"Bokhanko, Andrey S" <[EMAIL PROTECTED]> writes:
> Hi,
>
> As I learned from experience, gcc always assume independence between
> memory references in the following program:
>
> typedef struct {
> int m1;
> int m2;
> } S1;
>
> void foo(S1 *p1, S1 *p2) {
> ... = p1->m1;
> ... = p2->m2;
> }
>
> ...even if -fno-strict-aliasing (an option disabling ansi-aliasing
> rules) supplied.
>
> I wonder, is there a way to force gcc not to assume independence in
> the example shown above?
A dirty trick using volatile may work:
int foo(S1 *p1, S1 *p2) {
((S1 volatile*)p1)->m2 = 10;
return ((S1 volatile*)p2)->m1;
}
However, provided that GCC guarantees (unlike C standard) that type
punning through unions will work, it seems that GCC should not assume
p1->m2 and p2->m1 never alias:
typedef struct {
int pad;
S1 s;
} X;
typedef union {
S1 s;
X x;
} U;
U u;
If it is guaranteed that
int boo() {
u.s.m2 = 10;
return u.x.s.m1;
}
works, then
f(&u.s, &u.x.s);
where
int f(S1 *p1, S1 *p2) {
p1->m2 = 10;
return p2->m1;
}
should better work the same way, isn't it?
-- Sergei.
