| <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/memchr-2.5.0/src/memmem/genericsimd.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>genericsimd.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../../normalize.css"><link rel="stylesheet" href="../../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../../ayu.css" disabled><link rel="stylesheet" href="../../../dark.css" disabled><link rel="stylesheet" href="../../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../../storage.js"></script><script defer src="../../../source-script.js"></script><script defer src="../../../source-files.js"></script><script defer src="../../../main.js"></script><noscript><link rel="stylesheet" href="../../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../../memchr/index.html"><div class="logo-container"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../../memchr/index.html"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span> |
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| </pre><pre class="rust"><code><span class="kw">use </span>core::mem::size_of; |
| |
| <span class="kw">use </span><span class="kw">crate</span>::memmem::{util::memcmp, vector::Vector, NeedleInfo}; |
| |
| <span class="doccomment">/// 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. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">const </span>MIN_NEEDLE_LEN: usize = <span class="number">2</span>; |
| |
| <span class="doccomment">/// 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? |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">const </span>MAX_NEEDLE_LEN: usize = <span class="number">32</span>; |
| |
| <span class="doccomment">/// 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. |
| </span><span class="attribute">#[derive(Clone, Copy, Debug)] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>Forward { |
| rare1i: u8, |
| rare2i: u8, |
| } |
| |
| <span class="kw">impl </span>Forward { |
| <span class="doccomment">/// Create a new "generic simd" forward searcher. If one could not be |
| /// created from the given inputs, then None is returned. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>new(ninfo: <span class="kw-2">&</span>NeedleInfo, needle: <span class="kw-2">&</span>[u8]) -> <span class="prelude-ty">Option</span><Forward> { |
| <span class="kw">let </span>(rare1i, rare2i) = ninfo.rarebytes.as_rare_ordered_u8(); |
| <span class="comment">// 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.) |
| </span><span class="kw">if </span>needle.len() < MIN_NEEDLE_LEN |
| || needle.len() > MAX_NEEDLE_LEN |
| || rare1i == rare2i |
| { |
| <span class="kw">return </span><span class="prelude-val">None</span>; |
| } |
| <span class="prelude-val">Some</span>(Forward { rare1i, rare2i }) |
| } |
| |
| <span class="doccomment">/// 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. |
| </span><span class="attribute">#[inline(always)] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>min_haystack_len<V: Vector>(<span class="kw-2">&</span><span class="self">self</span>) -> usize { |
| <span class="self">self</span>.rare2i <span class="kw">as </span>usize + size_of::<V>() |
| } |
| } |
| |
| <span class="doccomment">/// 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.) |
| </span><span class="attribute">#[inline(always)] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">unsafe fn </span>fwd_find<V: Vector>( |
| fwd: <span class="kw-2">&</span>Forward, |
| haystack: <span class="kw-2">&</span>[u8], |
| needle: <span class="kw-2">&</span>[u8], |
| ) -> <span class="prelude-ty">Option</span><usize> { |
| <span class="comment">// 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. |
| </span><span class="kw">if </span>haystack.len() < needle.len() { |
| <span class="kw">return </span><span class="prelude-val">None</span>; |
| } |
| <span class="kw">let </span>min_haystack_len = fwd.min_haystack_len::<V>(); |
| <span class="macro">assert!</span>(haystack.len() >= min_haystack_len, <span class="string">"haystack too small"</span>); |
| <span class="macro">debug_assert!</span>(needle.len() <= haystack.len()); |
| <span class="macro">debug_assert!</span>( |
| needle.len() >= MIN_NEEDLE_LEN, |
| <span class="string">"needle must be at least {} bytes"</span>, |
| MIN_NEEDLE_LEN, |
| ); |
| <span class="macro">debug_assert!</span>( |
| needle.len() <= MAX_NEEDLE_LEN, |
| <span class="string">"needle must be at most {} bytes"</span>, |
| MAX_NEEDLE_LEN, |
| ); |
| |
| <span class="kw">let </span>(rare1i, rare2i) = (fwd.rare1i <span class="kw">as </span>usize, fwd.rare2i <span class="kw">as </span>usize); |
| <span class="kw">let </span>rare1chunk = V::splat(needle[rare1i]); |
| <span class="kw">let </span>rare2chunk = V::splat(needle[rare2i]); |
| |
| <span class="kw">let </span>start_ptr = haystack.as_ptr(); |
| <span class="kw">let </span>end_ptr = start_ptr.add(haystack.len()); |
| <span class="kw">let </span>max_ptr = end_ptr.sub(min_haystack_len); |
| <span class="kw">let </span><span class="kw-2">mut </span>ptr = start_ptr; |
| |
| <span class="comment">// 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. |
| </span><span class="kw">while </span>ptr <= max_ptr { |
| <span class="kw">let </span>m = fwd_find_in_chunk( |
| fwd, needle, ptr, end_ptr, rare1chunk, rare2chunk, !<span class="number">0</span>, |
| ); |
| <span class="kw">if let </span><span class="prelude-val">Some</span>(chunki) = m { |
| <span class="kw">return </span><span class="prelude-val">Some</span>(matched(start_ptr, ptr, chunki)); |
| } |
| ptr = ptr.add(size_of::<V>()); |
| } |
| <span class="kw">if </span>ptr < end_ptr { |
| <span class="kw">let </span>remaining = diff(end_ptr, ptr); |
| <span class="macro">debug_assert!</span>( |
| remaining < min_haystack_len, |
| <span class="string">"remaining bytes should be smaller than the minimum haystack \ |
| length of {}, but there are {} bytes remaining"</span>, |
| min_haystack_len, |
| remaining, |
| ); |
| <span class="kw">if </span>remaining < needle.len() { |
| <span class="kw">return </span><span class="prelude-val">None</span>; |
| } |
| <span class="macro">debug_assert!</span>( |
| max_ptr < ptr, |
| <span class="string">"after main loop, ptr should have exceeded max_ptr"</span>, |
| ); |
| <span class="kw">let </span>overlap = diff(ptr, max_ptr); |
| <span class="macro">debug_assert!</span>( |
| overlap > <span class="number">0</span>, |
| <span class="string">"overlap ({}) must always be non-zero"</span>, |
| overlap, |
| ); |
| <span class="macro">debug_assert!</span>( |
| overlap < size_of::<V>(), |
| <span class="string">"overlap ({}) cannot possibly be >= than a vector ({})"</span>, |
| overlap, |
| size_of::<V>(), |
| ); |
| <span class="comment">// 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. |
| </span><span class="kw">let </span>mask = !((<span class="number">1 </span><< overlap) - <span class="number">1</span>); |
| ptr = max_ptr; |
| <span class="kw">let </span>m = fwd_find_in_chunk( |
| fwd, needle, ptr, end_ptr, rare1chunk, rare2chunk, mask, |
| ); |
| <span class="kw">if let </span><span class="prelude-val">Some</span>(chunki) = m { |
| <span class="kw">return </span><span class="prelude-val">Some</span>(matched(start_ptr, ptr, chunki)); |
| } |
| } |
| <span class="prelude-val">None |
| </span>} |
| |
| <span class="doccomment">/// 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()). |
| </span><span class="attribute">#[inline(always)] |
| </span><span class="kw">unsafe fn </span>fwd_find_in_chunk<V: Vector>( |
| fwd: <span class="kw-2">&</span>Forward, |
| needle: <span class="kw-2">&</span>[u8], |
| ptr: <span class="kw-2">*const </span>u8, |
| end_ptr: <span class="kw-2">*const </span>u8, |
| rare1chunk: V, |
| rare2chunk: V, |
| mask: u32, |
| ) -> <span class="prelude-ty">Option</span><usize> { |
| <span class="kw">let </span>chunk0 = V::load_unaligned(ptr.add(fwd.rare1i <span class="kw">as </span>usize)); |
| <span class="kw">let </span>chunk1 = V::load_unaligned(ptr.add(fwd.rare2i <span class="kw">as </span>usize)); |
| |
| <span class="kw">let </span>eq0 = chunk0.cmpeq(rare1chunk); |
| <span class="kw">let </span>eq1 = chunk1.cmpeq(rare2chunk); |
| |
| <span class="kw">let </span><span class="kw-2">mut </span>match_offsets = eq0.and(eq1).movemask() & mask; |
| <span class="kw">while </span>match_offsets != <span class="number">0 </span>{ |
| <span class="kw">let </span>offset = match_offsets.trailing_zeros() <span class="kw">as </span>usize; |
| <span class="kw">let </span>ptr = ptr.add(offset); |
| <span class="kw">if </span>end_ptr.sub(needle.len()) < ptr { |
| <span class="kw">return </span><span class="prelude-val">None</span>; |
| } |
| <span class="kw">let </span>chunk = core::slice::from_raw_parts(ptr, needle.len()); |
| <span class="kw">if </span>memcmp(needle, chunk) { |
| <span class="kw">return </span><span class="prelude-val">Some</span>(offset); |
| } |
| match_offsets &= match_offsets - <span class="number">1</span>; |
| } |
| <span class="prelude-val">None |
| </span>} |
| |
| <span class="doccomment">/// 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.) |
| </span><span class="attribute">#[cold] |
| #[inline(never)] |
| </span><span class="kw">fn </span>matched(start_ptr: <span class="kw-2">*const </span>u8, ptr: <span class="kw-2">*const </span>u8, chunki: usize) -> usize { |
| diff(ptr, start_ptr) + chunki |
| } |
| |
| <span class="doccomment">/// Subtract `b` from `a` and return the difference. `a` must be greater than |
| /// or equal to `b`. |
| </span><span class="kw">fn </span>diff(a: <span class="kw-2">*const </span>u8, b: <span class="kw-2">*const </span>u8) -> usize { |
| <span class="macro">debug_assert!</span>(a >= b); |
| (a <span class="kw">as </span>usize) - (b <span class="kw">as </span>usize) |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../../" data-current-crate="memchr" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
