| /*! |
| Generic crate-internal routines for the "packed pair" SIMD algorithm. |
| |
| The "packed pair" algorithm is based on the [generic SIMD] algorithm. The main |
| difference is that it (by default) uses a background distribution of byte |
| frequencies to heuristically select the pair of bytes to search for. |
| |
| [generic SIMD]: http://0x80.pl/articles/simd-strfind.html#first-and-last |
| */ |
| |
| use crate::{ |
| arch::all::{is_equal_raw, packedpair::Pair}, |
| ext::Pointer, |
| vector::{MoveMask, Vector}, |
| }; |
| |
| /// A generic architecture dependent "packed pair" finder. |
| /// |
| /// This finder picks two bytes that it believes have high predictive power |
| /// for indicating an overall match of a needle. Depending on whether |
| /// `Finder::find` or `Finder::find_prefilter` is used, it reports offsets |
| /// where the needle matches or could match. In the prefilter case, candidates |
| /// are reported whenever the [`Pair`] of bytes given matches. |
| /// |
| /// This is architecture dependent because it uses specific vector operations |
| /// to look for occurrences of the pair of bytes. |
| /// |
| /// This type is not meant to be exported and is instead meant to be used as |
| /// the implementation for architecture specific facades. Why? Because it's a |
| /// bit of a quirky API that requires `inline(always)` annotations. And pretty |
| /// much everything has safety obligations due (at least) to the caller needing |
| /// to inline calls into routines marked with |
| /// `#[target_feature(enable = "...")]`. |
| #[derive(Clone, Copy, Debug)] |
| pub(crate) struct Finder<V> { |
| pair: Pair, |
| v1: V, |
| v2: V, |
| min_haystack_len: usize, |
| } |
| |
| impl<V: Vector> Finder<V> { |
| /// Create a new pair searcher. The searcher returned can either report |
| /// exact matches of `needle` or act as a prefilter and report candidate |
| /// positions of `needle`. |
| /// |
| /// # Safety |
| /// |
| /// Callers must ensure that whatever vector type this routine is called |
| /// with is supported by the current environment. |
| /// |
| /// Callers must also ensure that `needle.len() >= 2`. |
| #[inline(always)] |
| pub(crate) unsafe fn new(needle: &[u8], pair: Pair) -> Finder<V> { |
| let max_index = pair.index1().max(pair.index2()); |
| let min_haystack_len = |
| core::cmp::max(needle.len(), usize::from(max_index) + V::BYTES); |
| let v1 = V::splat(needle[usize::from(pair.index1())]); |
| let v2 = V::splat(needle[usize::from(pair.index2())]); |
| Finder { pair, v1, v2, min_haystack_len } |
| } |
| |
| /// Searches the given haystack for the given needle. The needle given |
| /// should be the same as the needle that this finder was initialized |
| /// with. |
| /// |
| /// # Panics |
| /// |
| /// When `haystack.len()` is less than [`Finder::min_haystack_len`]. |
| /// |
| /// # Safety |
| /// |
| /// Since this is meant to be used with vector functions, callers need to |
| /// specialize this inside of a function with a `target_feature` attribute. |
| /// Therefore, callers must ensure that whatever target feature is being |
| /// used supports the vector functions that this function is specialized |
| /// for. (For the specific vector functions used, see the Vector trait |
| /// implementations.) |
| #[inline(always)] |
| pub(crate) unsafe fn find( |
| &self, |
| haystack: &[u8], |
| needle: &[u8], |
| ) -> Option<usize> { |
| assert!( |
| haystack.len() >= self.min_haystack_len, |
| "haystack too small, should be at least {} but got {}", |
| self.min_haystack_len, |
| haystack.len(), |
| ); |
| |
| let all = V::Mask::all_zeros_except_least_significant(0); |
| let start = haystack.as_ptr(); |
| let end = start.add(haystack.len()); |
| let max = end.sub(self.min_haystack_len); |
| let mut cur = start; |
| |
| // N.B. I did experiment with unrolling the loop to deal with size(V) |
| // bytes at a time and 2*size(V) bytes at a time. The double unroll |
| // was marginally faster while the quadruple unroll was unambiguously |
| // slower. In the end, I decided the complexity from unrolling wasn't |
| // worth it. I used the memmem/krate/prebuilt/huge-en/ benchmarks to |
| // compare. |
| while cur <= max { |
| if let Some(chunki) = self.find_in_chunk(needle, cur, end, all) { |
| return Some(matched(start, cur, chunki)); |
| } |
| cur = cur.add(V::BYTES); |
| } |
| if cur < end { |
| let remaining = end.distance(cur); |
| debug_assert!( |
| remaining < self.min_haystack_len, |
| "remaining bytes should be smaller than the minimum haystack \ |
| length of {}, but there are {} bytes remaining", |
| self.min_haystack_len, |
| remaining, |
| ); |
| if remaining < needle.len() { |
| return None; |
| } |
| debug_assert!( |
| max < cur, |
| "after main loop, cur should have exceeded max", |
| ); |
| let overlap = cur.distance(max); |
| debug_assert!( |
| overlap > 0, |
| "overlap ({}) must always be non-zero", |
| overlap, |
| ); |
| debug_assert!( |
| overlap < V::BYTES, |
| "overlap ({}) cannot possibly be >= than a vector ({})", |
| overlap, |
| V::BYTES, |
| ); |
| // The mask has all of its bits set except for the first N least |
| // significant bits, where N=overlap. This way, any matches that |
| // occur in find_in_chunk within the overlap are automatically |
| // ignored. |
| let mask = V::Mask::all_zeros_except_least_significant(overlap); |
| cur = max; |
| let m = self.find_in_chunk(needle, cur, end, mask); |
| if let Some(chunki) = m { |
| return Some(matched(start, cur, chunki)); |
| } |
| } |
| None |
| } |
| |
| /// Searches the given haystack for offsets that represent candidate |
| /// matches of the `needle` given to this finder's constructor. The offsets |
| /// returned, if they are a match, correspond to the starting offset of |
| /// `needle` in the given `haystack`. |
| /// |
| /// # Panics |
| /// |
| /// When `haystack.len()` is less than [`Finder::min_haystack_len`]. |
| /// |
| /// # Safety |
| /// |
| /// Since this is meant to be used with vector functions, callers need to |
| /// specialize this inside of a function with a `target_feature` attribute. |
| /// Therefore, callers must ensure that whatever target feature is being |
| /// used supports the vector functions that this function is specialized |
| /// for. (For the specific vector functions used, see the Vector trait |
| /// implementations.) |
| #[inline(always)] |
| pub(crate) unsafe fn find_prefilter( |
| &self, |
| haystack: &[u8], |
| ) -> Option<usize> { |
| assert!( |
| haystack.len() >= self.min_haystack_len, |
| "haystack too small, should be at least {} but got {}", |
| self.min_haystack_len, |
| haystack.len(), |
| ); |
| |
| let start = haystack.as_ptr(); |
| let end = start.add(haystack.len()); |
| let max = end.sub(self.min_haystack_len); |
| let mut cur = start; |
| |
| // N.B. I did experiment with unrolling the loop to deal with size(V) |
| // bytes at a time and 2*size(V) bytes at a time. The double unroll |
| // was marginally faster while the quadruple unroll was unambiguously |
| // slower. In the end, I decided the complexity from unrolling wasn't |
| // worth it. I used the memmem/krate/prebuilt/huge-en/ benchmarks to |
| // compare. |
| while cur <= max { |
| if let Some(chunki) = self.find_prefilter_in_chunk(cur) { |
| return Some(matched(start, cur, chunki)); |
| } |
| cur = cur.add(V::BYTES); |
| } |
| if cur < end { |
| // This routine immediately quits if a candidate match is found. |
| // That means that if we're here, no candidate matches have been |
| // found at or before 'ptr'. Thus, we don't need to mask anything |
| // out even though we might technically search part of the haystack |
| // that we've already searched (because we know it can't match). |
| cur = max; |
| if let Some(chunki) = self.find_prefilter_in_chunk(cur) { |
| return Some(matched(start, cur, chunki)); |
| } |
| } |
| None |
| } |
| |
| /// Search for an occurrence of our byte pair from the needle in the chunk |
| /// pointed to by cur, with the end of the haystack pointed to by end. |
| /// When an occurrence is found, memcmp is run to check if a match occurs |
| /// at the corresponding position. |
| /// |
| /// `mask` should have bits set corresponding the positions in the chunk |
| /// in which matches are considered. This is only used for the last vector |
| /// load where the beginning of the vector might have overlapped with the |
| /// last load in the main loop. The mask lets us avoid visiting positions |
| /// that have already been discarded as matches. |
| /// |
| /// # Safety |
| /// |
| /// It must be safe to do an unaligned read of size(V) bytes starting at |
| /// both (cur + self.index1) and (cur + self.index2). It must also be safe |
| /// to do unaligned loads on cur up to (end - needle.len()). |
| #[inline(always)] |
| unsafe fn find_in_chunk( |
| &self, |
| needle: &[u8], |
| cur: *const u8, |
| end: *const u8, |
| mask: V::Mask, |
| ) -> Option<usize> { |
| let index1 = usize::from(self.pair.index1()); |
| let index2 = usize::from(self.pair.index2()); |
| let chunk1 = V::load_unaligned(cur.add(index1)); |
| let chunk2 = V::load_unaligned(cur.add(index2)); |
| let eq1 = chunk1.cmpeq(self.v1); |
| let eq2 = chunk2.cmpeq(self.v2); |
| |
| let mut offsets = eq1.and(eq2).movemask().and(mask); |
| while offsets.has_non_zero() { |
| let offset = offsets.first_offset(); |
| let cur = cur.add(offset); |
| if end.sub(needle.len()) < cur { |
| return None; |
| } |
| if is_equal_raw(needle.as_ptr(), cur, needle.len()) { |
| return Some(offset); |
| } |
| offsets = offsets.clear_least_significant_bit(); |
| } |
| None |
| } |
| |
| /// Search for an occurrence of our byte pair from the needle in the chunk |
| /// pointed to by cur, with the end of the haystack pointed to by end. |
| /// When an occurrence is found, memcmp is run to check if a match occurs |
| /// at the corresponding position. |
| /// |
| /// # Safety |
| /// |
| /// It must be safe to do an unaligned read of size(V) bytes starting at |
| /// both (cur + self.index1) and (cur + self.index2). It must also be safe |
| /// to do unaligned reads on cur up to (end - needle.len()). |
| #[inline(always)] |
| unsafe fn find_prefilter_in_chunk(&self, cur: *const u8) -> Option<usize> { |
| let index1 = usize::from(self.pair.index1()); |
| let index2 = usize::from(self.pair.index2()); |
| let chunk1 = V::load_unaligned(cur.add(index1)); |
| let chunk2 = V::load_unaligned(cur.add(index2)); |
| let eq1 = chunk1.cmpeq(self.v1); |
| let eq2 = chunk2.cmpeq(self.v2); |
| |
| let offsets = eq1.and(eq2).movemask(); |
| if !offsets.has_non_zero() { |
| return None; |
| } |
| Some(offsets.first_offset()) |
| } |
| |
| /// Returns the pair of offsets (into the needle) used to check as a |
| /// predicate before confirming whether a needle exists at a particular |
| /// position. |
| #[inline] |
| pub(crate) fn pair(&self) -> &Pair { |
| &self.pair |
| } |
| |
| /// Returns the minimum haystack length that this `Finder` can search. |
| /// |
| /// Providing a haystack to this `Finder` shorter than this length is |
| /// guaranteed to result in a panic. |
| #[inline(always)] |
| pub(crate) fn min_haystack_len(&self) -> usize { |
| self.min_haystack_len |
| } |
| } |
| |
| /// Accepts a chunk-relative offset and returns a haystack relative offset. |
| /// |
| /// This used to be marked `#[cold]` and `#[inline(never)]`, but I couldn't |
| /// observe a consistent measureable difference between that and just inlining |
| /// it. So we go with inlining it. |
| /// |
| /// # Safety |
| /// |
| /// Same at `ptr::offset_from` in addition to `cur >= start`. |
| #[inline(always)] |
| unsafe fn matched(start: *const u8, cur: *const u8, chunki: usize) -> usize { |
| cur.distance(start) + chunki |
| } |
| |
| // If you're looking for tests, those are run for each instantiation of the |
| // above code. So for example, see arch::x86_64::sse2::packedpair. |
