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Diffstat (limited to 'vendor/memchr/src/arch/generic/packedpair.rs')
| -rw-r--r-- | vendor/memchr/src/arch/generic/packedpair.rs | 317 |
1 files changed, 317 insertions, 0 deletions
diff --git a/vendor/memchr/src/arch/generic/packedpair.rs b/vendor/memchr/src/arch/generic/packedpair.rs new file mode 100644 index 0000000..8d97cf2 --- /dev/null +++ b/vendor/memchr/src/arch/generic/packedpair.rs @@ -0,0 +1,317 @@ +/*! +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. |