On Sun, Aug 23, 2020 at 04:21:30PM +0100, Matthew Wilcox wrote:
> On Wed, Aug 19, 2020 at 10:14:11AM +0200, Giuseppe Scrivano wrote:
> > >> - cft->file_offset = offsetof(struct hugetlb_cgroup,
> > >> events_file[idx]),
> > >> + cft->file_offset = offsetof(struct hugetlb_cgroup,
> > >> events_file[idx]);
> > >> cft->flags = CFTYPE_NOT_ON_ROOT;
> >
> > I think in this case having two expressions as part of the same
> > statement is equivalent to having two separate statements. Both
> > cft->file_offset and cft->flags get the expected value.
>
> That's not how the comma operator works.
>
> It will evaluate offsetof(struct hugetlb_cgroup, events_file[idx]) and
> then discard the result. Since it has no side-effects, this is effectively
> doing:
>
> cft->file_offset = cft->flags = CFTYPE_NOT_ON_ROOT;
_oh_. I tested this. I'm wrong because the comma operator is at lower
precedence than assignment.
Testcase:
struct a {
int x;
int y;
};
void g(struct a *a) {
a->x = 1,
a->y = 0;
}
void h(struct a *a) {
a->x = (1,
a->y = 0);
}
test.c: In function ‘h’:
test.c:12:12: warning: left-hand operand of comma expression has no effect
[-Wunused-value]
12 | a->x = (1,
| ^
0000000000000000 <g>:
0: 48 c7 07 01 00 00 00 movq $0x1,(%rdi)
7: c3 retq
8: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1)
f: 00
0000000000000010 <h>:
10: 48 c7 07 00 00 00 00 movq $0x0,(%rdi)
17: c3 retq
So there's no bug here! It's just confusing, so should be fixed.
(I think Andrew was confused too ;-)
