Failing test

[Veedrac]

This test fails

let haystack = vec![0, 2, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
let needle = 2;
assert_eq!(count(&haystack[0..], needle), naive_count(&haystack[0..], needle));

giving a count of 2 rather than 1. The errors are with

(((x ^ needles).wrapping_sub(LO)) & !x & HI) >> 7

Firstly, this should be

let x = x ^ needles;
((x.wrapping_sub(LO)) & !x & HI) >> 7

Secondly, this doesn't actually work for counting - the x.wrapping_sub(LO) sets the high bit correctly but can overflow into the next value if the next byte is 1 - wrapping_sub will decrement that to zero and the subtraction will continue past.

It's fairly simple to deal with by using

!((((v & !HI) + !HI) | v) >> 7) & LO

instead, but it'd be interesting to see if you don't have to spend the extra operations to get that. It doesn't seem to affect speed all too much, though.