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Diffstat (limited to 'vendor/memchr/src/memmem/prefilter/genericsimd.rs')
-rw-r--r-- | vendor/memchr/src/memmem/prefilter/genericsimd.rs | 207 |
1 files changed, 207 insertions, 0 deletions
diff --git a/vendor/memchr/src/memmem/prefilter/genericsimd.rs b/vendor/memchr/src/memmem/prefilter/genericsimd.rs new file mode 100644 index 000000000..1a6e38734 --- /dev/null +++ b/vendor/memchr/src/memmem/prefilter/genericsimd.rs @@ -0,0 +1,207 @@ +use core::mem::size_of; + +use crate::memmem::{ + prefilter::{PrefilterFnTy, PrefilterState}, + vector::Vector, + NeedleInfo, +}; + +/// The implementation of the forward vector accelerated candidate finder. +/// +/// This is inspired by the "generic SIMD" algorithm described here: +/// http://0x80.pl/articles/simd-strfind.html#algorithm-1-generic-simd +/// +/// The main difference is that this is just a prefilter. That is, it reports +/// candidates once they are seen and doesn't attempt to confirm them. Also, +/// the bytes this routine uses to check for candidates are selected based on +/// an a priori background frequency distribution. This means that on most +/// haystacks, this will on average spend more time in vectorized code than you +/// would if you just selected the first and last bytes of the needle. +/// +/// Note that a non-prefilter variant of this algorithm can be found in the +/// parent module, but it only works on smaller needles. +/// +/// `prestate`, `ninfo`, `haystack` and `needle` are the four prefilter +/// function parameters. `fallback` is a prefilter that is used if the haystack +/// is too small to be handled with the given vector size. +/// +/// This routine is not safe because it is intended for callers to specialize +/// this with a particular vector (e.g., __m256i) and then call it with the +/// relevant target feature (e.g., avx2) enabled. +/// +/// # Panics +/// +/// If `needle.len() <= 1`, then this panics. +/// +/// # 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<V: Vector>( + prestate: &mut PrefilterState, + ninfo: &NeedleInfo, + haystack: &[u8], + needle: &[u8], + fallback: PrefilterFnTy, +) -> Option<usize> { + assert!(needle.len() >= 2, "needle must be at least 2 bytes"); + let (rare1i, rare2i) = ninfo.rarebytes.as_rare_ordered_usize(); + let min_haystack_len = rare2i + size_of::<V>(); + if haystack.len() < min_haystack_len { + return fallback(prestate, ninfo, haystack, needle); + } + + 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; + + let rare1chunk = V::splat(needle[rare1i]); + let rare2chunk = V::splat(needle[rare2i]); + + // 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 = find_in_chunk2(ptr, rare1i, rare2i, rare1chunk, rare2chunk); + if let Some(chunki) = m { + return Some(matched(prestate, start_ptr, ptr, chunki)); + } + ptr = ptr.add(size_of::<V>()); + } + if ptr < end_ptr { + // 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). + ptr = max_ptr; + let m = find_in_chunk2(ptr, rare1i, rare2i, rare1chunk, rare2chunk); + if let Some(chunki) = m { + return Some(matched(prestate, start_ptr, ptr, chunki)); + } + } + prestate.update(haystack.len()); + None +} + +// Below are two different techniques for checking whether a candidate +// match exists in a given chunk or not. find_in_chunk2 checks two bytes +// where as find_in_chunk3 checks three bytes. The idea behind checking +// three bytes is that while we do a bit more work per iteration, we +// decrease the chances of a false positive match being reported and thus +// make the search faster overall. This actually works out for the +// memmem/krate/prebuilt/huge-en/never-all-common-bytes benchmark, where +// using find_in_chunk3 is about 25% faster than find_in_chunk2. However, +// it turns out that find_in_chunk2 is faster for all other benchmarks, so +// perhaps the extra check isn't worth it in practice. +// +// For now, we go with find_in_chunk2, but we leave find_in_chunk3 around +// to make it easy to switch to and benchmark when possible. + +/// Search for an occurrence of two rare bytes from the needle in the current +/// chunk pointed to by ptr. +/// +/// rare1chunk and rare2chunk correspond to vectors with the rare1 and rare2 +/// bytes repeated in each 8-bit lane, respectively. +/// +/// # Safety +/// +/// It must be safe to do an unaligned read of size(V) bytes starting at both +/// (ptr + rare1i) and (ptr + rare2i). +#[inline(always)] +unsafe fn find_in_chunk2<V: Vector>( + ptr: *const u8, + rare1i: usize, + rare2i: usize, + rare1chunk: V, + rare2chunk: V, +) -> Option<usize> { + let chunk0 = V::load_unaligned(ptr.add(rare1i)); + let chunk1 = V::load_unaligned(ptr.add(rare2i)); + + let eq0 = chunk0.cmpeq(rare1chunk); + let eq1 = chunk1.cmpeq(rare2chunk); + + let match_offsets = eq0.and(eq1).movemask(); + if match_offsets == 0 { + return None; + } + Some(match_offsets.trailing_zeros() as usize) +} + +/// Search for an occurrence of two rare bytes and the first byte (even if one +/// of the rare bytes is equivalent to the first byte) from the needle in the +/// current chunk pointed to by ptr. +/// +/// firstchunk, rare1chunk and rare2chunk correspond to vectors with the first, +/// rare1 and rare2 bytes repeated in each 8-bit lane, respectively. +/// +/// # Safety +/// +/// It must be safe to do an unaligned read of size(V) bytes starting at ptr, +/// (ptr + rare1i) and (ptr + rare2i). +#[allow(dead_code)] +#[inline(always)] +unsafe fn find_in_chunk3<V: Vector>( + ptr: *const u8, + rare1i: usize, + rare2i: usize, + firstchunk: V, + rare1chunk: V, + rare2chunk: V, +) -> Option<usize> { + let chunk0 = V::load_unaligned(ptr); + let chunk1 = V::load_unaligned(ptr.add(rare1i)); + let chunk2 = V::load_unaligned(ptr.add(rare2i)); + + let eq0 = chunk0.cmpeq(firstchunk); + let eq1 = chunk1.cmpeq(rare1chunk); + let eq2 = chunk2.cmpeq(rare2chunk); + + let match_offsets = eq0.and(eq1).and(eq2).movemask(); + if match_offsets == 0 { + return None; + } + Some(match_offsets.trailing_zeros() as usize) +} + +/// Accepts a chunk-relative offset and returns a haystack relative offset +/// after updating the prefilter state. +/// +/// Why do we use this unlineable function when a search completes? Well, +/// I don't know. Really. Obviously this function was not here initially. +/// When doing profiling, the codegen for the inner loop here looked bad and +/// I didn't know why. There were a couple extra 'add' instructions and an +/// extra 'lea' instruction that I couldn't explain. I hypothesized that the +/// optimizer was having trouble untangling the hot code in the loop from the +/// code that deals with a candidate match. By putting the latter into an +/// unlineable function, it kind of forces the issue and it had the intended +/// effect: codegen improved measurably. It's good for a ~10% improvement +/// across the board on the memmem/krate/prebuilt/huge-en/ benchmarks. +#[cold] +#[inline(never)] +fn matched( + prestate: &mut PrefilterState, + start_ptr: *const u8, + ptr: *const u8, + chunki: usize, +) -> usize { + let found = diff(ptr, start_ptr) + chunki; + prestate.update(found); + found +} + +/// 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) +} |