[Bug libstdc++/94295] use __builtin_operator_new and __builtin_operator_delete when available

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94295

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Richard S., is there any reason to use the built-ins for the constant
evaluation case? I assume not. Currently std::allocator does:

      [[nodiscard,__gnu__::__always_inline__]]
      constexpr _Tp*
      allocate(size_t __n)
      {
#ifdef __cpp_lib_is_constant_evaluated
        if (std::is_constant_evaluated())
          return static_cast<_Tp*>(::operator new(__n * sizeof(_Tp)));
#endif
        return __allocator_base<_Tp>::allocate(__n, 0);
      }

and my assumption is that there is no reason to change this code, because the
benefits of __builtin_operator_new are only for run-time uses.

The calls to ::operator new in __allocator_base<_Tp>::allocate can use the
built-in though e.g.

--- a/libstdc++-v3/include/ext/new_allocator.h
+++ b/libstdc++-v3/include/ext/new_allocator.h
@@ -97,6 +97,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { return std::__addressof(__x); }
 #endif

+#if __has_builtin(__builtin_operator_new) >= 201802L
+# define _GLIBCXX_OPERATOR_NEW __builtin_operator_new
+# define _GLIBCXX_OPERATOR_DELETE __builtin_operator_delete
+#else
+# define _GLIBCXX_OPERATOR_NEW ::operator new
+# define _GLIBCXX_OPERATOR_DELETE ::operator delete
+#endif
+
       // NB: __n is permitted to be 0.  The C++ standard says nothing
       // about what the return value is when __n == 0.
       _GLIBCXX_NODISCARD _Tp*
@@ -121,34 +129,38 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        if (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
          {
            std::align_val_t __al = std::align_val_t(alignof(_Tp));
-           return static_cast<_Tp*>(::operator new(__n * sizeof(_Tp), __al));
+           return static_cast<_Tp*>(_GLIBCXX_OPERATOR_NEW(__n * sizeof(_Tp),
+                                                          __al));
          }
 #endif
-       return static_cast<_Tp*>(::operator new(__n * sizeof(_Tp)));
+       return static_cast<_Tp*>(_GLIBCXX_OPERATOR_NEW(__n * sizeof(_Tp)));
       }

       // __p is not permitted to be a null pointer.
       void
       deallocate(_Tp* __p, size_type __t __attribute__ ((__unused__)))
       {
+#if __cpp_sized_deallocation
+# define _GLIBCXX_SIZED_DEALLOC(p) p, __t * sizeof(_Tp)
+#else
+# define _GLIBCXX_SIZED_DEALLOC(p) p
+#endif
+
 #if __cpp_aligned_new
        if (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
          {
-           ::operator delete(__p,
-# if __cpp_sized_deallocation
-                             __t * sizeof(_Tp),
-# endif
-                             std::align_val_t(alignof(_Tp)));
+           _GLIBCXX_OPERATOR_DELETE(_GLIBCXX_SIZED_DEALLOC(__p),
+                                    std::align_val_t(alignof(_Tp)));
            return;
          }
 #endif
-       ::operator delete(__p
-#if __cpp_sized_deallocation
-                         , __t * sizeof(_Tp)
-#endif
-                        );
+       _GLIBCXX_OPERATOR_DELETE(_GLIBCXX_SIZED_DEALLOC(__p));
       }

+#undef _GLIBCXX_SIZED_DEALLOC
+#undef _GLIBCXX_OPERATOR_DELETE
+#undef _GLIBCXX_OPERATOR_NEW
+
 #if __cplusplus <= 201703L
       size_type
       max_size() const _GLIBCXX_USE_NOEXCEPT



I see no benefit to using __builtin_operator_new in
std::pmr::new_delete_resource either, because  that will usually be used
through virtual calls to std::pmr::memory_resource::do_allocate, and the actual
call to ::operator new is inside libstdc++.so, not visible to the compiler.