[NFC] Replace HEADER_GLOBAL with inline (C++17).

HEADER_GLOBAL was using non-standard attributes to achieve what C++17
now permits with a keyword.  Use the standard formulation.

Update the README to note that gcc is still not recommended, but because
of its poor codegen for 128-bit atomic compare and exchange, rather than
because it doesn't support the attribute used for HEADER_GLOBAL.

README.md

@@ -50,9 +50,8 @@ To use snmalloc on NetBSD, you must either acquire a `libatomic` implementation
 
 snmalloc has very few dependencies, CMake, Ninja, Clang 6.0 or later and a C++17
 standard library.
-Building with GCC is currently not recommended because GCC lacks support for the
-`selectany` attribute to specify variables in a COMDAT.  
-It will build with GCC-7, but some of global variables will be preemptible.
+Building with GCC is currently not recommended because GCC emits calls to
+libatomic for 128-bit atomic operations.
 
 To build a debug configuration:
 ```

src/ds/defines.h

@@ -3,7 +3,6 @@
 #ifdef _MSC_VER
 #  define ALWAYSINLINE __forceinline
 #  define NOINLINE __declspec(noinline)
-#  define HEADER_GLOBAL __declspec(selectany)
 #  define likely(x) !!(x)
 #  define unlikely(x) !!(x)
 #  define SNMALLOC_SLOW_PATH NOINLINE
@@ -17,13 +16,6 @@
 #  define SNMALLOC_SLOW_PATH NOINLINE
 #  define SNMALLOC_FAST_PATH inline ALWAYSINLINE
 #  define SNMALLOC_PURE __attribute__((const))
-#  ifdef __clang__
-#    define HEADER_GLOBAL __attribute__((selectany))
-#  else
-//  GCC does not support selectany, weak is almost the correct
-//  attribute, but leaves the global variable preemptible.
-#    define HEADER_GLOBAL __attribute__((weak))
-#  endif
 #endif
 
 #ifndef __has_builtin

src/mem/largealloc.h

@@ -405,5 +405,5 @@ namespace snmalloc
    * The memory provider that will be used if no other provider is explicitly
    * passed as an argument.
    */
-  HEADER_GLOBAL GlobalVirtual default_memory_provider;
+  inline GlobalVirtual default_memory_provider;
 } // namespace snmalloc

src/mem/threadalloc.h

@@ -18,7 +18,7 @@ namespace snmalloc
    * replace itself with the thread-local allocator, allocating one if
    * required.  This avoids a branch on the fast path.
    */
-  HEADER_GLOBAL Alloc GlobalPlaceHolder(
+  inline Alloc GlobalPlaceHolder(
     default_memory_provider, SNMALLOC_DEFAULT_PAGEMAP(), nullptr, true);
 
 #ifdef SNMALLOC_EXTERNAL_THREAD_ALLOC

src/pal/pal_windows.h

@@ -231,8 +231,8 @@ namespace snmalloc
     }
 #  endif
   };
-  HEADER_GLOBAL std::atomic<uint64_t> PALWindows::pressure_epoch;
-  HEADER_GLOBAL std::atomic<bool> PALWindows::registered_for_notifications;
-  HEADER_GLOBAL HANDLE PALWindows::lowMemoryObject;
+  inline std::atomic<uint64_t> PALWindows::pressure_epoch;
+  inline std::atomic<bool> PALWindows::registered_for_notifications;
+  inline HANDLE PALWindows::lowMemoryObject;
 }
 #endif