On Sun, 2005-08-21 at 20:32 +0200, Falk Hueffner wrote:
> Hi,
>
> I'm trying to implement a tree pass that warns about bad array
> accesses as suggested for PR 8268 by Jeff Law. However, I have trouble
> with the following:
>
> char digit_vector[5];
> const char *ggc_alloc_string(int length) {
> return digit_vector + ((length - 17) * 2);
> }
>
> this translates to:
>
> ggc_alloc_string (length)
> {
> const char * D.1292;
> int D.1293;
> long unsigned int D.1294;
> char * D.1295;
> char * D.1296;
>
> D.1293 = length * 2;
> D.1294 = (long unsigned int) D.1293;
> D.1295 = (char *) D.1294;
> D.1296 = &digit_vector + -34B; <-----------
> D.1292 = D.1295 + D.1296;
> return D.1292;
> }
>
> that is, a pointer is formed that wouldn't be legal to form from C,
> and we end up with
>
> return (char *) (long unsigned int) (length * 2) +
> &digit_vector[-000000022];
IIRC creating an invalid pointer is OK -- dereferencing the pointer is
what's bad. You need to focus on array accesses, pointer dereferences
and the like, not pointer generation.
Warning for pointer generation is going to be a *lot* harder and I
suspect will always result in more false positives.
> producing a warning. Is that correct GIMPLE? If so, I fear it simply
> isn't possible to do this kind of warnings after gimplification, and,
> if at all possible, would have to be done in the front-end after all.
Putting these warnings in the front-end is IMHO wrong. They
belong in the generic parts of the compiler.
Jeff
