Re: malloc attributes and realloc

[Geoff Keating <geoffk at geoffk dot org>]

"Kaveh R. Ghazi" <ghazi@caip.rutgers.edu> writes:

>  > From: "Joseph S. Myers" <jsm@polyomino.org.uk>
>  > 
>  > On Fri, 2 Jan 2004, Kaveh R. Ghazi wrote:
>  > 
>  > > Hum, that is surprising.  If we can't rely on pointer ordering, then
>  > > libiberty/alloca.c wouldn't work since it uses exactly this ordering
>  > > to detect the stack growth direction and also to detect when storage
>  > > can be freed.
>  > 
>  > We clearly ought to document exactly what extensions to pointers we
>  > support beyond what the C standard specifies.
> 
> If by "we support" you mean "GCC supports", my gist goes beyond that.
> 
> The pointer comparison code in alloca.c is the way we support the
> alloca() function for NON-gcc compilers.  Therefore pointer comparison
> works on _every_ stage1 compiler used to bootstrap gcc going back to
> the oldest traditional C compiler up until today's ISO C.  That's a
> lot of compilers supporting this so-called extension which is why
> (other than for segmented arches) I find it surprising we cannot rely
> on this behavior. ;-)

Note that stage1 is done without optimisation.  I seem to remember
that at -O0, GCC doesn't do aliasing analysis at all.

-- 
- Geoffrey Keating <geoffk@geoffk.org>