[Bug c/61236] GCC 4.9 generates incorrect object code

["pinskia at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61236

--- Comment #15 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Mukund Sivaraman from comment #14)
> (In reply to Jakub Jelinek from comment #10)
> > But the compiler doesn't know there that x is NULL.  The compiler sees a
> 
> See comment #3. It generates 2 codepaths, one where (nalloc == 0) and
> another where (nalloc != 0). For the former, it deletes the if statement and
> isc_mem_put() call at the free_rdatas label completely:
> 
>  free_rdatas:
>         if (x != NULL)
>                 isc_mem_put(mctx, x, nalloc * sizeof(struct xrdata));
> 	return (result);
> }
> 
> and instead reduces free_rdata's definition to:
> 
>  free_rdatas:
> 	return (result);
> }
> 
> How does the compiler do that if it has not inferred that x is NULL there?
> 
> OTOH, you're the compiler developers, so if you say it doesn't know that x
> is NULL, then that is that. :) Maybe the part of compiler code that does
> this doesn't know it.
> 
> Note that despite all this discussion of correctness, this optimization is
> counter intuitive and will bite developers. There should at least be
> warnings where they could be generated.
> 
> The point about correctness with C standards is taken and agreed.
> 
> See what is happening from a programmer's point of view: an explicit NULL
> check is deleted. There are no warnings about qsort() used with NULL
> arguments where it seems the compiler could warn (see above). Also consider
> the use of notnull as an API annotation change by 3rd party libraries, which
> can make caller code buggy without any way to notice it.
> 
> At the very least, if it is possible to detect that the pointer is NULL by
> static analysis and it is being passed to a function that has the notnull
> attribute, please warn mentioning inferences being made.
The warning did not make it into gcc 4.9 due to the patches to do the warning
were not ready. Gcc 4.10 should warn about it. If it does not then that is a
bug.