1/*!
2Generic crate-internal routines for the "packed pair" SIMD algorithm.
3
4The "packed pair" algorithm is based on the [generic SIMD] algorithm. The main
5difference is that it (by default) uses a background distribution of byte
6frequencies to heuristically select the pair of bytes to search for.
7
8[generic SIMD]: http://0x80.pl/articles/simd-strfind.html#first-and-last
9*/
10
11use crate::{
12 arch::all::{is_equal_raw, packedpair::Pair},
13 ext::Pointer,
14 vector::{MoveMask, Vector},
15};
16
17/// A generic architecture dependent "packed pair" finder.
18///
19/// This finder picks two bytes that it believes have high predictive power
20/// for indicating an overall match of a needle. Depending on whether
21/// `Finder::find` or `Finder::find_prefilter` is used, it reports offsets
22/// where the needle matches or could match. In the prefilter case, candidates
23/// are reported whenever the [`Pair`] of bytes given matches.
24///
25/// This is architecture dependent because it uses specific vector operations
26/// to look for occurrences of the pair of bytes.
27///
28/// This type is not meant to be exported and is instead meant to be used as
29/// the implementation for architecture specific facades. Why? Because it's a
30/// bit of a quirky API that requires `inline(always)` annotations. And pretty
31/// much everything has safety obligations due (at least) to the caller needing
32/// to inline calls into routines marked with
33/// `#[target_feature(enable = "...")]`.
34#[derive(Clone, Copy, Debug)]
35pub(crate) struct Finder<V> {
36 pair: Pair,
37 v1: V,
38 v2: V,
39 min_haystack_len: usize,
40}
41
42impl<V: Vector> Finder<V> {
43 /// Create a new pair searcher. The searcher returned can either report
44 /// exact matches of `needle` or act as a prefilter and report candidate
45 /// positions of `needle`.
46 ///
47 /// # Safety
48 ///
49 /// Callers must ensure that whatever vector type this routine is called
50 /// with is supported by the current environment.
51 ///
52 /// Callers must also ensure that `needle.len() >= 2`.
53 #[inline(always)]
54 pub(crate) unsafe fn new(needle: &[u8], pair: Pair) -> Finder<V> {
55 let max_index = pair.index1().max(pair.index2());
56 let min_haystack_len =
57 core::cmp::max(needle.len(), usize::from(max_index) + V::BYTES);
58 let v1 = V::splat(needle[usize::from(pair.index1())]);
59 let v2 = V::splat(needle[usize::from(pair.index2())]);
60 Finder { pair, v1, v2, min_haystack_len }
61 }
62
63 /// Searches the given haystack for the given needle. The needle given
64 /// should be the same as the needle that this finder was initialized
65 /// with.
66 ///
67 /// # Panics
68 ///
69 /// When `haystack.len()` is less than [`Finder::min_haystack_len`].
70 ///
71 /// # Safety
72 ///
73 /// Since this is meant to be used with vector functions, callers need to
74 /// specialize this inside of a function with a `target_feature` attribute.
75 /// Therefore, callers must ensure that whatever target feature is being
76 /// used supports the vector functions that this function is specialized
77 /// for. (For the specific vector functions used, see the Vector trait
78 /// implementations.)
79 #[inline(always)]
80 pub(crate) unsafe fn find(
81 &self,
82 haystack: &[u8],
83 needle: &[u8],
84 ) -> Option<usize> {
85 assert!(
86 haystack.len() >= self.min_haystack_len,
87 "haystack too small, should be at least {} but got {}",
88 self.min_haystack_len,
89 haystack.len(),
90 );
91
92 let all = V::Mask::all_zeros_except_least_significant(0);
93 let start = haystack.as_ptr();
94 let end = start.add(haystack.len());
95 let max = end.sub(self.min_haystack_len);
96 let mut cur = start;
97
98 // N.B. I did experiment with unrolling the loop to deal with size(V)
99 // bytes at a time and 2*size(V) bytes at a time. The double unroll
100 // was marginally faster while the quadruple unroll was unambiguously
101 // slower. In the end, I decided the complexity from unrolling wasn't
102 // worth it. I used the memmem/krate/prebuilt/huge-en/ benchmarks to
103 // compare.
104 while cur <= max {
105 if let Some(chunki) = self.find_in_chunk(needle, cur, end, all) {
106 return Some(matched(start, cur, chunki));
107 }
108 cur = cur.add(V::BYTES);
109 }
110 if cur < end {
111 let remaining = end.distance(cur);
112 debug_assert!(
113 remaining < self.min_haystack_len,
114 "remaining bytes should be smaller than the minimum haystack \
115 length of {}, but there are {} bytes remaining",
116 self.min_haystack_len,
117 remaining,
118 );
119 if remaining < needle.len() {
120 return None;
121 }
122 debug_assert!(
123 max < cur,
124 "after main loop, cur should have exceeded max",
125 );
126 let overlap = cur.distance(max);
127 debug_assert!(
128 overlap > 0,
129 "overlap ({}) must always be non-zero",
130 overlap,
131 );
132 debug_assert!(
133 overlap < V::BYTES,
134 "overlap ({}) cannot possibly be >= than a vector ({})",
135 overlap,
136 V::BYTES,
137 );
138 // The mask has all of its bits set except for the first N least
139 // significant bits, where N=overlap. This way, any matches that
140 // occur in find_in_chunk within the overlap are automatically
141 // ignored.
142 let mask = V::Mask::all_zeros_except_least_significant(overlap);
143 cur = max;
144 let m = self.find_in_chunk(needle, cur, end, mask);
145 if let Some(chunki) = m {
146 return Some(matched(start, cur, chunki));
147 }
148 }
149 None
150 }
151
152 /// Searches the given haystack for offsets that represent candidate
153 /// matches of the `needle` given to this finder's constructor. The offsets
154 /// returned, if they are a match, correspond to the starting offset of
155 /// `needle` in the given `haystack`.
156 ///
157 /// # Panics
158 ///
159 /// When `haystack.len()` is less than [`Finder::min_haystack_len`].
160 ///
161 /// # Safety
162 ///
163 /// Since this is meant to be used with vector functions, callers need to
164 /// specialize this inside of a function with a `target_feature` attribute.
165 /// Therefore, callers must ensure that whatever target feature is being
166 /// used supports the vector functions that this function is specialized
167 /// for. (For the specific vector functions used, see the Vector trait
168 /// implementations.)
169 #[inline(always)]
170 pub(crate) unsafe fn find_prefilter(
171 &self,
172 haystack: &[u8],
173 ) -> Option<usize> {
174 assert!(
175 haystack.len() >= self.min_haystack_len,
176 "haystack too small, should be at least {} but got {}",
177 self.min_haystack_len,
178 haystack.len(),
179 );
180
181 let start = haystack.as_ptr();
182 let end = start.add(haystack.len());
183 let max = end.sub(self.min_haystack_len);
184 let mut cur = start;
185
186 // N.B. I did experiment with unrolling the loop to deal with size(V)
187 // bytes at a time and 2*size(V) bytes at a time. The double unroll
188 // was marginally faster while the quadruple unroll was unambiguously
189 // slower. In the end, I decided the complexity from unrolling wasn't
190 // worth it. I used the memmem/krate/prebuilt/huge-en/ benchmarks to
191 // compare.
192 while cur <= max {
193 if let Some(chunki) = self.find_prefilter_in_chunk(cur) {
194 return Some(matched(start, cur, chunki));
195 }
196 cur = cur.add(V::BYTES);
197 }
198 if cur < end {
199 // This routine immediately quits if a candidate match is found.
200 // That means that if we're here, no candidate matches have been
201 // found at or before 'ptr'. Thus, we don't need to mask anything
202 // out even though we might technically search part of the haystack
203 // that we've already searched (because we know it can't match).
204 cur = max;
205 if let Some(chunki) = self.find_prefilter_in_chunk(cur) {
206 return Some(matched(start, cur, chunki));
207 }
208 }
209 None
210 }
211
212 /// Search for an occurrence of our byte pair from the needle in the chunk
213 /// pointed to by cur, with the end of the haystack pointed to by end.
214 /// When an occurrence is found, memcmp is run to check if a match occurs
215 /// at the corresponding position.
216 ///
217 /// `mask` should have bits set corresponding the positions in the chunk
218 /// in which matches are considered. This is only used for the last vector
219 /// load where the beginning of the vector might have overlapped with the
220 /// last load in the main loop. The mask lets us avoid visiting positions
221 /// that have already been discarded as matches.
222 ///
223 /// # Safety
224 ///
225 /// It must be safe to do an unaligned read of size(V) bytes starting at
226 /// both (cur + self.index1) and (cur + self.index2). It must also be safe
227 /// to do unaligned loads on cur up to (end - needle.len()).
228 #[inline(always)]
229 unsafe fn find_in_chunk(
230 &self,
231 needle: &[u8],
232 cur: *const u8,
233 end: *const u8,
234 mask: V::Mask,
235 ) -> Option<usize> {
236 let index1 = usize::from(self.pair.index1());
237 let index2 = usize::from(self.pair.index2());
238 let chunk1 = V::load_unaligned(cur.add(index1));
239 let chunk2 = V::load_unaligned(cur.add(index2));
240 let eq1 = chunk1.cmpeq(self.v1);
241 let eq2 = chunk2.cmpeq(self.v2);
242
243 let mut offsets = eq1.and(eq2).movemask().and(mask);
244 while offsets.has_non_zero() {
245 let offset = offsets.first_offset();
246 let cur = cur.add(offset);
247 if end.sub(needle.len()) < cur {
248 return None;
249 }
250 if is_equal_raw(needle.as_ptr(), cur, needle.len()) {
251 return Some(offset);
252 }
253 offsets = offsets.clear_least_significant_bit();
254 }
255 None
256 }
257
258 /// Search for an occurrence of our byte pair from the needle in the chunk
259 /// pointed to by cur, with the end of the haystack pointed to by end.
260 /// When an occurrence is found, memcmp is run to check if a match occurs
261 /// at the corresponding position.
262 ///
263 /// # Safety
264 ///
265 /// It must be safe to do an unaligned read of size(V) bytes starting at
266 /// both (cur + self.index1) and (cur + self.index2). It must also be safe
267 /// to do unaligned reads on cur up to (end - needle.len()).
268 #[inline(always)]
269 unsafe fn find_prefilter_in_chunk(&self, cur: *const u8) -> Option<usize> {
270 let index1 = usize::from(self.pair.index1());
271 let index2 = usize::from(self.pair.index2());
272 let chunk1 = V::load_unaligned(cur.add(index1));
273 let chunk2 = V::load_unaligned(cur.add(index2));
274 let eq1 = chunk1.cmpeq(self.v1);
275 let eq2 = chunk2.cmpeq(self.v2);
276
277 let offsets = eq1.and(eq2).movemask();
278 if !offsets.has_non_zero() {
279 return None;
280 }
281 Some(offsets.first_offset())
282 }
283
284 /// Returns the pair of offsets (into the needle) used to check as a
285 /// predicate before confirming whether a needle exists at a particular
286 /// position.
287 #[inline]
288 pub(crate) fn pair(&self) -> &Pair {
289 &self.pair
290 }
291
292 /// Returns the minimum haystack length that this `Finder` can search.
293 ///
294 /// Providing a haystack to this `Finder` shorter than this length is
295 /// guaranteed to result in a panic.
296 #[inline(always)]
297 pub(crate) fn min_haystack_len(&self) -> usize {
298 self.min_haystack_len
299 }
300}
301
302/// Accepts a chunk-relative offset and returns a haystack relative offset.
303///
304/// This used to be marked `#[cold]` and `#[inline(never)]`, but I couldn't
305/// observe a consistent measureable difference between that and just inlining
306/// it. So we go with inlining it.
307///
308/// # Safety
309///
310/// Same at `ptr::offset_from` in addition to `cur >= start`.
311#[inline(always)]
312unsafe fn matched(start: *const u8, cur: *const u8, chunki: usize) -> usize {
313 cur.distance(origin:start) + chunki
314}
315
316// If you're looking for tests, those are run for each instantiation of the
317// above code. So for example, see arch::x86_64::sse2::packedpair.
318