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

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* 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.


-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68