[OpenMP] Allow <math.h> to go first in C++-mode in target regions

[jdoerfert]

Summary

If we are in C++ mode and include <math.h> (not <cmath>) first, we still
need to make sure <cmath> is read first. The problem otherwise is that
we haven't seen the declarations of the math.h functions when the system
math.h includes our cmath overlay. However, our cmath overlay, or better
the underlying overlay, e.g. CUDA, uses the math.h functions. Since we
haven't declared them yet we get errors. CUDA avoids this by eagerly
declaring all math functions (in the device space) but we cannot do
this. Instead we break the dependence by forcing cmath to go first.

[JonChesterfield]

The cmath/math.h story makes me sad, but this is a good workaround. Thanks

clang/lib/Headers/openmp_wrappers/math.h

 /*===---- openmp_wrapper/math.h -------- OpenMP math.h intercept ------ c++ -===
  *
  * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  * See https://llvm.org/LICENSE.txt for license information.
  * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
  *
  *===-----------------------------------------------------------------------===
  */
 
+// If we are in C++ mode and include <math.h> (not <cmath>) first, we still need
+// to make sure <cmath> is read first. The problem otherwise is that we haven't
+// seen the declarations of the math.h functions when the system math.h includes
+// our cmath overlay. However, our cmath overlay, or better the underlying
+// overlay, e.g. CUDA, uses the math.h functions. Since we haven't declared them
+// yet we get errors. CUDA avoids this by eagerly declaring all math functions
+// (in the __device__ space) but we cannot do this. Instead we break the
+// dependence by forcing cmath to go first. While our cmath will in turn include
+// this file, the cmath guards will prevent recursion.
+#ifdef __cplusplus
+#include <cmath>
+#endif
+
 #ifndef __CLANG_OPENMP_MATH_H__
 #define __CLANG_OPENMP_MATH_H__
 
 #ifndef _OPENMP
 #error "This file is for OpenMP compilation only."
 #endif
 
 #include_next <math.h>

clang/test/Headers/Inputs/include/math.h

 #define HUGE_VALF (__builtin_huge_valf())
 
 /**
  * A positive double constant expression. HUGE_VAL evaluates
  * to +infinity. Used as an error value returned by the built-in
  * math functions.
  */
 #define HUGE_VAL (__builtin_huge_val())
+
+#ifdef __cplusplus
+#include <cmath>
+#endif

clang/test/Headers/nvptx_device_math_sincos.cpp

 // REQUIRES: nvptx-registered-target
 // RUN: %clang_cc1 -internal-isystem %S/Inputs/include -fopenmp -triple powerpc64le-unknown-unknown -fopenmp-targets=nvptx64-nvidia-cuda -emit-llvm-bc %s -o %t-ppc-host.bc
-// RUN: %clang_cc1 -internal-isystem %S/../../lib/Headers/openmp_wrappers -include __clang_openmp_device_functions.h -internal-isystem %S/Inputs/include -fopenmp -triple nvptx64-nvidia-cuda -aux-triple powerpc64le-unknown-unknown -fopenmp-targets=nvptx64-nvidia-cuda -emit-llvm %s -fopenmp-is-device -fopenmp-host-ir-file-path %t-ppc-host.bc -o - | FileCheck %s
+// RUN: %clang_cc1 -internal-isystem %S/../../lib/Headers/openmp_wrappers         -include __clang_openmp_device_functions.h -internal-isystem %S/Inputs/include -fopenmp -triple nvptx64-nvidia-cuda -aux-triple powerpc64le-unknown-unknown -fopenmp-targets=nvptx64-nvidia-cuda -emit-llvm %s -fopenmp-is-device -fopenmp-host-ir-file-path %t-ppc-host.bc -o - | FileCheck %s
+// RUN: %clang_cc1 -internal-isystem %S/../../lib/Headers/openmp_wrappers -DCMATH -include __clang_openmp_device_functions.h -internal-isystem %S/Inputs/include -fopenmp -triple nvptx64-nvidia-cuda -aux-triple powerpc64le-unknown-unknown -fopenmp-targets=nvptx64-nvidia-cuda -emit-llvm %s -fopenmp-is-device -fopenmp-host-ir-file-path %t-ppc-host.bc -o - | FileCheck %s
 
+#ifdef CMATH
 #include <cmath>
+#else
+#include <math.h>
+#endif
 
 // 4 calls to sincos(f), all translated to __nv_sincos calls:
 
 // CHECK-NOT: _Z.sincos
 // CHECK: call void @__nv_sincos(double
 // CHECK-NOT: _Z.sincos
 // CHECK: call void @__nv_sincosf(float
 // CHECK-NOT: _Z.sincos