -o - | FileCheck --check-prefixes=CHECK-NORMAL %s
+! RUN: %flang_fc1 -emit-fir %s -o - | FileCheck --check-prefixes=CHECK-NORMAL %s
+
+function test_real4(x)
+  real :: x, test_real4
+  test_real4 = atan(x)
+end function
+
+! CHECK-RUNTIME: {{%[A-Za-z0-9._]+}} = fir.call @atanf({{.*}}) {{.*}}: (f32) -> f32
+! CHECK-NORMAL: {{%[A-Za-z0-9._]+}} = math.atan {{.*}} {{.*}}: f32
+
+function test_real8(x)
+  real(8) :: x, test_real8
+  test_real8 = atan(x)
+end function
+
+! CHECK-RUNTIME: {{%[A-Za-z0-9._]+}} = fir.call @atan({{.*}}) {{.*}}: (f64) -> f64
+! CHECK-NORMAL: {{%[A-Za-z0-9._]+}} = math.atan {{.*}} {{.*}}: f64
+
+function test_complex4(x)
+  complex :: x, test_complex4
+  test_complex4 = atan(x)
+end function
+
+! CHECK-RUNTIME: {{%[A-Za-z0-9._]+}} = fir.call @catanf({{.*}}) {{.*}}: (!fir.complex<4>) -> !fir.complex<4>
+! CHECK-NORMAL: {{%[A-Za-z0-9._]+}} = fir.call @catanf({{.*}}) {{.*}}: (!fir.complex<4>) -> !fir.complex<4>
+
+function test_complex8(x)
+  complex(kind=8) :: x, test_complex8
+  test_complex8 = atan(x)
+end function
+
+! CHECK-RUNTIME: {{%[A-Za-z0-9._]+}} = fir.call @catan({{.*}}) {{.*}}: (!fir.complex<8>) -> !fir.complex<8>
+! CHECK-NORMAL: {{%[A-Za-z0-9._]+}} = fir.call @catan({{.*}}) {{.*}}: (!fir.complex<8>) -> !fir.complex<8>
+
+function test_real4_2(y, x)
+  real :: y, x, test_real4_2
+  test_real4_2 = atan(y, x)
+end function
+
+! CHECK-RUNTIME: {{%[A-Za-z0-9._]+}} = fir.call @atan2f({{.*}}) {{.*}}: (f32, f32) -> f32
+! CHECK-NORMAL: {{%[A-Za-z0-9._]+}} = math.atan2 {{.*}} {{.*}}: f32
+
+function test_real8_2(y, x)
+  real(8) :: y, x, test_real8_2
+  test_real8_2 = atan(y, x)
+end function
+
+! CHECK-RUNTIME: {{%[A-Za-z0-9._]+}} = fir.call @atan2({{.*}}) {{.*}}: (f64, f64) -> f64
+! CHECK-NORMAL: {{%[A-Za-z0-9._]+}} = math.atan2 {{.*}} {{.*}}: f64

flang/test/Lower/Intrinsics/atan_real16.f90

@@ -3,7 +3,18 @@
 ! RUN: bbc --math-runtime=precise -emit-fir %s -o - | FileCheck %s
 ! RUN: %flang_fc1 -emit-fir %s -o - | FileCheck %s
 
+function test_real16(x)
+  real(16) :: x, test_real16
+  test_real16 = atan(x)
+end function real16
+
+! CHECK-LABEL: @_QPtest_real16
 ! CHECK: fir.call @_FortranAAtanF128({{.*}}){{.*}}: (f128) -> f128
-  real(16) :: a, b
-  b = atan(a)
-end
+
+function test_real16_2(y, x)
+  real(16) :: y, x, test_real16
+  test_real16 = atan(y, x)
+end function real16_2
+
+! CHECK-LABEL: @_QPtest_real16
+! CHECK: fir.call @_FortranAAtan2F128({{.*}}){{.*}}: (f128, f128) -> f128