[cmake] Improve pthread_[gs]etname_np detection code

[labath]

Summary

Due to some android peculiarities, in some build configurations
(statically linked executables targeting older release) we could detect
the presence of these functions (because they are present in libc.a,
where check_library_exists searches), but then fail to build because the
headers did not include the definition.

This attempts to remedy that by upgrading the check_library_exists to
check_symbol_exists, which will check that the function is declared too.

I am hoping that a more thorough check will make the messy #ifdef we
have accumulated in the code obsolete, so I optimistically try to remove
it.

[krytarowski]

Just a note that FreeBSD calls its function pthread_set_name_np.

NetBSD has pthread_getname_np() and pthread_setname_np() like Linux, but the setname variant has a different prototype:

int
pthread_setname_np(pthread_t thread, const char *name, void *arg);

[krytarowski]

FreeBSD also uses pthread_np.h header.

[labath]

Yeah, so far these checks are only actually used on linux. Do you need them on Free/NetBSD or can we just assume they will always be there?

[krytarowski]

This patch looks fine, not sure if we can improve it.

cmake/config-ix.cmake

 check_symbol_exists(__GLIBC__ stdio.h LLVM_USING_GLIBC)
 if( LLVM_USING_GLIBC )
   add_definitions( -D_GNU_SOURCE )
   list(APPEND CMAKE_REQUIRED_DEFINITIONS "-D_GNU_SOURCE")
 endif()
 # This check requires _GNU_SOURCE
 check_symbol_exists(sched_getaffinity sched.h HAVE_SCHED_GETAFFINITY)
 check_symbol_exists(CPU_COUNT sched.h HAVE_CPU_COUNT)
-if(HAVE_LIBPTHREAD)
-  check_library_exists(pthread pthread_getname_np "" HAVE_PTHREAD_GETNAME_NP)
-  check_library_exists(pthread pthread_setname_np "" HAVE_PTHREAD_SETNAME_NP)
-elseif(PTHREAD_IN_LIBC)
-  check_library_exists(c pthread_getname_np "" HAVE_PTHREAD_GETNAME_NP)
-  check_library_exists(c pthread_setname_np "" HAVE_PTHREAD_SETNAME_NP)
+if (NOT PURE_WINDOWS)
+  if (LLVM_PTHREAD_LIB)
+    list(APPEND CMAKE_REQUIRED_LIBRARIES ${LLVM_PTHREAD_LIB})
+  endif()
+  check_symbol_exists(pthread_getname_np pthread.h HAVE_PTHREAD_GETNAME_NP)
+  check_symbol_exists(pthread_setname_np pthread.h HAVE_PTHREAD_SETNAME_NP)
+  if (LLVM_PTHREAD_LIB)
+    list(REMOVE_ITEM CMAKE_REQUIRED_LIBRARIES ${LLVM_PTHREAD_LIB})
+  endif()
 endif()
 
 # available programs checks
 function(llvm_find_program name)
   string(TOUPPER ${name} NAME)
   string(REGEX REPLACE "\\." "_" NAME ${NAME})
 
   find_program(LLVM_PATH_${NAME} NAMES ${ARGV})

lib/Support/Unix/Threading.inc

#if defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
   return;
 #elif defined(__NetBSD__)
   constexpr uint32_t len = get_max_thread_name_length_impl();
   char buf[len];
   ::pthread_getname_np(::pthread_self(), buf, len);
 
   Name.append(buf, buf + strlen(buf));
 #elif defined(__linux__)
-#if (defined(__GLIBC__) && defined(_GNU_SOURCE)) || defined(__ANDROID__)
 #if HAVE_PTHREAD_GETNAME_NP
   constexpr uint32_t len = get_max_thread_name_length_impl();
   char Buffer[len] = {'\0'};  // FIXME: working around MSan false positive.
   if (0 == ::pthread_getname_np(::pthread_self(), Buffer, len))
     Name.append(Buffer, Buffer + strlen(Buffer));
 #endif
 #endif
-#endif
 }