[Bug c/21474] New: missed optimizations when comparing address to NULL

[Bug c/21474] New: missed optimizations when comparing address to NULL

[trt at acm dot org]

The program below gives some expressions which gcc could, but does not, evaluate
to true.  E.g. gcc considers &p->a true, even when a is the first element
of the struct (is this a bug?), but does not consider &p->b[3] to be true.

struct foo {int a, b[10];};
 
int subr(int i, struct foo *p)
{
    int x[10];
 
#if 0
    // gcc folds this
    if (&p->a) return 1;
#else
    // but not these
    if (&p->b[3]) return 1;
 
    if (&x[3] != 0) return 1;
    if (&x[i] != 0) return 1;  // not sure if this one is safe to fold
#endif
    return 0;
}

-- 
           Summary: missed optimizations when comparing address to NULL
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: trt at acm dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21474

[Bug middle-end/21474] missed optimizations when comparing address to NULL

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-05-09 17:26 -------
Actually I don't think it is safe to fold any of these.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |middle-end


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21474

[Bug middle-end/21474] missed optimizations when comparing address to NULL

[trt at acm dot org]

------- Additional Comments From trt at acm dot org  2005-05-12 15:08 -------
I think it is reasonable to assume the address of an auto variable is non-NULL,
and so the address of anything in the local "int x[10];"  is non-NULL.
So gcc can (and does) fold "if (x) ..." and "if (&x[0]) ..."

gcc does not fold "if (&x[3]) ..." due to the the quirk that
that it is represented as x+3 and fold does not recognize that to be non-NULL.

Now consider "if (&x[i])".  The only legal values for i are 0..10,
which precludes any value of `i' that might cause &x[i] to be NULL.
I suppose if x were a pointer, instead of an array, then we wouldn't know
the legal range of values for `i'. But whatever the legal range happens to be
would still (I think) preclude values which could cause &x[i] to be NULL.

The argument for 'if (&p->b[3])' is more convoluted.
Suppose p is non-NULL, then surely this address should be considered non-NULL
for basically the same reason that &x[3] above is considered to be non-NULL.
Suppose instead that p is NULL, then surely a non-zero offset added to p
yields a non-NULL value.

That leaves "if (&p->a)" which gcc folds even though a's offset is zero.
I think this is arguably a bug.  But if no one reports it as a bug ...
well I think this one could be argued either way.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21474