Local variables reordering and 'asm volatile("" ::: "memory");'

[Ayrat Gaskarov]

But it would be a real pain to add asm volatile for every possible
variable and computations that were previously (and after) done (may
be even from callers up a stacktrace). Could we somehow avoid this?

вс, 8 сент. 2019 г. в 23:48, Alexander Monakov <amonakov@ispras.ru>:
>
> On Sun, 8 Sep 2019, Ayrat Gaskarov wrote:
>
> > mutex_lock()
> > function_to_lock()
> > some_really_slow_not_observable_function()
> > mutex_unlock()
> >
> > How could we avoid this? For example if
> > some_really_slow_not_observable_function is long running loop using
> > only local variables.
>
> One possibility is by tying control flow and data flow in the inline asm.
> Suppose your some_really_slow_not_observable_function returns an int, then
>
>   int ret = some_really_slow_not_observable_function();
>   asm volatile ("" :: "g"(ret), "X" (&mut) : "memory");
>
>   mutex_lock(&mut);
>   func_under_lock();
>   mutex_unlock(&mut);
>
>   <use ret>
>
> enforces order by creating a dependency between the value returned from
> the first call and mutex state used in the following call. A similar barrier
> can be placed between the unlock and the use of 'ret':
>
>   asm volatile ("" : "=g"(ret) : "X" (&mut) : "memory");
>
> (in general empty volatile asms with non-empty constraints is a nice tool for
> limiting what the compiler may do)
>
> Alexander



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