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.
