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


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).


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