https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95353
Martin Sebor <msebor at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Blocks| |88443
Assignee|unassigned at gcc dot gnu.org |msebor at gcc dot
gnu.org
Status|NEW |ASSIGNED
--- Comment #5 from Martin Sebor <msebor at gcc dot gnu.org> ---
The warning is due to a limitation of the compute_objsize() function. A small
"supported" test case (one that doesn't depend on a trailing array of non-zero
size being treated as a flexible array member) that I think reproduces the
Binutils warning is below. In this case the function doesn't work hard enough
to determine that the pointer points to a trailing array member and instead
uses the the array's actual size. It needs to be improved or preferably
rewritten as discussed in pr94335 comment 7.
As suggested, using a flexible array member instead of the one-element (or
zero-length) array avoids the warning.
$ cat z.c && gcc -O2 -S -Wall -fdump-tree-strlen=/dev/stdout z.c
struct S {
char n, a[0];
};
void f (struct S *p)
{
char *q = p->a;
q[1] = 1; // no warning
}
void g (struct S *p, int i)
{
char *q = p->a + i;
q[1] = 1; // spurious -Wstringop-overflow
}
;; Function f (f, funcdef_no=0, decl_uid=1933, cgraph_uid=1, symbol_order=0)
;; 1 loops found
;;
;; Loop 0
;; header 0, latch 1
;; depth 0, outer -1
;; nodes: 0 1 2
;; 2 succs { 1 }
f (struct S * p)
{
<bb 2> [local count: 1073741824]:
MEM[(char *)p_1(D) + 2B] = 1;
return;
}
;; Function g (g, funcdef_no=1, decl_uid=1938, cgraph_uid=2, symbol_order=1)
;; 1 loops found
;;
;; Loop 0
;; header 0, latch 1
;; depth 0, outer -1
;; nodes: 0 1 2
;; 2 succs { 1 }
z.c: In function ‘g’:
z.c:15:8: warning: writing 1 byte into a region of size 0
[-Wstringop-overflow=]
15 | q[1] = 1; // spurious -Wstringop-overflow
| ~~~~~^~~
g (struct S * p, int i)
{
char * q;
char[0:] * _1;
sizetype _2;
<bb 2> [local count: 1073741824]:
_1 = &p_3(D)->a; <<< doesn't consider that a is a trailing array
_2 = (sizetype) i_4(D);
q_5 = _1 + _2;
MEM[(char *)q_5 + 1B] = 1; <<< warning here
return;
}
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88443
[Bug 88443] [meta-bug] bogus/missing -Wstringop-overflow warnings
