[Bug tree-optimization/78035] Inconsistency between address comparison and alias analysis

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78035

--- Comment #10 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 2 Nov 2016, krister.walfridsson at gmail dot com wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78035
> 
> krister.walfridsson at gmail dot com changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                  CC|                            |krister.walfridsson at gmail dot c
>                    |                            |om
> 
> --- Comment #9 from krister.walfridsson at gmail dot com ---
> Doesn't this just introduce more inconsistencies in the compiler? For example
> 
>     extern int a;
>     extern int b;
> 
>     int foo(void)
>     {
>         a = 1;
>         b = 5;
>         a++;
>         return &a != &b;
>     }
> 
> optimizes to
> 
>     foo:
>         movl    $a, %eax
>         movl    $5, b(%rip)
>         movl    $2, a(%rip)
>         cmpq    $b, %rax
>         setne   %al
>         movzbl  %al, %eax
>         ret
> 
> That is, the accesses to a and b are optimized as if they are distinct, even
> though the compiler keeps the comparison of the addresses.
> 
> I cannot think of a reasonable use case where you must handle comparisons of
> the addresses as currently implemented while allowing other optimizations as if
> the objects are distinct, so I'd say the bug from the original description is
> that we were "being too conservative in bar"...

I opened this bug for the inconsistency in address comparisons which
now can utilize points-to analysis.  This part is fixed now.

Yes, for alias analysis we are less conservative than when optimizing
address comparisons ... but it appears that this is on purpose.