* Konstantin Kharlamov:
> On 07.01.2019 13:35, Florian Weimer wrote:
>> * Marc Glisse:
>>
>>> On Mon, 7 Jan 2019, Konstantin Kharlamov wrote:
>>>
>>>> In most projects a definite pattern that's unlikely to be executed
>>>> is a PRINT_ERR macro which is basically a wrapper around fprintf()
>>>> call. E.g.
>>>>
>>>> if (some_error) {
>>>> PRINT_ERR("ERR");
>>>> // do cleanup
>>>> return;
>>>> }
>>>>
>>>> I wonder, is there a way to hint GCC that, whenever that code
>>>> appears, whatever branch was prior to that is unlikely to be
>>>> executed?
>>>
>>> Make PRINT_ERR a function with __attribute__((cold)).
>>
>> But this does something completely differently. On some targets, it
>> produces unbearably slow code because GCC expects the code to never run
>> in practice. __builtin_expect does not have this effect.
>
> So, you would not recommend using that attribute for
> functions-loggers?
No, at least not on i386 and x86-64. It may be okay if the logging
action ends up in an actual system call (due to its overhead), but if
the logging-active path ends up suppressing the message (after calling a
string function), it could lead to artifacts showing up in profiling.
> Ultimately I was asking because I was thinking of
> contributing such optimization to arbitrary projects I happen to use,
> such as libinput, wine, etc (list is completely offhand, I didn't look
> at wine code in particular, and they're on feature-freeze ATM anyway).
Currently, you need to use __builtin_expect.
I'll try to improve the documentation of the attribute.
Thanks,
Florian
