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libstdc++: Avoid negating a size_t [pr 94747] 

Although the code here is well formed, it doesn't show intent well.
The reason checkers trigger on this is that it is a cause of real
bugs.  So, negate a ptrdiff_t instead.

	* libsupc++/dyncast.cc (__dynamic_cast): Cast offsetof to
	ptrdiff_t before negation, to show intent more clearly.
This commit is contained in:
Nathan Sidwell 2020-05-04 10:06:40 -07:00
parent bb6ce54220
commit e6b31fc717
2 changed files with 12 additions and 5 deletions
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6
libstdc++-v3/ChangeLog
View File

@ -1,3 +1,9 @@
2020-05-04 Nathan Sidwell <nathan@acm.org>
PR libstdc++/94747
* libsupc++/dyncast.cc (__dynamic_cast): Cast offsetof to
ptrdiff_t before negation, to show intent more clearly.
2020-05-04 Jonathan Wakely <jwakely@redhat.com>
PR libstdc++/94936

11
libstdc++-v3/libsupc++/dyncast.cc
View File

@ -49,8 +49,8 @@ __dynamic_cast (const void *src_ptr, // object started from
{
const void *vtable = *static_cast <const void *const *> (src_ptr);
const vtable_prefix *prefix =
adjust_pointer <vtable_prefix> (vtable,
-offsetof (vtable_prefix, origin));
(adjust_pointer <vtable_prefix>
(vtable, -ptrdiff_t (offsetof (vtable_prefix, origin))));
const void *whole_ptr =
adjust_pointer <void> (src_ptr, prefix->whole_object);
const __class_type_info *whole_type = prefix->whole_type;
@ -63,8 +63,8 @@ __dynamic_cast (const void *src_ptr, // object started from
// segfault later trying to use a vbase offset that doesn't exist.
const void *whole_vtable = *static_cast <const void *const *> (whole_ptr);
const vtable_prefix *whole_prefix =
adjust_pointer <vtable_prefix> (whole_vtable,
-offsetof (vtable_prefix, origin));
(adjust_pointer <vtable_prefix>
(whole_vtable, -ptrdiff_t (offsetof (vtable_prefix, origin))));
if (whole_prefix->whole_type != whole_type)
return NULL;
@ -75,7 +75,8 @@ __dynamic_cast (const void *src_ptr, // object started from
if (contained_public_p (result.dst2src))
// Src is known to be a public base of dst.
return const_cast <void *> (result.dst_ptr);
if (contained_public_p (__class_type_info::__sub_kind (result.whole2src & result.whole2dst)))
if (contained_public_p (__class_type_info::__sub_kind
(result.whole2src & result.whole2dst)))
// Both src and dst are known to be public bases of whole. Found a valid
// cross cast.
return const_cast <void *> (result.dst_ptr);