Re:Counting set bits in an interesting way
by tikhonj on Saturday April 30, 2022 @12:36PM
(#31217219)
That's true, but in any scenario where it mattered, I'd hate to rely on it. Compilers have the idiom recognition pass as a sort of hack for speeding up existing codebases but it's ultimately just a heuristic. When I write code, I would much prefer to be explicit about things like that.
[ Parent
| on HN ]
You should be explicit, but equally profile before worrying about this stuff.
Re:Counting set bits in an interesting way
by ycombobreaker on Saturday April 30, 2022 @01:15PM
(#31217496)
Purely from a maintenance perspective I would rather somebody use popcnt instructions (if available) over hand-rolling a bit counting algorithm.
Re:Counting set bits in an interesting way
by tialaramex on Saturday April 30, 2022 @02:49PM
(#31218231)
Exactly. Write your intention first. Only write something else if you had a measurable performance problem and the change fixed it. If you didn't measure, that wasn't a performance improvement, it just was wanking.
Let the compiler, and library writers take care of most of the work of translating your intention into good runtime performance and only intervene when they don't get the job done.
The point is that writing "__builtin_popcount" makes your intentions clearer than implementing popcount as a loop. The guaranteed performance is downstream of the clarity: since the code is more explicit, the compiler doesn't have to rely on heuristics to guess that you want popcount, so it will always do the right thing.