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
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