On Wed, Feb 17, 2021 at 02:11:43PM -0700, Martin Sebor wrote:
> On 2/17/21 1:47 PM, Jakub Jelinek wrote:
> > On Wed, Feb 17, 2021 at 01:27:55PM -0700, Martin Sebor wrote:
> >
> > Not in this patch, but I've looked at what maxobjsize is and wonder why
> > the roundtrip tree -> HOST_WIDE_INT -> offset_int:
> > const offset_int maxobjsize = tree_to_shwi (max_object_size ());
> > Can't it be
> > const offset_int maxobjsize = wi::to_offset (max_object_size ());
> > ?
>
> Yes, that's what it is elsewhere so this should do the same. I've
> changed it.
Ok.
> > Doesn't arrbounds[1] == 0 mean there will be warnings for any accesses?
> > For eltsize == 0 I think you shouldn't warn when nelts isn't known,
> > instead of always warning, arr[100000000] will have the same address as
> > arr[0] ...
>
> This branch is entered for VLAs of zero-length arrays where we want
> to warn, like this:
>
> void f (void*);
>
> void g (int n)
> {
> int a[n][0];
> ((int*)a)[0] = 0;
> f (a);
> }
For this you do want to warn, but not the way you warn with the patch:
xxx.c: In function ‘g’:
xxx.c:6:12: warning: array subscript 0 is outside array bounds of
‘int[<Uec60>][0]’ [-Warray-bounds]
6 | ((int*)a)[0] = 0;
| ~~~~~~~~~^~~
xxx.c:5:7: note: while referencing ‘a’
5 | int a[n][0];
| ^
The message doesn't make it clear which of the two subscripts is out of
bounds, yes, for [0] it would be outside of bounds, but for the VLA index
no index < n would be outside of bounds.
Consider a different (GNU C, in C++ struct S has non-zero size) testcase:
void f (void*);
void g (int n)
{
struct S {} a[n];
((int*)a)[0] = 0;
f (a);
}
yyy.c:6:12: warning: array subscript 0 is outside array bounds of ‘struct
S[<Ucc60>]’ [-Warray-bounds]
6 | ((int*)a)[0] = 0;
| ~~~~~~~~~^~~
yyy.c:5:15: note: while referencing ‘a’
5 | struct S {} a[n];
| ^
I bet that means you are really complaining about the VLA bound rather than
the [0] bound even in the first case, because the wording is otherwise the
same. And for g (154) the array subscript 0 is certainly not a problem,
so the warning would need to be worded differently in that case.
Jakub
