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0 pointsYesBox3d ago0 comments

> A cache miss is going to mean anywhere from a 100 to 1000 cycle penalty. That blows out any sort of hit you take cutting your cycles from 3 to 1.

A good example of this is using std::vector<bool> vs. std::vector<uint8_t> in the debug build vs release build.

vector<bool> is much slower to access (it's a dynamic bitset). If you have a hot part of the code that frequently touches a vector<bool>, you'll see a multiple X slowdown in the debug build.

However, in the release build, there is no performance difference between the two (for me at least, I'm making a fairly complicated game). The cache misses bury it.

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fragmede3d ago

Fascinating, that's counterintuitive. I'd think the point of using vector <bool> is because the compiler would optimize it to be a bit field which is fewer bits and thus smaller and thus faster than using vector <uint_t8>. How did you come to figure that out?

YesBoxOP3d ago

I dont know how it's implemented by the standard/compiler (not my domain). The performance differences are well documented though.

I've used both in my pathing code and tested each in debug/release.

Even if the std:: implementation was as fast as possible, you're still adding bit manipulation on top of accessing the element, so it will be slower no matter what you do.

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