Re: malloc attributes and realloc

[Daniel Berlin <dberlin at dberlin dot org>]

On Jan 2, 2004, at 11:58 AM, Joseph S. Myers wrote:

> On Fri, 2 Jan 2004, Daniel Berlin wrote:
>
> > 1. heap allocation functions  return pointers that don't point to
> > anything.
> > 2. pointer destroying operations destroy pointers irreversibly.
> >
> > If #1 does not hold for realloc, it should not be attribute malloc.
> > The fact that it has worked okay so far doesn't mean it will continue
> > to work in the future.  If you change the definition of attribute
> > malloc so that #1 doesn't hold for them, it makes it useless for
> > points-to analysis use in determining heap allocation functions.  
> > Which
> > may or may not be okay by everyone, i don't know.
>
> My current draft documentation change for attribute malloc (for this 
> issue
> and PR 3414) is
>
>  The @code{malloc} attribute is used to tell the compiler that a 
> function
> -may be treated as if it were the malloc function.  The compiler 
> assumes
> -that calls to malloc result in pointers that cannot alias anything.
> +may be treated as if any non-@code{NULL} pointer it returns cannot
> +alias any other pointer valid when the function returns.
>  This will often improve optimization.
> +Standard functions with this property include @code{malloc},
> +@code{calloc} and @code{realloc}.
>
> Does this description (non-NULL pointers don't alias anything valid 
> when
> the allocation function returns) accurately describe what is 
> appropriate
> for the points-to analysis, or not?
Yes, it does.

>  (And quite what does the malloc
> attribute do at present, given the lack of points-to analysis in GCC?)
Absolutely nothing, AFAIK. :)
I'm dreading the __attribute__ (malloc) related bugs that will happen 
if I include this optimization in tree-ssa's PTA.
--Dan