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

Nick Desaulniers
2020-05-05T18:28:06Z

On Tue, May 5, 2020 at 11:13 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> 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); \

Changing
void *func = READ_ONCE(name.func); \
to
void *func = &READ_ONCE(name.func); \
produces the tail call. Not sure if that's relevant/what you were
looking for/even correct (haven't thought to hard about the
implications of that change; juggling other stuff ATM)

> (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

--
Thanks,
~Nick Desaulniers