Hi guys,
On Wed, Jun 12, 2019 at 10:31:37PM +0200, Vincent Bernat wrote:
> ? 12 juin 2019 20:47 +02, Tim Duesterhus :
>
> > - write(2, trash.area, trash.data);
> > + shut_your_big_mouth_gcc(write(2, trash.area, trash.data));
Now merged, thanks Tim!
> An alternative not discussed in 89efaed6b67b (which introduced this
> function) is to use "(void)!":
>
> (void)!write(2, trash.area, trash.data);
>
> For an entertaining discussion, see:
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425
>
> Of course, like the "if (foo()) /* do nothing */;", this can be broken
> in the future.
Oh that's a very interesting lecture. And indeed I wouldn't trust it
for a long time, as it can become quite easy for future compilers to
detect that the transformed variable is useless. There's actually a
trick as well consisting in using two variables each assigned to the
other one. The compiler cannot detect anymore that any of them is not
used :
static inline void unused(int ret)
{
int a, b;
// mark all variables as read and written
a = ret;
b = a;
a = b;
}
Yes that's ugly as well, I strongly prefer this in practice :
static inline void unused(int ret)
{
asm("" ::r(ret));
}
At least I'm certain it doesn't emit any code.
Willy
