Re: Compiled code crashes storing to read-only location

[Waldek Hebisch <hebisch at math dot uni dot wroc dot pl>]

Geoffrey Keating wrote:
> Waldek Hebisch <hebisch@math.uni.wroc.pl> writes:
> 
> > The following program:
> > 
> > typedef struct {int c, l; char ch[3];} pstr;
> > const pstr ao = {2, 2, "OK"};
> > const pstr * const a = &ao;
> > 
> > int main(void)
> > {
> >         if (a->ch[a->l]) {
> >                 a->ch[a->l] = 0;
> >         }
> >         return 0;
> > }
> > 
> > segfaults when compiled using `-O' (or better optimisation). Without
> > optimisation the store is not executed and the program runs fine. 
> > 
> > I tried CVS gcc (updated DEC 14, 04:28 CET 2003) on Athlon-XP running
> > Debian Woody. The problem does not appear in 20031029 snapshot, 
> > but is present in 20031105 snapshot.
> > 
> > By the way, I have some doubts about corretness of the program, however
> > my original problem is in GNU Pascal. Constant Pascal strings are stored
> > in .rodata section and compiler makes sure that the string is null
> > terminated. On the other hand variable strings need not be null terminated.
> > When convering Pascal strings to C strings GPC generates code to null
> > terminate the strings which are not already null terminated. The C
> > program above tries to reproduce the code GPC produces.
> 
> This program isn't valid C, it should be 
> 
>       ((char *)a->ch)[a->l] = 0;
> 
> but with that change, it is perfectly valid.  Could you file a bug
> report in bugzilla?
> 

Corrected version still segfaults. Reported as 'middle-end/13400':
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13400

The problem goes away when I revert the following patch:

http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ifcvt.c.diff?cvsroot=gcc&r1=1.128
&r2=1.129

which is supposed to fix:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10817


-- 
                              Waldek Hebisch
hebisch@math.uni.wroc.pl