[Bug tree-optimization/65752] Too strong optimizations int -> pointer casts

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752

--- Comment #34 from rguenther at suse dot de <rguenther at suse dot de> ---
On Sat, 23 May 2015, gil.hur at sf dot snu.ac.kr wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752
> 
> --- Comment #33 from Chung-Kil Hur <gil.hur at sf dot snu.ac.kr> ---
> Dear Richard,
> 
> Thanks for the detailed response.
> 
> I have a suggestion for a solution of the problem, which is based on my paper
> to appear at PLDI 2015.
> 
> * A Formal C Memory Model Supporting Integer-Pointer Casts.
>   Jeehoon Kang, Chung-Kil Hur, William Mansky, Dmitri Garbuzov, Steve
> Zdancewic, Viktor Vafeiadis.
>   http://sf.snu.ac.kr/gil.hur/publications/intptrcast.pdf
> 
> The suggestion is simple.
> You do not need to turn off the phiopt optimizations.
> We propose to slightly change the following assumption.
> 
> > PTA considers that all pointers coming from integer constants
> > point to global memory only.
> 
> Here, if you change this as follows, you can solve the problem.
> 
> * All pointers coming from integer constants can point to only global memory
> and
>   local variables whose addresses have been cast to integers.

Ok, so you basically add a 2nd class of "escaping".  So in GCC PTA
terms you'd add a new ESCAPE-like 'INTEGER' variable with

INTEGER = NONLOCAL

and add

INTEGER = x

constraints for each

.. = (integer-type) &x

conversion and for the reverse

 ptr = (pointer-type) i

add

ptr = INTEGER

> Also, we expect that this would not decrease the optimization performance of
> GCC very much because those variables whose addresses have been cast to
> integers tend to be escaped (e.g. passed to a hash function, or stored in the
> memory).

Well - the above basically makes _all_ pointers converted from integers
point to non-local memory, it also basically globs all pointers
converted from integers into a single equivalence class.

So I think you underestimate the effect on optimization (but I may
overestimate the effect on optimization of not simply making all
pointers converted from integers point to all globals and all
address-taken locals, aka ANYTHING in GCC PTA terms)