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[Bug tree-optimization/38835] field-insensitive PTA causes libstdc++ miscompiles
- From: "rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 16 Jan 2009 18:27:35 -0000
- Subject: [Bug tree-optimization/38835] field-insensitive PTA causes libstdc++ miscompiles
- References: <bug-38835-10053@http.gcc.gnu.org/bugzilla/>
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
------- Comment #3 from rguenther at suse dot de 2009-01-16 18:27 -------
Subject: Re: field-insensitive PTA causes
libstdc++ miscompiles
On Fri, 16 Jan 2009, pinskia at gcc dot gnu dot org wrote:
> ------- Comment #2 from pinskia at gcc dot gnu dot org 2009-01-16 18:22 -------
> return reinterpret_cast<_Tp*>(reinterpret_cast<_UIntPtrType>(this)
> + _M_diff);
>
>
> (const struct _Fwd_list_node_base *) this p+ (long unsigned int) ((const
> struct _Relative_pointer_impl *) this)->_M_diff
>
>
> That seems fine. In fact If we are failing because of aliasing issues, then we
> have a bigger issue really because the code could have been written as:
> return reinterpret_cast<_Tp*>(reinterpret_cast<char*>(this) +
> _M_diff);
>
> And still be defined so I don't think this is really PR 36227.
No, char * is not allowed to change objects either. What the above code
does is use the pointer -> intptr -> pointer rule by storing a pointer
as a relative offset to the store, like so:
void *p;
intptr_t iptr = (intptr_t)&iptr - (intptr_t)p;
and reconstructing p via
(void *)((intptr_t)&iptr + iptr);
which is perfectly valid, but we break that because we fold the latter
to (void *)(&iptr + iptr) which is of course invalid.
Richard.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38835