Re: [PATCH][C] Change array size overflow check

[Richard Guenther <rguenther at suse dot de>]

On Mon, 2 May 2011, Joseph S. Myers wrote:

> On Mon, 2 May 2011, Richard Guenther wrote:
> 
> > This changes the code that deals with too large array sizes to
> > use int_fits_type_p instead of relying on the TREE_OVERFLOW setting
> > of the tree folder.  The latter will break once we don't treat
> > sizetypes specially (and they keep being unsigned).
> > 
> > Bootstrapped and tested on x86_64-unknown-linux-gnu, ok for trunk?
> 
> An array size in C or C++ ought to be considered to overflow (and so give 
> an error if the size is compile-time constant) if the size of the array in 
> bytes is greater than or equal to half the address space, because it is 
> then no longer possible to compute differences between all array elements, 
> and pointers to just past the end of the array, reliably as ptrdiff_t 
> values (cf. PR 45779).  Thus, overflow in a signed rather than unsigned 
> type is what's relevant.
> 
> I don't know if there's a relevant testcase in the testsuite, but the 
> patch is OK with the addition of a testcase such as
> 
> /* { dg-do compile } */
> /* { dg-options "" } */
> 
> typedef __SIZE_TYPE__ size_t;
> 
> extern char a[((size_t)-1 >> 1) + 1]; /* { dg-error "too large" } */
> extern char b[((size_t)-1 >> 1)];
> extern int c[(((size_t)-1 >> 1) + 1) / sizeof(int)]; /* { dg-error "too large" } */
> extern int d[((size_t)-1 >> 1) / sizeof(int)];
> 
> supposing it passes.

We do have similar testcases in gcc.dg/large-size-array*.c, but not
exactly testing the bound.  The above testcase also complains with
-pedantic about

t.c:3:13: error: size of array ‘a’ is too large
t.c:4:1: error: overflow in constant expression [-Woverflow]
t.c:5:12: error: size of array ‘c’ is too large
t.c:6:1: error: overflow in constant expression [-Woverflow]

with and without the patch.  I can add -Wno-overflow to the flags.
Ok with that?

Thanks,
Richard.