1use core::mem::size_of;
2
3use crate::memmem::{util::memcmp, vector::Vector, NeedleInfo};
4
5/// The minimum length of a needle required for this algorithm. The minimum
6/// is 2 since a length of 1 should just use memchr and a length of 0 isn't
7/// a case handled by this searcher.
8pub(crate) const MIN_NEEDLE_LEN: usize = 2;
9
10/// The maximum length of a needle required for this algorithm.
11///
12/// In reality, there is no hard max here. The code below can handle any
13/// length needle. (Perhaps that suggests there are missing optimizations.)
14/// Instead, this is a heuristic and a bound guaranteeing our linear time
15/// complexity.
16///
17/// It is a heuristic because when a candidate match is found, memcmp is run.
18/// For very large needles with lots of false positives, memcmp can make the
19/// code run quite slow.
20///
21/// It is a bound because the worst case behavior with memcmp is multiplicative
22/// in the size of the needle and haystack, and we want to keep that additive.
23/// This bound ensures we still meet that bound theoretically, since it's just
24/// a constant. We aren't acting in bad faith here, memcmp on tiny needles
25/// is so fast that even in pathological cases (see pathological vector
26/// benchmarks), this is still just as fast or faster in practice.
27///
28/// This specific number was chosen by tweaking a bit and running benchmarks.
29/// The rare-medium-needle, for example, gets about 5% faster by using this
30/// algorithm instead of a prefilter-accelerated Two-Way. There's also a
31/// theoretical desire to keep this number reasonably low, to mitigate the
32/// impact of pathological cases. I did try 64, and some benchmarks got a
33/// little better, and others (particularly the pathological ones), got a lot
34/// worse. So... 32 it is?
35pub(crate) const MAX_NEEDLE_LEN: usize = 32;
36
37/// The implementation of the forward vector accelerated substring search.
38///
39/// This is extremely similar to the prefilter vector module by the same name.
40/// The key difference is that this is not a prefilter. Instead, it handles
41/// confirming its own matches. The trade off is that this only works with
42/// smaller needles. The speed up here is that an inlined memcmp on a tiny
43/// needle is very quick, even on pathological inputs. This is much better than
44/// combining a prefilter with Two-Way, where using Two-Way to confirm the
45/// match has higher latency.
46///
47/// So why not use this for all needles? We could, and it would probably work
48/// really well on most inputs. But its worst case is multiplicative and we
49/// want to guarantee worst case additive time. Some of the benchmarks try to
50/// justify this (see the pathological ones).
51///
52/// The prefilter variant of this has more comments. Also note that we only
53/// implement this for forward searches for now. If you have a compelling use
54/// case for accelerated reverse search, please file an issue.
55#[derive(Clone, Copy, Debug)]
56pub(crate) struct Forward {
57    rare1i: u8,
58    rare2i: u8,
59}
60
61impl Forward {
62    /// Create a new "generic simd" forward searcher. If one could not be
63    /// created from the given inputs, then None is returned.
64    pub(crate) fn new(ninfo: &NeedleInfo, needle: &[u8]) -> Option<Forward> {
65        let (rare1i, rare2i) = ninfo.rarebytes.as_rare_ordered_u8();
66        // If the needle is too short or too long, give up. Also, give up
67        // if the rare bytes detected are at the same position. (It likely
68        // suggests a degenerate case, although it should technically not be
69        // possible.)
70        if needle.len() < MIN_NEEDLE_LEN
71            || needle.len() > MAX_NEEDLE_LEN
72            || rare1i == rare2i
73        {
74            return None;
75        }
76        Some(Forward { rare1i, rare2i })
77    }
78
79    /// Returns the minimum length of haystack that is needed for this searcher
80    /// to work for a particular vector. Passing a haystack with a length
81    /// smaller than this will cause `fwd_find` to panic.
82    #[inline(always)]
83    pub(crate) fn min_haystack_len<V: Vector>(&self) -> usize {
84        self.rare2i as usize + size_of::<V>()
85    }
86}
87
88/// Searches the given haystack for the given needle. The needle given should
89/// be the same as the needle that this searcher was initialized with.
90///
91/// # Panics
92///
93/// When the given haystack has a length smaller than `min_haystack_len`.
94///
95/// # Safety
96///
97/// Since this is meant to be used with vector functions, callers need to
98/// specialize this inside of a function with a `target_feature` attribute.
99/// Therefore, callers must ensure that whatever target feature is being used
100/// supports the vector functions that this function is specialized for. (For
101/// the specific vector functions used, see the Vector trait implementations.)
102#[inline(always)]
103pub(crate) unsafe fn fwd_find<V: Vector>(
104    fwd: &Forward,
105    haystack: &[u8],
106    needle: &[u8],
107) -> Option<usize> {
108    // It would be nice if we didn't have this check here, since the meta
109    // searcher should handle it for us. But without this, I don't think we
110    // guarantee that end_ptr.sub(needle.len()) won't result in UB. We could
111    // put it as part of the safety contract, but it makes it more complicated
112    // than necessary.
113    if haystack.len() < needle.len() {
114        return None;
115    }
116    let min_haystack_len = fwd.min_haystack_len::<V>();
117    assert!(haystack.len() >= min_haystack_len, "haystack too small");
118    debug_assert!(needle.len() <= haystack.len());
119    debug_assert!(
120        needle.len() >= MIN_NEEDLE_LEN,
121        "needle must be at least {} bytes",
122        MIN_NEEDLE_LEN,
123    );
124    debug_assert!(
125        needle.len() <= MAX_NEEDLE_LEN,
126        "needle must be at most {} bytes",
127        MAX_NEEDLE_LEN,
128    );
129
130    let (rare1i, rare2i) = (fwd.rare1i as usize, fwd.rare2i as usize);
131    let rare1chunk = V::splat(needle[rare1i]);
132    let rare2chunk = V::splat(needle[rare2i]);
133
134    let start_ptr = haystack.as_ptr();
135    let end_ptr = start_ptr.add(haystack.len());
136    let max_ptr = end_ptr.sub(min_haystack_len);
137    let mut ptr = start_ptr;
138
139    // N.B. I did experiment with unrolling the loop to deal with size(V)
140    // bytes at a time and 2*size(V) bytes at a time. The double unroll was
141    // marginally faster while the quadruple unroll was unambiguously slower.
142    // In the end, I decided the complexity from unrolling wasn't worth it. I
143    // used the memmem/krate/prebuilt/huge-en/ benchmarks to compare.
144    while ptr <= max_ptr {
145        let m = fwd_find_in_chunk(
146            fwd, needle, ptr, end_ptr, rare1chunk, rare2chunk, !0,
147        );
148        if let Some(chunki) = m {
149            return Some(matched(start_ptr, ptr, chunki));
150        }
151        ptr = ptr.add(size_of::<V>());
152    }
153    if ptr < end_ptr {
154        let remaining = diff(end_ptr, ptr);
155        debug_assert!(
156            remaining < min_haystack_len,
157            "remaining bytes should be smaller than the minimum haystack \
158             length of {}, but there are {} bytes remaining",
159            min_haystack_len,
160            remaining,
161        );
162        if remaining < needle.len() {
163            return None;
164        }
165        debug_assert!(
166            max_ptr < ptr,
167            "after main loop, ptr should have exceeded max_ptr",
168        );
169        let overlap = diff(ptr, max_ptr);
170        debug_assert!(
171            overlap > 0,
172            "overlap ({}) must always be non-zero",
173            overlap,
174        );
175        debug_assert!(
176            overlap < size_of::<V>(),
177            "overlap ({}) cannot possibly be >= than a vector ({})",
178            overlap,
179            size_of::<V>(),
180        );
181        // The mask has all of its bits set except for the first N least
182        // significant bits, where N=overlap. This way, any matches that
183        // occur in find_in_chunk within the overlap are automatically
184        // ignored.
185        let mask = !((1 << overlap) - 1);
186        ptr = max_ptr;
187        let m = fwd_find_in_chunk(
188            fwd, needle, ptr, end_ptr, rare1chunk, rare2chunk, mask,
189        );
190        if let Some(chunki) = m {
191            return Some(matched(start_ptr, ptr, chunki));
192        }
193    }
194    None
195}
196
197/// Search for an occurrence of two rare bytes from the needle in the chunk
198/// pointed to by ptr, with the end of the haystack pointed to by end_ptr. When
199/// an occurrence is found, memcmp is run to check if a match occurs at the
200/// corresponding position.
201///
202/// rare1chunk and rare2chunk correspond to vectors with the rare1 and rare2
203/// bytes repeated in each 8-bit lane, respectively.
204///
205/// mask should have bits set corresponding the positions in the chunk in which
206/// matches are considered. This is only used for the last vector load where
207/// the beginning of the vector might have overlapped with the last load in
208/// the main loop. The mask lets us avoid visiting positions that have already
209/// been discarded as matches.
210///
211/// # Safety
212///
213/// It must be safe to do an unaligned read of size(V) bytes starting at both
214/// (ptr + rare1i) and (ptr + rare2i). It must also be safe to do unaligned
215/// loads on ptr up to (end_ptr - needle.len()).
216#[inline(always)]
217unsafe fn fwd_find_in_chunk<V: Vector>(
218    fwd: &Forward,
219    needle: &[u8],
220    ptr: *const u8,
221    end_ptr: *const u8,
222    rare1chunk: V,
223    rare2chunk: V,
224    mask: u32,
225) -> Option<usize> {
226    let chunk0 = V::load_unaligned(ptr.add(fwd.rare1i as usize));
227    let chunk1 = V::load_unaligned(ptr.add(fwd.rare2i as usize));
228
229    let eq0 = chunk0.cmpeq(rare1chunk);
230    let eq1 = chunk1.cmpeq(rare2chunk);
231
232    let mut match_offsets = eq0.and(eq1).movemask() & mask;
233    while match_offsets != 0 {
234        let offset = match_offsets.trailing_zeros() as usize;
235        let ptr = ptr.add(offset);
236        if end_ptr.sub(needle.len()) < ptr {
237            return None;
238        }
239        let chunk = core::slice::from_raw_parts(ptr, needle.len());
240        if memcmp(needle, chunk) {
241            return Some(offset);
242        }
243        match_offsets &= match_offsets - 1;
244    }
245    None
246}
247
248/// Accepts a chunk-relative offset and returns a haystack relative offset
249/// after updating the prefilter state.
250///
251/// See the same function with the same name in the prefilter variant of this
252/// algorithm to learned why it's tagged with inline(never). Even here, where
253/// the function is simpler, inlining it leads to poorer codegen. (Although
254/// it does improve some benchmarks, like prebuiltiter/huge-en/common-you.)
255#[cold]
256#[inline(never)]
257fn matched(start_ptr: *const u8, ptr: *const u8, chunki: usize) -> usize {
258    diff(ptr, start_ptr) + chunki
259}
260
261/// Subtract `b` from `a` and return the difference. `a` must be greater than
262/// or equal to `b`.
263fn diff(a: *const u8, b: *const u8) -> usize {
264    debug_assert!(a >= b);
265    (a as usize) - (b as usize)
266}
267