On Mon, Jul 07, 2025 at 09:07:14AM -0400, Siddhesh Poyarekar wrote:
> On 2025-07-07 08:48, Jakub Jelinek wrote:
> > > The return value of .ACCESS_WITH_SIZE clobbering PTR (that subsequently
> > > gets
> > > passed to __builtin_dynamic_object_size) should be sufficient to fully
> > > prevent the reordering, it shouldn't have to clobber &PTR, I think.
> >
> > The original use of .ACCESS_WITH_SIZE was designed for FAMs, for those
> > it IMHO does the right thing, it is a pass through first arg function
> > which attaches size information to the passed as well as returned pointer.
> > That pointer is &FAM, so address of the containing structure plus offsetof
> > of the FAM first element.
>
> So that ought to be &a->fam[0] right, and not &a->fam? It means the same
> for a FAM, so why not specify it as &a->fam[0] (or simply a->fam)? That will
> be consistent with when fam is a pointer.
The only difference between &a->fam[0] and &a->fam is not the value (that is
the same), just the type in one case say int *, in the other int [0:] *.
At least in GIMPLE pointer conversions are useless, so what exact type of
the argument is doesn't matter that much, but it matters e.g. when you're
dereferencing it.
Now, for
int n; int (*p)[n] __attribute__((counted_by(n)));
f->p has the int [n] * type, so guess that is what you want to use.
Jakub
