Can you use builtin_expect with switch statement

Question

I have a switch statement in some time-critical code. I was trying to optimize it with __builtin_expect, but it does not seem to be working. I'm wondering if anyone can tell me if I'm missing some detail, or if the compiler simply does not optimize for it. I have tried the following on my host machine:

int main() {
    volatile int v=0;
    long i = 0;

    for (i=0; i<1000000000L; i++) {
            switch(__builtin_expect(v, EXPT)) {
            case 7:
                    v=7;
                    break;
            default:
                    v=7;
                    break;
            }
    }
    return v;
}

Then I compile and run as follows:

~/code/builtinexpect> gcc bie.c -o bie -D EXPT=0 && time ./bie 

real    0m2.092s  
user    0m2.086s
sys     0m0.000s
~/code/builtinexpect> gcc bie.c -o bie -D EXPT=7 && time ./bie 

real    0m2.092s
user    0m2.086s
sys     0m0.000s

I am using GCC version 4.5.1.

How does the produced assembly code look like in both cases? — Leeor, Feb 06 '14 at 19:30

score 6 · Accepted Answer · edited Aug 30 '16 at 23:46

6

GCC does not support this on any architecture I'm aware of. If you have a switch statement which strongly favors a particular case, your best recourse is to do an if ... else switch ... statement. This would result in the optimization you're looking for.

edited Aug 30 '16 at 23:46

RandomDSdevel

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answered Feb 13 '14 at 04:22

blackghost

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GCC seems to have added support: – alx - recommends codidact Aug 09 '22 at 12:34

score 3 · Answer 2 · edited Dec 21 '15 at 22:33

3

Both case branches (and therefore all cases) do the same thing, so the compiler is free to replace the whole switch statement with v=7. Even if it doesn't (without optimization), would you expect any real difference in timing?

~~But more to the point, __builtin_expect evaluates as (v == EXPT), either (0) or (1), so case 7: will never be taken.~~

edited Dec 21 '15 at 22:33

Quuxplusone

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answered Feb 06 '14 at 19:35

Brett Hale

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He didn't use compiler optimizations, so the code should differ and may cause forced mispredictions, but nice catch about the boolean-ness! – Leeor Feb 06 '14 at 19:45
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@Leeor - I like to think a branch-predictor would get it mostly right with a billion iterations:) – Brett Hale Feb 06 '14 at 19:54
1

I would hope for a decent __builtin_expect to be strong enough to force the HW not to. Those HW folks need to know their place :) – Leeor Feb 06 '14 at 20:19
1

Thanks for the replies. I shaved down the example to post here to only two case statements (though I had tried it with more, with differing side-effects, but got the same results). The real-life scenerio is run millions of times per second, and even a 1% improvement would be significant. – John Feb 07 '14 at 14:05
3

According to [gcc documentation](http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html), "The return value is the value of exp, which should be an integral expression" (it says it _expects_ `exp == c`, but does not say it returns the result of this)... I just tried it, and case 1 and 0 are never hit if I initialize to -1. – John Feb 07 '14 at 14:14

Can you use builtin_expect with switch statement

2 Answers2