Re: [google] Add options to pattern match function name for hotness attributes

[Dehao Chen <dehao at google dot com>]

On Sat, Jun 2, 2012 at 9:17 PM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> > Well, perhaps the feature would be more useful with command line option allowing
>> > to add an attribute instead of having special case command line options for individual
>> > cases.
>>
>> Hi, Honza, I'm not sure what you mean by "command line option allowing
>> to add an attribution". But this seems what this patch does: it adds
>> attributes to functions with specified name patterns.
>
> I mean that if we want to go this route, we may want to allow this interface to i.e. add
> always_inline attrbute or noinline or such. I can imagine this to be useful especially for
> languages that has no attributes.

This annotation is mainly for static branch prediction. Some functions
may be very cold, but it may still be good to inline it.

>
>>
>> > ?I guess the problem would be also solvable with header included after
>> > the library adding the attributes to the declarations
>> >
>> > typeof (printf) printf __attribute__ ((cold));
>>
>> Do you mean that for each specific application, we extract a common
>> header file to include all function annotations, and all source files
>> will include this header file?
>
> Yes, something like that...
> Where do you use the likely_hot/likely_cold logic?

For really cold functions, we predict the branch to be 0.04% taken.
However, some callee functions are not that extremely cold, and
predicting them as 0.04% is too aggressive. Likely_cold is for such
functions, and we predict them as 25% taken (at least we can predict
the direction right). Vise versa for likely_hot annotation.

Thanks,
Dehao

>
> Honza
>>
>> Thanks,
>> Dehao
>>
>> >
>> > Honza
>> >>
>> >> thanks,
>> >>
>> >> David
>> >> > Honza