Re: [patch 9/9] Scheduler profiling - Use conditional calls

Mathieu Desnoyers
2007-05-31T22:07:51Z

* Andi Kleen (andi@firstfloor.org) wrote:
> Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> writes:
> > }
> > - profile_hit(SCHED_PROFILING, __builtin_return_address(0));
> > + cond_call(profile_on,
> > + profile_hit(SCHED_PROFILING, __builtin_return_address(0)));
>
> Would it be possible to use a syntax like
>
> if (unlikely_cond_call(variable)) { (or better name)
> ...
> }
>
> instead? I think that would be much nicer to read than having
> code in a macro argument
>

I see your point, but there is a level of control on the branch I would
lack by doing so: the ability to put the call in either the if or else
branch. It is an optimization on i386.

I could do it by defining my home-made if() :

cond_if (cond_call_name) {
code
}

The macro cond_if could then expand (this is a simplified example) in either in

if (cond)

or

if (cond)
else

Also, I live in the expectation that, someday, the gcc guys will be nice
enough to add some kind of support for a nop-based jump that would
require code patching to put a jump instead. If it ever happens, my
macro could evolve into this for newer compiler versions, which I could
not do with the if() statement you are proposing.