[libcxx-commits] [libcxx] [libc++] Inline fast path for`exception_ptr` copy constructor & destructor (PR #165909)

[Nikolas Klauser via libcxx-commits]

================
@@ -30,6 +30,28 @@ _LIBCPP_PUSH_MACROS
 
 #ifndef _LIBCPP_ABI_MICROSOFT
 
+// Previously, parts of exception_ptr were defined out-of-line, which prevented
+// useful compiler optimizations. Changing the out-of-line definitions to inline
+// definitions is an ABI break, however. To prevent this, we have to make sure
+// the symbols remain available in the libc++ library, in addition to being
+// defined inline here in this header.
+// To this end, we use _LIBCPP_EXPORTED_FROM_LIB_INLINEABLE macro:
+// The macro is defined as empty for src/exception.cpp, forcing the definitions of
+// the functions to be emitted and included in the library. When users of libc++
+// compile their code, the __gnu_inline__ attribute will suppress generation of
+// these functions while making their definitions available for inlining.
+#  ifdef _LIBCPP_EMIT_CODE_FOR_EXCEPTION_PTR
+#    define _LIBCPP_EXPORTED_FROM_LIB_INLINEABLE _LIBCPP_EXPORTED_FROM_ABI
+#  else
+#    if !__has_cpp_attribute(__gnu__::__gnu_inline__)
+#      error "GNU inline attribute is not supported"
+#    endif
+#    define _LIBCPP_EXPORTED_FROM_LIB_INLINEABLE [[__gnu__::__gnu_inline__]] inline
+#  endif
----------------
philnik777 wrote:

I think it would be simpler to read if you just do
```c++
#ifdef _LIBCPP_BUILDING_LIBRARY
  exception_ptr(const exception_ptr&);
  ...
#else
  _LIBCPP_HIDE_FROM_ABI exception_ptr(const exception_ptr& __other) { ... }
#endif
```

https://github.com/llvm/llvm-project/pull/165909