https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91168
Martin Sebor <msebor at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |diagnostic
Status|UNCONFIRMED |RESOLVED
CC| |msebor at gcc dot gnu.org
Resolution|--- |WONTFIX
--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
When reporting bugs please provide all the information requested here:
https://www.gnu.org/software/gcc/bugs/#need
The warning was introduced as a result of r262893. It's caused by a confluence
of two factors:
1) the upper bound of the loop having a limited range (i.e., a subrange of
ptrdiff_t/size_t -- compiling with -m32 suppresses it), and
2) the optimizer's necessary assumption that operator new may modify this->N.
The avoid the warning either use N_ as the upper bound of the loop or change
the type of S::N to size_t.
The only way for GCC to avoid the warning would be to avoid assuming (for the
purposes of the warning only) that operator new and other similar functions(*)
clobber memory. That might be possible but I'm not sure how intrusive a change
it would be or whether it's a good idea. So for now, I'm going to resolve this
as WONTFIX.
[*] Another function that GCC assumes clobbers memory is printf, even though in
most cases it doesn't. It assumes that because the function can, in fact,
clobber memory via the %n format directive and GCC doesn't scan the format
string to determine whether or not it contains the directive. (The function
can also clobber memory indirectly by calling a registered printf hook but
that's outside the scope of what GCC needs to consider).
$ cat pr91168.c && gcc -O2 -S -Wall -Wextra -Wpedantic pr91168.c
int f (int *n)
{
const int a[] = { 1, 2 };
int x = 0;
*n = 2;
__builtin_printf ("n = %p\n", (void*)n);
for (int i = 1; i < *n - 1; i++)
x = a[i + 1];
return x;
}
pr91168.c: In function ‘f’:
pr91168.c:10:10: warning: array subscript 2 is above array bounds of ‘const
int[2]’ [-Warray-bounds]
10 | x = a[i + 1];
| ~^~~~~~~
pr91168.c:3:13: note: while referencing ‘a’
3 | const int a[] = { 1, 2 };
| ^
