Using __builtin_expect() in the body of unlikely branch

[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? 
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).