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Diffstat (limited to 'vendor/memchr/src/memmem/genericsimd.rs')
-rw-r--r-- | vendor/memchr/src/memmem/genericsimd.rs | 266 |
1 files changed, 266 insertions, 0 deletions
diff --git a/vendor/memchr/src/memmem/genericsimd.rs b/vendor/memchr/src/memmem/genericsimd.rs new file mode 100644 index 000000000..28bfdab88 --- /dev/null +++ b/vendor/memchr/src/memmem/genericsimd.rs @@ -0,0 +1,266 @@ +use core::mem::size_of; + +use crate::memmem::{util::memcmp, vector::Vector, NeedleInfo}; + +/// The minimum length of a needle required for this algorithm. The minimum +/// is 2 since a length of 1 should just use memchr and a length of 0 isn't +/// a case handled by this searcher. +pub(crate) const MIN_NEEDLE_LEN: usize = 2; + +/// The maximum length of a needle required for this algorithm. +/// +/// In reality, there is no hard max here. The code below can handle any +/// length needle. (Perhaps that suggests there are missing optimizations.) +/// Instead, this is a heuristic and a bound guaranteeing our linear time +/// complexity. +/// +/// It is a heuristic because when a candidate match is found, memcmp is run. +/// For very large needles with lots of false positives, memcmp can make the +/// code run quite slow. +/// +/// It is a bound because the worst case behavior with memcmp is multiplicative +/// in the size of the needle and haystack, and we want to keep that additive. +/// This bound ensures we still meet that bound theoretically, since it's just +/// a constant. We aren't acting in bad faith here, memcmp on tiny needles +/// is so fast that even in pathological cases (see pathological vector +/// benchmarks), this is still just as fast or faster in practice. +/// +/// This specific number was chosen by tweaking a bit and running benchmarks. +/// The rare-medium-needle, for example, gets about 5% faster by using this +/// algorithm instead of a prefilter-accelerated Two-Way. There's also a +/// theoretical desire to keep this number reasonably low, to mitigate the +/// impact of pathological cases. I did try 64, and some benchmarks got a +/// little better, and others (particularly the pathological ones), got a lot +/// worse. So... 32 it is? +pub(crate) const MAX_NEEDLE_LEN: usize = 32; + +/// The implementation of the forward vector accelerated substring search. +/// +/// This is extremely similar to the prefilter vector module by the same name. +/// The key difference is that this is not a prefilter. Instead, it handles +/// confirming its own matches. The trade off is that this only works with +/// smaller needles. The speed up here is that an inlined memcmp on a tiny +/// needle is very quick, even on pathological inputs. This is much better than +/// combining a prefilter with Two-Way, where using Two-Way to confirm the +/// match has higher latency. +/// +/// So why not use this for all needles? We could, and it would probably work +/// really well on most inputs. But its worst case is multiplicative and we +/// want to guarantee worst case additive time. Some of the benchmarks try to +/// justify this (see the pathological ones). +/// +/// The prefilter variant of this has more comments. Also note that we only +/// implement this for forward searches for now. If you have a compelling use +/// case for accelerated reverse search, please file an issue. +#[derive(Clone, Copy, Debug)] +pub(crate) struct Forward { + rare1i: u8, + rare2i: u8, +} + +impl Forward { + /// Create a new "generic simd" forward searcher. If one could not be + /// created from the given inputs, then None is returned. + pub(crate) fn new(ninfo: &NeedleInfo, needle: &[u8]) -> Option<Forward> { + let (rare1i, rare2i) = ninfo.rarebytes.as_rare_ordered_u8(); + // If the needle is too short or too long, give up. Also, give up + // if the rare bytes detected are at the same position. (It likely + // suggests a degenerate case, although it should technically not be + // possible.) + if needle.len() < MIN_NEEDLE_LEN + || needle.len() > MAX_NEEDLE_LEN + || rare1i == rare2i + { + return None; + } + Some(Forward { rare1i, rare2i }) + } + + /// Returns the minimum length of haystack that is needed for this searcher + /// to work for a particular vector. Passing a haystack with a length + /// smaller than this will cause `fwd_find` to panic. + #[inline(always)] + pub(crate) fn min_haystack_len<V: Vector>(&self) -> usize { + self.rare2i as usize + size_of::<V>() + } +} + +/// Searches the given haystack for the given needle. The needle given should +/// be the same as the needle that this searcher was initialized with. +/// +/// # Panics +/// +/// When the given haystack has a length smaller than `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 fwd_find<V: Vector>( + fwd: &Forward, + haystack: &[u8], + needle: &[u8], +) -> Option<usize> { + // It would be nice if we didn't have this check here, since the meta + // searcher should handle it for us. But without this, I don't think we + // guarantee that end_ptr.sub(needle.len()) won't result in UB. We could + // put it as part of the safety contract, but it makes it more complicated + // than necessary. + if haystack.len() < needle.len() { + return None; + } + let min_haystack_len = fwd.min_haystack_len::<V>(); + assert!(haystack.len() >= min_haystack_len, "haystack too small"); + debug_assert!(needle.len() <= haystack.len()); + debug_assert!( + needle.len() >= MIN_NEEDLE_LEN, + "needle must be at least {} bytes", + MIN_NEEDLE_LEN, + ); + debug_assert!( + needle.len() <= MAX_NEEDLE_LEN, + "needle must be at most {} bytes", + MAX_NEEDLE_LEN, + ); + + let (rare1i, rare2i) = (fwd.rare1i as usize, fwd.rare2i as usize); + let rare1chunk = V::splat(needle[rare1i]); + let rare2chunk = V::splat(needle[rare2i]); + + let start_ptr = haystack.as_ptr(); + let end_ptr = start_ptr.add(haystack.len()); + let max_ptr = end_ptr.sub(min_haystack_len); + let mut ptr = start_ptr; + + // 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 ptr <= max_ptr { + let m = fwd_find_in_chunk( + fwd, needle, ptr, end_ptr, rare1chunk, rare2chunk, !0, + ); + if let Some(chunki) = m { + return Some(matched(start_ptr, ptr, chunki)); + } + ptr = ptr.add(size_of::<V>()); + } + if ptr < end_ptr { + let remaining = diff(end_ptr, ptr); + debug_assert!( + remaining < min_haystack_len, + "remaining bytes should be smaller than the minimum haystack \ + length of {}, but there are {} bytes remaining", + min_haystack_len, + remaining, + ); + if remaining < needle.len() { + return None; + } + debug_assert!( + max_ptr < ptr, + "after main loop, ptr should have exceeded max_ptr", + ); + let overlap = diff(ptr, max_ptr); + debug_assert!( + overlap > 0, + "overlap ({}) must always be non-zero", + overlap, + ); + debug_assert!( + overlap < size_of::<V>(), + "overlap ({}) cannot possibly be >= than a vector ({})", + overlap, + size_of::<V>(), + ); + // 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 = !((1 << overlap) - 1); + ptr = max_ptr; + let m = fwd_find_in_chunk( + fwd, needle, ptr, end_ptr, rare1chunk, rare2chunk, mask, + ); + if let Some(chunki) = m { + return Some(matched(start_ptr, ptr, chunki)); + } + } + None +} + +/// Search for an occurrence of two rare bytes from the needle in the chunk +/// pointed to by ptr, with the end of the haystack pointed to by end_ptr. When +/// an occurrence is found, memcmp is run to check if a match occurs at the +/// corresponding position. +/// +/// rare1chunk and rare2chunk correspond to vectors with the rare1 and rare2 +/// bytes repeated in each 8-bit lane, respectively. +/// +/// 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 +/// (ptr + rare1i) and (ptr + rare2i). It must also be safe to do unaligned +/// loads on ptr up to (end_ptr - needle.len()). +#[inline(always)] +unsafe fn fwd_find_in_chunk<V: Vector>( + fwd: &Forward, + needle: &[u8], + ptr: *const u8, + end_ptr: *const u8, + rare1chunk: V, + rare2chunk: V, + mask: u32, +) -> Option<usize> { + let chunk0 = V::load_unaligned(ptr.add(fwd.rare1i as usize)); + let chunk1 = V::load_unaligned(ptr.add(fwd.rare2i as usize)); + + let eq0 = chunk0.cmpeq(rare1chunk); + let eq1 = chunk1.cmpeq(rare2chunk); + + let mut match_offsets = eq0.and(eq1).movemask() & mask; + while match_offsets != 0 { + let offset = match_offsets.trailing_zeros() as usize; + let ptr = ptr.add(offset); + if end_ptr.sub(needle.len()) < ptr { + return None; + } + let chunk = core::slice::from_raw_parts(ptr, needle.len()); + if memcmp(needle, chunk) { + return Some(offset); + } + match_offsets &= match_offsets - 1; + } + None +} + +/// Accepts a chunk-relative offset and returns a haystack relative offset +/// after updating the prefilter state. +/// +/// See the same function with the same name in the prefilter variant of this +/// algorithm to learned why it's tagged with inline(never). Even here, where +/// the function is simpler, inlining it leads to poorer codegen. (Although +/// it does improve some benchmarks, like prebuiltiter/huge-en/common-you.) +#[cold] +#[inline(never)] +fn matched(start_ptr: *const u8, ptr: *const u8, chunki: usize) -> usize { + diff(ptr, start_ptr) + chunki +} + +/// Subtract `b` from `a` and return the difference. `a` must be greater than +/// or equal to `b`. +fn diff(a: *const u8, b: *const u8) -> usize { + debug_assert!(a >= b); + (a as usize) - (b as usize) +} |