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Re: Using __builtin_expect() in the body of unlikely branch


* 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.

Thanks,
Florian


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