[PATCH] libstdc++: Use __builtin_operator_new when available [PR94295]

[Jonathan Wakely]

Clang provides __builtin_operator_new and __builtin_operator_delete,
which have the same semantics as ::operator new and ::operator delete
except that the compiler is allowed to elide calls to them. This changes
std::allocator to use those built-in functions so that memory allocated
by std::allocator can be optimized away when using Clang. This avoids an
abstraction penalty for using std::allocator to allocate storage rather
than a new-expression.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/94295
	* include/ext/new_allocator.h (_GLIBCXX_OPERATOR_NEW)
	(_GLIBCXX_OPERATOR_DELETE, _GLIBCXX_SIZED_DEALLOC): Define.
	(allocator::allocate, allocator::deallocate): Use new macros.

Tested powerpc64le-linux.

The macros are ugly, but using #ifdef is unavoidable here. I think
this is less ugly than the laternative ways to do this.

Any objections to pushing this, or any better ideas for doing this?


-------------- next part --------------
commit f8c86589f2eab42c0888390bea7bceb8dc31d92b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Jul 19 16:43:11 2021

    libstdc++: Use __builtin_operator_new when available [PR94295]
    
    Clang provides __builtin_operator_new and __builtin_operator_delete,
    which have the same semantics as ::operator new and ::operator delete
    except that the compiler is allowed to elide calls to them. This changes
    std::allocator to use those built-in functions so that memory allocated
    by std::allocator can be optimized away when using Clang. This avoids an
    abstraction penalty for using std::allocator to allocate storage rather
    than a new-expression.
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/94295
            * include/ext/new_allocator.h (_GLIBCXX_OPERATOR_NEW)
            (_GLIBCXX_OPERATOR_DELETE, _GLIBCXX_SIZED_DEALLOC): Define.
            (allocator::allocate, allocator::deallocate): Use new macros.

diff --git a/libstdc++-v3/include/ext/new_allocator.h b/libstdc++-v3/include/ext/new_allocator.h
index 3fb893be152..7c48c820c62 100644
--- 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__)))
+      deallocate(_Tp* __p, size_type __n __attribute__ ((__unused__)))
       {
+#if __cpp_sized_deallocation
+# define _GLIBCXX_SIZED_DEALLOC(p, n) (p), (n) * sizeof(_Tp)
+#else
+# define _GLIBCXX_SIZED_DEALLOC(p, n) (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, __n),
+				     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, __n));
       }
 
+#undef _GLIBCXX_SIZED_DEALLOC
+#undef _GLIBCXX_OPERATOR_DELETE
+#undef _GLIBCXX_OPERATOR_NEW
+
 #if __cplusplus <= 201703L
       size_type
       max_size() const _GLIBCXX_USE_NOEXCEPT