libc: Use slow path in fenv in C++

C++ exposes cfenv functions via using ::func. Our name-mangling
mechanism rewrites all function calls causing symbols such as
std::feclearexcept to be transformed into std::__feclearexcept_int.
Since no such function exists, compilation fails.

The using ::feclearexpect declarations themselves are unaffected because
they are not function calls, which further exposes the mismatch

As a result, enable the fast path only for C and fall back to the slow
path in C++.

Reviewed by:    kib
Fixes:          5bc64b7d41
MFC after:      2 weeks
Sponsored by:   The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57450

lib/msun/aarch64/fenv.h

@@ -96,6 +96,15 @@ int feenableexcept(int);
 int fedisableexcept(int);
 int fegetexcept(void);
 
+/*
+ * C permits a standard library function to also be exposed as a function-like
+ * macro (C23 7.1.4), and msun uses that here to inline the fast path.  C++
+ * forbids it: <cfenv> imports these names into namespace std (using
+ * ::feclearexcept; etc.), so std::feclearexcept() and friends must denote the
+ * actual functions.  Expose the inlining macros to C only; C++ uses the real
+ * extern functions (defined in the matching lib/msun/<arch>/fenv.c).
+ */
+#ifndef __cplusplus
 #define	feclearexcept(a)	__feclearexcept_int(a)
 #define	fegetexceptflag(e, a)	__fegetexceptflag_int(e, a)
 #define	fesetexceptflag(e, a)	__fesetexceptflag_int(e, a)
@@ -110,6 +119,7 @@ int fegetexcept(void);
 #define	feenableexcept(a)	__feenableexcept_int(a)
 #define	fedisableexcept(a)	__fedisableexcept_int(a)
 #define	fegetexcept()		__fegetexcept_int()
+#endif /* !__cplusplus */
 
 __fenv_static inline int
 __feclearexcept_int(int __excepts)

lib/msun/arm/fenv.h

@@ -129,6 +129,15 @@ int fedisableexcept(int);
 int fegetexcept(void);
 #endif
 
+/*
+ * C permits a standard library function to also be exposed as a function-like
+ * macro (C23 7.1.4), and msun uses that here to inline the fast path.  C++
+ * forbids it: <cfenv> imports these names into namespace std (using
+ * ::feclearexcept; etc.), so std::feclearexcept() and friends must denote the
+ * actual functions.  Expose the inlining macros to C only; C++ uses the real
+ * extern functions (defined in the matching lib/msun/<arch>/fenv.c).
+ */
+#ifndef __cplusplus
 #define	feclearexcept(a)	__feclearexcept_int(a)
 #define	fegetexceptflag(e, a)	__fegetexceptflag_int(e, a)
 #define	fesetexceptflag(e, a)	__fesetexceptflag_int(e, a)
@@ -145,6 +154,7 @@ int fegetexcept(void);
 #define	fedisableexcept(a)	__fedisableexcept_int(a)
 #define	fegetexcept()		__fegetexcept_int()
 #endif
+#endif /* !__cplusplus */
 
 __fenv_static inline int
 __feclearexcept_int(int __excepts)

lib/msun/powerpc/fenv.h

@@ -123,6 +123,15 @@ int feholdexcept(fenv_t *);
 int fesetenv(const fenv_t *);
 int feupdateenv(const fenv_t *);
 
+/*
+ * C permits a standard library function to also be exposed as a function-like
+ * macro (C23 7.1.4), and msun uses that here to inline the fast path.  C++
+ * forbids it: <cfenv> imports these names into namespace std (using
+ * ::feclearexcept; etc.), so std::feclearexcept() and friends must denote the
+ * actual functions.  Expose the inlining macros to C only; C++ uses the real
+ * extern functions (defined in the matching lib/msun/<arch>/fenv.c).
+ */
+#ifndef __cplusplus
 #define	feclearexcept(a)	__feclearexcept_int(a)
 #define	fegetexceptflag(e, a)	__fegetexceptflag_int(e, a)
 #define	fesetexceptflag(e, a)	__fesetexceptflag_int(e, a)
@@ -134,6 +143,7 @@ int feupdateenv(const fenv_t *);
 #define	feholdexcept(e)		__feholdexcept_int(e)
 #define	fesetenv(e)		__fesetenv_int(e)
 #define	feupdateenv(e)		__feupdateenv_int(e)
+#endif /* !__cplusplus */
 
 __fenv_static inline int
 __feclearexcept_int(int __excepts)
@@ -266,8 +276,10 @@ __feupdateenv_int(const fenv_t *__envp)
 int feenableexcept(int);
 int fedisableexcept(int);
 
+#ifndef __cplusplus	/* see the note above; C++ uses the real functions */
 #define	feenableexcept(a)	__feenableexcept_int(a)
 #define	fedisableexcept(a)	__fedisableexcept_int(a)
+#endif
 
 __fenv_static inline int
 __feenableexcept_int(int __mask)

lib/msun/riscv/fenv.h

@@ -91,6 +91,15 @@ int feholdexcept(fenv_t *);
 int fesetenv(const fenv_t *);
 int feupdateenv(const fenv_t *);
 
+/*
+ * C permits a standard library function to also be exposed as a function-like
+ * macro (C23 7.1.4), and msun uses that here to inline the fast path.  C++
+ * forbids it: <cfenv> imports these names into namespace std (using
+ * ::feclearexcept; etc.), so std::feclearexcept() and friends must denote the
+ * actual functions.  Expose the inlining macros to C only; C++ uses the real
+ * extern functions (defined in the matching lib/msun/<arch>/fenv.c).
+ */
+#ifndef __cplusplus
 #define	feclearexcept(a)	__feclearexcept_int(a)
 #define	fegetexceptflag(e, a)	__fegetexceptflag_int(e, a)
 #define	fesetexceptflag(e, a)	__fesetexceptflag_int(e, a)
@@ -102,6 +111,7 @@ int feupdateenv(const fenv_t *);
 #define	feholdexcept(e)		__feholdexcept_int(e)
 #define	fesetenv(e)		__fesetenv_int(e)
 #define	feupdateenv(e)		__feupdateenv_int(e)
+#endif /* !__cplusplus */
 
 __fenv_static inline int
 __feclearexcept_int(int __excepts)
@@ -224,8 +234,10 @@ __feupdateenv_int(const fenv_t *__envp)
 int feenableexcept(int);
 int fedisableexcept(int);
 
+#ifndef __cplusplus	/* see the note above; C++ uses the real functions */
 #define	feenableexcept(a)	__feenableexcept_int(a)
 #define	fedisableexcept(a)	__fedisableexcept_int(a)
+#endif
 
 __fenv_static inline int
 __feenableexcept_int(int __mask __unused)

lib/msun/x86/fenv.h

@@ -150,12 +150,22 @@ int fesetround(int);
 int fegetround(void);
 int fesetenv(const fenv_t *);
 
+/*
+ * C permits a standard library function to also be exposed as a function-like
+ * macro (C23 7.1.4), and msun uses that here to inline the fast path.  C++
+ * forbids it: <cfenv> imports these names into namespace std (using
+ * ::feclearexcept; etc.), so std::feclearexcept() and friends must denote the
+ * actual functions.  Expose the inlining macros to C only; C++ uses the real
+ * extern functions (defined in the matching lib/msun/<arch>/fenv.c).
+ */
+#ifndef __cplusplus
 #define	feclearexcept(a)	__feclearexcept_int(a)
 #define	fegetexceptflag(e, a)	__fegetexceptflag_int(e, a)
 #define	fetestexcept(a)		__fetestexcept_int(a)
 #define	fesetround(a)		__fesetround_int(a)
 #define	fegetround()		__fegetround_int()
 #define	fesetenv(a)		__fesetenv_int(a)
+#endif /* !__cplusplus */
 
 #ifdef __i386__