Guard a feature that unsupported by old GCC

[twoh]

Summary

As @david2050 commented, changes introduced by https://reviews.llvm.org/D56397 break builds for older compilers
which don't support __has(_cpp)_attribute. This is a fix for the break.

[kkwli0]

See https://reviews.llvm.org/D57848. I think we can close either one.

[twoh]

@kkwli0 Thanks for letting me know! But wouldn't https://reviews.llvm.org/D57848 always fall to "else" case added if the compiler is not GCC?

[Hahnfeld]

In D57851#1387935, @twoh wrote:

@kkwli0 Thanks for letting me know! But wouldn't https://reviews.llvm.org/D57848 always fall to "else" case added if the compiler is not GCC?

I agree, Clang defines __GNUC__ to value 4 for compatibility reasons. Likewise, this change doesn't handle compilers other than GCC that don't support __has_cpp_attribute (yet).

I'd propose to take the solution that libcxx, LLVMSupport, and LLVMDemangle use:

#ifndef __has_cpp_attribute
#define __has_cpp_attribute(__x) 0
#endif

Also Kelvin is right that _GNUC_VER is not defined here (it's from libcxx). Maybe we should take the macros from LLVM (except the non-C++ case)? That would also give us a correct checking for gnu::fallthrough, but remove __attribute__((__fallthrough__)) completely.

[kkwli0]

I close https://reviews.llvm.org/D57848 and consolidate the review and patch in this one.

[twoh]

Addressing comments from @Hahnfeld. Thanks!

[Hahnfeld]

Excellent!

openmp/runtime/src/kmp_os.h

 #endif /* CACHE_LINE */
 
 #define KMP_CACHE_PREFETCH(ADDR) /* nothing */
 
 // Define attribute that indicates that the fall through from the previous 
 // case label is intentional and should not be diagnosed by a compiler
 //   Code from libcxx/include/__config
 // Use a function like macro to imply that it must be followed by a semicolon
+#if !KMP_COMPILER_GCC || __GNUC__ >= 5
 #if __cplusplus > 201402L && __has_cpp_attribute(fallthrough)
 #  define KMP_FALLTHROUGH() [[fallthrough]]
 #elif __has_cpp_attribute(clang::fallthrough)
 #  define KMP_FALLTHROUGH() [[clang::fallthrough]]
 #elif __has_attribute(fallthough) || _GNUC_VER >= 700
 #  define KMP_FALLTHROUGH() __attribute__((__fallthrough__))
 #else
 #  define KMP_FALLTHROUGH() ((void)0)
 #endif
+#else // !KMP_COMPILER_GCC || __GNUC__ >= 5
+#define KMP_FALLTHROUGH() ((void)0)
+#endif // !KMP_COMPILER_GCC || __GNUC__ >= 5
 
 // Define attribute that indicates a function does not return
 #if __cplusplus >= 201103L
 #define KMP_NORETURN [[noreturn]]
 #elif KMP_OS_WINDOWS
 #define KMP_NORETURN __declspec(noreturn)
 #else
 #define KMP_NORETURN __attribute__((noreturn))