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/*!
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.
*/
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.