Re: [PATCH v4 14/18] static_call: Add static_cond_call()

[Nick Desaulniers <ndesaulniers@google.com>]

On Tue, May 5, 2020 at 2:36 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
>
> HJ, Nick,
>
> Any chance any of you can see a way to make your respective compilers
> not emit utter junk for this?
>
> On Mon, May 04, 2020 at 10:14:45PM +0200, Peter Zijlstra wrote:
>
> > https://godbolt.org/z/SDRG2q

Woah, a godbolt link! Now we're speaking the same language.  What were
you expecting? Us to remove the conditional check that a volatile read
wasn't NULL? (Not using READ_ONCE, produces the direct tail call I
suspect you're looking for, but am unsure if that's what you meant,
and understand that's not a solution).  I am simultaneously impressed
and disgusted by this btw, cool stuff.

i.e.
void *func = &name.func; \
rather than
void *func = READ_ONCE(name.func); \
(I'm surprised that `&name.func;` and `name.func;` also produce
different results).

> >
> > ---
> > #include <stddef.h>
> >
> >
> > #define READ_ONCE(var)                (*((volatile typeof(var) *)&(var)))
> > #define WRITE_ONCE(var, val)  (*((volatile typeof(var) *)&(var)) = (val))
> >
> > struct static_call_key {
> >       void *func;
> > };
> >
> > #define DECLARE_STATIC_CALL(name, func)       \
> >       extern struct static_call_key name; \
> >       extern typeof(func) __SCT__##name;
> >
> > #define DEFINE_STATIC_COND_CALL(name, _func) \
> >       DECLARE_STATIC_CALL(name, _func) \
> >       struct static_call_key name = { \
> >               .func = NULL, \
> >       }
> >
> > static void __static_call_nop(void)
> > {
> > }
> >
> > #define __static_cond_call(name) \
> > ({ \
> >       void *func = READ_ONCE(name.func); \
> >       if (!func) \
> >               func = &__static_call_nop; \
> >       (typeof(__SCT__##name)*)func; \
> > })
> >
> > #define static_cond_call(name) (void)__static_cond_call(name)
> >
> > static void inline static_call_update(struct static_call_key *call, void *func)
> > {
> >       WRITE_ONCE(call->func, func);
> > }
> >
> > volatile int _x;
> >
> > void bar(int x)
> > {
> >       _x = x;
> > }
> >
> > DEFINE_STATIC_COND_CALL(foo, bar);
> >
> > void ponies(int x)
> > {
> >       static_cond_call(foo)(x);
> > }



-- 
Thanks,
~Nick Desaulniers