1 // These routines are meant to be optimized specifically for low latency as
2 // compared to the equivalent routines offered by std. (Which may invoke the
3 // dynamic linker and call out to libc, which introduces a bit more latency
4 // than we'd like.)
5
6 /// Returns true if and only if needle is a prefix of haystack.
7 #[inline(always)]
is_prefix(haystack: &[u8], needle: &[u8]) -> bool8 pub(crate) fn is_prefix(haystack: &[u8], needle: &[u8]) -> bool {
9 needle.len() <= haystack.len() && memcmp(&haystack[..needle.len()], needle)
10 }
11
12 /// Returns true if and only if needle is a suffix of haystack.
13 #[inline(always)]
is_suffix(haystack: &[u8], needle: &[u8]) -> bool14 pub(crate) fn is_suffix(haystack: &[u8], needle: &[u8]) -> bool {
15 needle.len() <= haystack.len()
16 && memcmp(&haystack[haystack.len() - needle.len()..], needle)
17 }
18
19 /// Return true if and only if x.len() == y.len() && x[i] == y[i] for all
20 /// 0 <= i < x.len().
21 ///
22 /// Why not just use actual memcmp for this? Well, memcmp requires calling out
23 /// to libc, and this routine is called in fairly hot code paths. Other than
24 /// just calling out to libc, it also seems to result in worse codegen. By
25 /// rolling our own memcmp in pure Rust, it seems to appear more friendly to
26 /// the optimizer.
27 ///
28 /// We mark this as inline always, although, some callers may not want it
29 /// inlined for better codegen (like Rabin-Karp). In that case, callers are
30 /// advised to create a non-inlineable wrapper routine that calls memcmp.
31 #[inline(always)]
memcmp(x: &[u8], y: &[u8]) -> bool32 pub(crate) fn memcmp(x: &[u8], y: &[u8]) -> bool {
33 if x.len() != y.len() {
34 return false;
35 }
36 // If we don't have enough bytes to do 4-byte at a time loads, then
37 // fall back to the naive slow version.
38 //
39 // TODO: We could do a copy_nonoverlapping combined with a mask instead
40 // of a loop. Benchmark it.
41 if x.len() < 4 {
42 for (&b1, &b2) in x.iter().zip(y) {
43 if b1 != b2 {
44 return false;
45 }
46 }
47 return true;
48 }
49 // When we have 4 or more bytes to compare, then proceed in chunks of 4 at
50 // a time using unaligned loads.
51 //
52 // Also, why do 4 byte loads instead of, say, 8 byte loads? The reason is
53 // that this particular version of memcmp is likely to be called with tiny
54 // needles. That means that if we do 8 byte loads, then a higher proportion
55 // of memcmp calls will use the slower variant above. With that said, this
56 // is a hypothesis and is only loosely supported by benchmarks. There's
57 // likely some improvement that could be made here. The main thing here
58 // though is to optimize for latency, not throughput.
59
60 // SAFETY: Via the conditional above, we know that both `px` and `py`
61 // have the same length, so `px < pxend` implies that `py < pyend`.
62 // Thus, derefencing both `px` and `py` in the loop below is safe.
63 //
64 // Moreover, we set `pxend` and `pyend` to be 4 bytes before the actual
65 // end of of `px` and `py`. Thus, the final dereference outside of the
66 // loop is guaranteed to be valid. (The final comparison will overlap with
67 // the last comparison done in the loop for lengths that aren't multiples
68 // of four.)
69 //
70 // Finally, we needn't worry about alignment here, since we do unaligned
71 // loads.
72 unsafe {
73 let (mut px, mut py) = (x.as_ptr(), y.as_ptr());
74 let (pxend, pyend) = (px.add(x.len() - 4), py.add(y.len() - 4));
75 while px < pxend {
76 let vx = (px as *const u32).read_unaligned();
77 let vy = (py as *const u32).read_unaligned();
78 if vx != vy {
79 return false;
80 }
81 px = px.add(4);
82 py = py.add(4);
83 }
84 let vx = (pxend as *const u32).read_unaligned();
85 let vy = (pyend as *const u32).read_unaligned();
86 vx == vy
87 }
88 }
89