Rust Performance Pitfalls

Post by [deleted] » Sat, Jun 10, 2017, 5:16 AM UTC

jblow wrote: post #103

Lerc wrote: post #59

I have been doing some exploration of how well Rust optimizes Iterators and have been quite impressed. Writing a iterator to provide the individual bits supplied by an iterator of bytes means you can count them with fn count_bits >(it : I) -> i32{ let mut a=0; for i in it { if i {a+=1}; } return a; } Counting bits in an array of bytes would need something like this let p:[u8;6] = [1,2,54,2,3,6]; let result = count_bi…

Okay, wait. Every modern CPU has a popcount instruction, so any hand-coded implementation would use that, meaning the compiler output is actually pretty bad in an absolute sense. But if you find popcount too "magical", the commonly-known fast way to count bits is via masking, shifts and adds, so that you do it in log(n) steps. Which also would perform much better than this solution. So what you're really saying is "t…