https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95594
Bug ID: 95594
Summary: overzealous -Wmaybe-uninitialized calling a function
with an empty range of const pointers
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
The recent enhancement to -Wuninitialized (PR 10138) is overly strict: it
triggers for calls to functions with a pair of pointers (at least one const)
delineating a slice of an array.
For example, while the warning doesn't trigger in g() below it does trigger in
h(), even though the call is safe. Passing such slices is not entirely
uncommon. For example, the C++ class member functions in() and out() are
declared like so:
result codevt::out (stateT& state,
const internT* from, const internT* from_end, const
internT*& from_next,
externT*to, externT* to_end, externT*& to_next) const;
and can be called with empty ranges.
A test case for the idiom is below:
$ cat q.c && /build/gcc-master/gcc/xgcc -B /build/gcc-master/gcc -O2 -S -Wall
-Wextra q.c
__attribute__ ((noipa)) void
f (void *to, const void *to_end)
{
__builtin_memset (to, 0, to_end - to);
}
void g (void)
{
int i;
f (&i, &i + 1); // no warning (good)
// ...
}
void h (void)
{
int a[2];
f (a, a); // -Wmaybe-uninitialized
// ...
}
q.c: In function ‘h’:
q.c:17:3: warning: ‘a’ may be used uninitialized [-Wmaybe-uninitialized]
17 | f (a, a); // -Wmaybe-uninitialized
| ^~~~~~~~
q.c:2:1: note: by argument 2 of type ‘const void *’ to ‘f’ declared here
2 | f (void *to, const void *to_end)
| ^
q.c:16:7: note: ‘a’ declared here
16 | int a[2];
| ^
