[libc++] Change workaround for init_priority((101)) outside of system headers

[ldionne]

Summary

We had previously been using a different workaround for pretending that
we were inside a system header, however it had some undesirable effects
on dependency parsing for build systems, as explained in [1].

This patch changes the workaround to use #pragma GCC system_header,
which shouldn't suffer from the same issue. Unfortunately, it is a lot
more verbose. The issue is that #pragma GCC system_header is ignored
when we are inside a source file, so we have to create a header just for
the sake of using it. IMO this seems like an artificial restriction
without much benefit, but investigating that is a different story.
For now, this should at least solve build system problems at the
cost of some readability.

[1]: https://reviews.llvm.org/D95972#3178968

[mstorsjo]

Thanks, this does seem to fix the issue. A small nit about the commit message though - it’s priority 100 that requires the workaround, 101 is the one that can be used freely.

[ldionne]

In D115334#3180002, @mstorsjo wrote:

Thanks, this does seem to fix the issue. A small nit about the commit message though - it’s priority 100 that requires the workaround, 101 is the one that can be used freely.

Thanks, will fix when landing.

libcxx/src/chrono.cpp

GetSystemTimeInit() {
     fp = (GetSystemTimeAsFileTimePtr)GetProcAddress(
         GetModuleHandleW(L"kernel32.dll"), "GetSystemTimePreciseAsFileTime");
     if (fp == nullptr)
       fp = GetSystemTimeAsFileTime;
   }
   GetSystemTimeAsFileTimePtr fp;
 };
 
-# 83 "chrono.cpp" 1 3
-GetSystemTimeInit GetSystemTimeAsFileTimeFunc _LIBCPP_INIT_PRIORITY_MAX;
-# 85 "chrono.cpp" 2
+// Pretend we're inside a system header so the compiler doesn't flag the use of the init_priority
+// attribute with a value that's reserved for the implementation (we're the implementation).
+#include "chrono_system_time_init.h"
 } // namespace
 
 #endif
 
 static system_clock::time_point __libcpp_system_clock_now() {
   // FILETIME is in 100ns units
   using filetime_duration =
       _VSTD::chrono::duration<__int64,

libcxx/src/chrono_system_time_init.h

+#pragma GCC system_header
+GetSystemTimeInit GetSystemTimeAsFileTimeFunc _LIBCPP_INIT_PRIORITY_MAX;
 No newline at end of file

libcxx/src/experimental/memory_resource.cpp

struct {
     __new_delete_memory_resource_imp new_delete_res;
     __null_memory_resource_imp       null_res;
   } resources;
   char dummy;
   _LIBCPP_CONSTEXPR_AFTER_CXX11 ResourceInitHelper() : resources() {}
   ~ResourceInitHelper() {}
 };
 
-# 79 "memory_resource.cpp" 1 3
-_LIBCPP_SAFE_STATIC ResourceInitHelper res_init _LIBCPP_INIT_PRIORITY_MAX;
-# 81 "memory_resource.cpp" 2
+// Pretend we're inside a system header so the compiler doesn't flag the use of the init_priority
+// attribute with a value that's reserved for the implementation (we're the implementation).
+#include "memory_resource_init_helper.h"
 
 } // end namespace
 
 
 memory_resource * new_delete_resource() noexcept {
     return &res_init.resources.new_delete_res;
 }
 

libcxx/src/experimental/memory_resource_init_helper.h

+#pragma GCC system_header
+_LIBCPP_SAFE_STATIC ResourceInitHelper res_init _LIBCPP_INIT_PRIORITY_MAX;
 No newline at end of file

libcxx/src/iostream.cpp

 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 _ALIGNAS_TYPE (wostream) _LIBCPP_FUNC_VIS char wclog[sizeof(wostream)]
 #if defined(_LIBCPP_ABI_MICROSOFT) && defined(__clang__)
 __asm__("?wclog@" _LIBCPP_ABI_NAMESPACE_STR "@std@@3V?$basic_ostream@_WU?$char_traits@_W@" _LIBCPP_ABI_NAMESPACE_STR "@std@@@12@A")
 #endif
 ;
 #endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
 
-// Hacky way to make the compiler believe that we're inside a system header so
-// it doesn't flag the use of the init_priority attribute with a value that's
-// reserved for the implementation (we're the implementation).
-# 80 "iostream.cpp" 1 3
-_LIBCPP_HIDDEN ios_base::Init __start_std_streams _LIBCPP_INIT_PRIORITY_MAX;
-# 82 "iostream.cpp" 2
+// Pretend we're inside a system header so the compiler doesn't flag the use of the init_priority
+// attribute with a value that's reserved for the implementation (we're the implementation).
+#include "iostream_init.h"
 
 // On Windows the TLS storage for locales needs to be initialized before we create
 // the standard streams, otherwise it may not be alive during program termination
 // when we flush the streams.
 static void force_locale_initialization() {
 #if defined(_LIBCPP_MSVCRT_LIKE)
   static bool once = []() {
     auto loc = newlocale(LC_ALL_MASK, "C", 0);

libcxx/src/iostream_init.h

+#pragma GCC system_header
+_LIBCPP_HIDDEN ios_base::Init __start_std_streams _LIBCPP_INIT_PRIORITY_MAX;
 No newline at end of file