Hi Bernd,
On Fri, Nov 27, 2015 at 09:26:40AM +0000, Bernd Edlinger wrote:
> On Tue, 17 Nov 2015 14:31:29, Jeff Law wrote:
> > The benefit is traditional asms do the expected thing. With no way to
> > describe dataflow, the only rational behaviour for a traditional asm is
> > that it has to be considered a
> use/clobber of memory and hard registers.
>
> I'd like to mention here, that there is also another use-case for a basic
> asms:
>
> It is most often used as a fairly portable memory barrier like this:
>
> x = 1;
> asm(""); // memory barrier
> y = 2;
>
> that is also the reason why every basic asm is implicitly a volatile asm.
But that is not a memory barrier, not as currently implemented anyway:
===
int a, b;
void f(void)
{
int j;
for (j = 0; j < 10; j++) {
a = 42;
asm("lolz");
b = 31;
}
}
===
does the asms in a loop, followed by the two stores. Making it
asm("lolz" ::: "memory"); works as you seem to expect.
It has behaved like this since at least 4.0 (the oldest compiler I have
around currently).
[ Yes I'm a broken record. ]
Segher
