[Bug tree-optimization/61964] [4.8/4.9/4.10 regression] krb5 database propagation enters infinite loop; reduced test case

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61964

--- Comment #9 from rguenther at suse dot de <rguenther at suse dot de> ---
On Thu, 31 Jul 2014, vries at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61964
> 
> vries at gcc dot gnu.org changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                  CC|                            |vries at gcc dot gnu.org
> 
> --- Comment #8 from vries at gcc dot gnu.org ---
> Using this patch on the example from the description field, I can modify the
> example on the command line:
> ...
> $ diff -u bug-orig.c bug-mod.c 
> --- bug-orig.c    2014-07-31 14:00:50.663275717 +0200
> +++ bug-mod.c    2014-07-31 14:01:49.403276412 +0200
> @@ -11,7 +11,7 @@
>    struct node *n = head->first;
>    struct head *h = &heads[k];
> 
> -  if (n->prev == (void *)h)
> +  if (FORCE n->prev == (void *)h)
>      h->first = n->next;
>    else
>      n->prev->next = n->next;
> ...
> 
> 1. -DFORCE="" gives the original
> 2. -DFORCE="1 ||" forces the condition to true
> 3. -DFORCE="0 &&" forces the confition to false
> 
> In this experiment, we don't use tree-tail-merge:
> ...
> $ gcc -DFORCE="1 ||" bug-mod.c -O2 -fno-strict-aliasing -fno-tree-tail-merge &&
> ./a.out ; echo $?
> 0
> $ gcc -DFORCE="1 ||" bug-mod.c -O2 -fstrict-aliasing -fno-tree-tail-merge &&
> ./a.out ; echo $?
> 0
> $ gcc -DFORCE="0 &&" bug-mod.c -O2 -fno-strict-aliasing -fno-tree-tail-merge &&
> ./a.out ; echo $?
> 0
> $ gcc -DFORCE="0 &&" bug-mod.c -O2 -fstrict-aliasing -fno-tree-tail-merge &&
> ./a.out ; echo $?
> 1
> ...
> 
> The last result seems to suggest that the example is not type-safe.
> 
> My understanding is that the problem is in the line:
>   n->prev->next = n->next;
> where we effectively do:
>   /* ((struct node*)&heads[2])->next = node.next */
> which is type-unsafe.

But that line is never executed at runtime (well, unless tail
merging comes along and makes it the only version present).

Richard.