| // These routines are meant to be optimized specifically for low latency as |
| // compared to the equivalent routines offered by std. (Which may invoke the |
| // dynamic linker and call out to libc, which introduces a bit more latency |
| // than we'd like.) |
| |
| /// Returns true if and only if needle is a prefix of haystack. |
| #[inline(always)] |
| pub(crate) fn is_prefix(haystack: &[u8], needle: &[u8]) -> bool { |
| needle.len() <= haystack.len() && memcmp(&haystack[..needle.len()], needle) |
| } |
| |
| /// Returns true if and only if needle is a suffix of haystack. |
| #[inline(always)] |
| pub(crate) fn is_suffix(haystack: &[u8], needle: &[u8]) -> bool { |
| needle.len() <= haystack.len() |
| && memcmp(&haystack[haystack.len() - needle.len()..], needle) |
| } |
| |
| /// Return true if and only if x.len() == y.len() && x[i] == y[i] for all |
| /// 0 <= i < x.len(). |
| /// |
| /// Why not just use actual memcmp for this? Well, memcmp requires calling out |
| /// to libc, and this routine is called in fairly hot code paths. Other than |
| /// just calling out to libc, it also seems to result in worse codegen. By |
| /// rolling our own memcmp in pure Rust, it seems to appear more friendly to |
| /// the optimizer. |
| /// |
| /// We mark this as inline always, although, some callers may not want it |
| /// inlined for better codegen (like Rabin-Karp). In that case, callers are |
| /// advised to create a non-inlineable wrapper routine that calls memcmp. |
| #[inline(always)] |
| pub(crate) fn memcmp(x: &[u8], y: &[u8]) -> bool { |
| if x.len() != y.len() { |
| return false; |
| } |
| // If we don't have enough bytes to do 4-byte at a time loads, then |
| // fall back to the naive slow version. |
| // |
| // TODO: We could do a copy_nonoverlapping combined with a mask instead |
| // of a loop. Benchmark it. |
| if x.len() < 4 { |
| for (&b1, &b2) in x.iter().zip(y) { |
| if b1 != b2 { |
| return false; |
| } |
| } |
| return true; |
| } |
| // When we have 4 or more bytes to compare, then proceed in chunks of 4 at |
| // a time using unaligned loads. |
| // |
| // Also, why do 4 byte loads instead of, say, 8 byte loads? The reason is |
| // that this particular version of memcmp is likely to be called with tiny |
| // needles. That means that if we do 8 byte loads, then a higher proportion |
| // of memcmp calls will use the slower variant above. With that said, this |
| // is a hypothesis and is only loosely supported by benchmarks. There's |
| // likely some improvement that could be made here. The main thing here |
| // though is to optimize for latency, not throughput. |
| |
| // SAFETY: Via the conditional above, we know that both `px` and `py` |
| // have the same length, so `px < pxend` implies that `py < pyend`. |
| // Thus, derefencing both `px` and `py` in the loop below is safe. |
| // |
| // Moreover, we set `pxend` and `pyend` to be 4 bytes before the actual |
| // end of of `px` and `py`. Thus, the final dereference outside of the |
| // loop is guaranteed to be valid. (The final comparison will overlap with |
| // the last comparison done in the loop for lengths that aren't multiples |
| // of four.) |
| // |
| // Finally, we needn't worry about alignment here, since we do unaligned |
| // loads. |
| unsafe { |
| let (mut px, mut py) = (x.as_ptr(), y.as_ptr()); |
| let (pxend, pyend) = (px.add(x.len() - 4), py.add(y.len() - 4)); |
| while px < pxend { |
| let vx = (px as *const u32).read_unaligned(); |
| let vy = (py as *const u32).read_unaligned(); |
| if vx != vy { |
| return false; |
| } |
| px = px.add(4); |
| py = py.add(4); |
| } |
| let vx = (pxend as *const u32).read_unaligned(); |
| let vy = (pyend as *const u32).read_unaligned(); |
| vx == vy |
| } |
| } |