From mboxrd@z Thu Jan  1 00:00:00 1970
From: der.herr@hofr.at (Nicholas Mc Guire)
Date: Wed, 18 Feb 2015 19:38:01 +0100
Subject: unlikely compiler flag propagation
In-Reply-To: <20150218182453.GB21740@kroah.com>
References: <CABuKBeL-96nLetmHSnawLuh-G=zMd6J8ffAvGnEpuJmsvBgx0Q@mail.gmail.com>
	<20150218182453.GB21740@kroah.com>
Message-ID: <20150218183801.GA1773@opentech.at>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

On Wed, 18 Feb 2015, Greg KH wrote:

> On Wed, Feb 18, 2015 at 07:09:47PM +0100, Matthias Brugger wrote:
> > Hi all,
> > 
> > I have a question about the unlikely compiler flag.
> > When a called function is only returns an error with the unlikely flag
> > set, should I set the unlikely compiler flag for the return value
> > check in the callee as well?
> > 
> > For example:
> > 
> > int function_one(int *list, int num_elements)
> > {
> >     int i;
> >     for (i =0; i < num_elements; i++) {
> >         if (unlikely(check_element(list + i)))
> >            return 1;
> >     }
> > 
> > [...]
> > 
> >     return 0;
> > }
> > 
> > int function_two(...)
> > {
> > [...]
> > 
> >      if (function_one(list, num))
> >          return -1;
> > }
> > 
> > 
> > So my question is, if function_two should instead implement:
> > if (unlikely(function_one(list, num))
> > 
> > Or does the unlikely compiler flag propagate to calling functions?
> 
> NEVER use unlikely/likely unless you can actually measure that it
> matters if you use it.  The compiler and processor is almost always
> better at making these types of guesses and predictions, so let it do
> the work instead.
> 
> As proof of this, there was a test of the kernel a year or so ago that
> measured the placement of the existing likely/unlikely markers in the
> kernel and 90% of the usages were wrong and actually slowed down the
> processor.
>

interesting - would you have a reference to some talk/paper/data/... ?
 
> So just don't use it, unless you can measure it.
>

thx!
hofrat