[Bug c++/23383] builtin array operator new is not marked with malloc attribute

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23383

--- Comment #19 from rguenther at suse dot de <rguenther at suse dot de> 2012-01-05 08:39:57 UTC ---
On Wed, 4 Jan 2012, xinliangli at gmail dot com wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23383
> 
> --- Comment #18 from davidxl <xinliangli at gmail dot com> 2012-01-04 17:11:26 UTC ---
> (In reply to comment #17)
> > On Wed, 4 Jan 2012, xinliangli at gmail dot com wrote:
> > 
> > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23383
> > > 
> > > --- Comment #16 from davidxl <xinliangli at gmail dot com> 2012-01-04 00:28:55 UTC ---
> > > A related topic - aliasing property of realloc -- the malloc attribute is not
> > > applied in the glibc header and the comment is like
> > > 
> > > /* __attribute_malloc__ is not used, because if realloc returns
> > >    the same pointer that was passed to it, aliasing needs to be allowed
> > >    between objects pointed by the old and new pointers.  *
> > > 
> > > 
> > > It is true that the realloc can return an address is physically aliased with
> > > the pointer passed to it -- but assuming no-alias by the compiler should cause
> > > no harm -- as all code motions/CSEs across the realloc call will not be
> > > possible because realloc may modify/use the memory location.
> > > 
> > > 
> > > Any comment on this? 
> > 
> > The malloc attribute assumes that the contents of the memory pointed
> > to by the return value is undefined, so the comment is inaccurate
> > but the malloc attribute can indeed be not used.
> 
> Which part of the optimizer takes advantage of the 'undefinedness' of returned
> memory?

points-to analysis.  It assumes that the returned blob of memory
points to nothing (yet).  So for

 int i;
 int **p = malloc (8);
 *p = &i;
 int **q = realloc (p, 8);

you'd get that *q points to nothing insteda of i.

> > We can explicitly handle REALLOC in the points-to code to honor the
> > fact that it is not (we do not at the moment).
> 
> ok.

which needs to be fixed here.

Richard.