[libc++] Use init_priority(100) when possible

[ldionne]

Summary

Priorities below 101 are reserved for the implementation, so that's what
we should be using here. That is unfortunately only supported on more
recent versions of Clang. See https://reviews.llvm.org/D31413 for details.

[aaron.ballman]

LGTM!

[ldionne]

Try to fix the build by pretending we're in a system header.

[ldionne]

libcxx/src/experimental/memory_resource.cpp

79

@arthur.j.odwyer Pinging you here since you wrote that code. Does this LGTY?

[• Quuxplusone]

libcxx/src/experimental/memory_resource.cpp

79

@ldionne: I just copied this line from experimental/... oh wait, this is experimental/... yeah, this was added by @EricWF in https://github.com/llvm/llvm-project/commit/4efaa30934e81 . IIUC, if we're sure this file is always compiled as C++14 at this point, then I don't even think it matters what we put here. In C++20 we should mark this variable constinit.

However, whatever you decide on, it'd be useful if you could add a review comment to D89057 .

[ldionne]

Poke CI, this should be good to go.

[ldionne]

Fix oversight in memory_resource.cpp

[mstorsjo]

CC @thakis

FYI, this change added an inconvenience in builds with clang-cl; the dummy preprocessor line directives (e.g. # 80 "iostream.cpp" 1 3) end up output as part of /showIncludes, which is what Ninja (and other tools) use for gathering info about the dependencies.

The command output contains this:

Note: including file:  C:/<builddir>/include/c++/v1\__undef_macros
[... more of the same ...]
Note: including file: iostream.cpp

The dependency on iostream.cpp is a dummy (the input file isn't found). This causes ninja builds to always rebuild iostream.cpp every time ninja is invoked, even when nothing has been touched. Running with ninja -d explain prints:

ninja explain: output iostream.cpp of phony edge with no inputs doesn't exist
ninja explain: iostream.cpp is dirty
ninja explain: src/CMakeFiles/cxx_shared.dir/iostream.cpp.obj is dirty

But I'm not sure if this is inconvenience enough to do something about it (I guess alternatives is to either use a windows specific mechanism directly, for setting a high enough priority for the constructors, or to just use the regular priority 101, so it doesn't need the dummy source markers).

[ldionne]

In D95972#3178968, @mstorsjo wrote:

CC @thakis

FYI, this change added an inconvenience in builds with clang-cl; the dummy preprocessor line directives (e.g. # 80 "iostream.cpp" 1 3) end up output as part of /showIncludes, which is what Ninja (and other tools) use for gathering info about the dependencies.

The command output contains this:

Note: including file:  C:/<builddir>/include/c++/v1\__undef_macros
[... more of the same ...]
Note: including file: iostream.cpp

The dependency on iostream.cpp is a dummy (the input file isn't found). This causes ninja builds to always rebuild iostream.cpp every time ninja is invoked, even when nothing has been touched. Running with ninja -d explain prints:

ninja explain: output iostream.cpp of phony edge with no inputs doesn't exist
ninja explain: iostream.cpp is dirty
ninja explain: src/CMakeFiles/cxx_shared.dir/iostream.cpp.obj is dirty

But I'm not sure if this is inconvenience enough to do something about it (I guess alternatives is to either use a windows specific mechanism directly, for setting a high enough priority for the constructors, or to just use the regular priority 101, so it doesn't need the dummy source markers).

FWIW, this workaround has always felt like a hack to me. Now that I think of it, perhaps we can push/pop the system header pragma around it instead?

libcxx/include/__config

 // Support for _FILE_OFFSET_BITS=64 landed gradually in Android, so the full set
 // of functions used in cstdio may not be available for low API levels when
 // using 64-bit file offsets on LP32.
 #if defined(__BIONIC__) && defined(__USE_FILE_OFFSET64) && __ANDROID_API__ < 24
 #define _LIBCPP_HAS_NO_FGETPOS_FSETPOS
 #endif
 
 #if __has_attribute(init_priority)
-# define _LIBCPP_INIT_PRIORITY_MAX __attribute__((init_priority(101)))
-#else
+    // TODO: Remove this once we drop support for building libc++ with old Clangs
+#   if (defined(_LIBCPP_CLANG_VER) && _LIBCPP_CLANG_VER < 1200) || \
+       (defined(__apple_build_version__) && __apple_build_version__ < 13000000)
+#     define _LIBCPP_INIT_PRIORITY_MAX __attribute__((init_priority(101)))
+#   else
+#     define _LIBCPP_INIT_PRIORITY_MAX __attribute__((init_priority(100)))
+#   endif
+#else
 # define _LIBCPP_INIT_PRIORITY_MAX
 #endif
 
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG

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() {}
 };
 
-// When compiled in C++14 this initialization should be a constant expression.
-// Only in C++11 is "init_priority" needed to ensure initialization order.
-#if _LIBCPP_STD_VER > 11
-_LIBCPP_SAFE_STATIC
-#endif
-ResourceInitHelper res_init _LIBCPP_INIT_PRIORITY_MAX;
+_LIBCPP_SAFE_STATIC ResourceInitHelper res_init;
 
 } // end namespace
 
 
 memory_resource * new_delete_resource() _NOEXCEPT {
     return &res_init.resources.new_delete_res;
 }
 

libcxx/src/iostream.cpp

 #endif
 ;
 _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
 ;
 
+// 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
 
 // 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);