https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98646
--- Comment #12 from Martin Sebor <msebor at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #10)
> So, the problem is IMHO that the warning about passing NULL to this is
> misplaced, it shouldn't be done in the FEs, but later when there was at
> least some chance of dead code removal.
That would be a hack. First, the this argument to a member function is just
like a plain argument to an ordinary nonnull function, so they all should be
treated the same. Second, there already is code in tree-ssa-ccp.c to handle
-Wnonnull, so nothing needs to be moved.
The -Wnonnull code could be removed from the front end, but that would
considerably compromise the warning: passing null pointers to inline member
functions would cease to be diagnosed. See for example the effect on the code
below. In C++ that wouldn't be a good deal.
$ cat u.C && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout u.C
struct S {
int i;
void f () { i = 0; };
void g ();
};
void f (void)
{
S *p = 0;
p->f (); // missing warning
}
void g (void)
{
S *p = 0;
p->g (); // -Wnonnull (good)
}
;; Function f (_Z1fv, funcdef_no=1, decl_uid=2357, cgraph_uid=2,
symbol_order=1) (executed once)
void f ()
{
<bb 2> [local count: 1073741824]:
MEM[(struct S *)0B].i ={v} 0;
__builtin_trap ();
}
u.C: In function ‘void g()’:
u.C:16:8: warning: ‘this’ pointer null [-Wnonnull]
16 | p->g (); // -Wnonnull (good)
| ~~~~~^~
u.C:4:8: note: in a call to non-static member function ‘void S::g()’
4 | void g ();
| ^
;; Function g (_Z1gv, funcdef_no=2, decl_uid=2360, cgraph_uid=3,
symbol_order=2)
void g ()
{
<bb 2> [local count: 1073741824]:
S::g (0B); [tail call]
return;
}
Also, removing the -Wnonnull handling from the FE wouldn't actually solve the
reported problem. It would just paper over it because the middle end warning
doesn't consider PHIs yet. If/when it's enhanced to consider them the warning
would come back again as is plain to see in the dump for the test case from
comment #11:
$ cat pr98646.C && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout pr98646.C
struct A { virtual ~A (); };
struct B { virtual ~B (); B* f (); };
struct C: A, B { virtual ~C (); void g () const; };
void f (B *p)
{
static_cast<C*>(p->f ())->g (); // bogus -Wnonnull
}
;; Function f (_Z1fP1B, funcdef_no=0, decl_uid=2430, cgraph_uid=1,
symbol_order=0)
Removing basic block 5
void f (struct B * p)
{
struct C * iftmp.0_1;
struct B * _5;
struct C * iftmp.0_6;
<bb 2> [local count: 1073741824]:
_5 = B::f (p_3(D));
if (_5 != 0B)
goto <bb 3>; [70.00%]
else
goto <bb 4>; [30.00%]
<bb 3> [local count: 751619281]:
iftmp.0_6 = _5 + 18446744073709551608;
<bb 4> [local count: 1073741824]:
# iftmp.0_1 = PHI <iftmp.0_6(3), _5(2)>
C::g (iftmp.0_1); [tail call] <<< should get a warning: pointer may be
null
return;
}
We can see that already today by running the analyzer on the test case:
pr98646.C: In function ‘void f(B*)’:
pr98646.C:8:31: warning: use of NULL where non-null expected [CWE-476]
[-Wanalyzer-null-argument]
8 | static_cast<C*>(p->f ())->g (); // bogus -Wnonnull
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
‘void f(B*)’: events 1-3
|
|
pr98646.C:4:38: note: argument 'this' of ‘void C::g() const’ must be non-null
4 | struct C: A, B { virtual ~C (); void g () const; };
| ^
Which is why, if we want to avoid these warnings, we need the front end to
annotate the implicit COND_EXPR somehow. It could be done by setting
TREE_NO_WARNING but I'd worry about it getting lost or unintentionally
suppressing important warnings. Adding a special internal function instead and
expanding it after the first -Wnonnull handler would be better.
