------- 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

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