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diff --git a/vendor/memchr/README.md b/vendor/memchr/README.md new file mode 100644 index 0000000..db00ebb --- /dev/null +++ b/vendor/memchr/README.md @@ -0,0 +1,196 @@ +memchr +====== +This library provides heavily optimized routines for string search primitives. + +[](https://github.com/BurntSushi/memchr/actions) +[](https://crates.io/crates/memchr) + +Dual-licensed under MIT or the [UNLICENSE](https://unlicense.org/). + + +### Documentation + +[https://docs.rs/memchr](https://docs.rs/memchr) + + +### Overview + +* The top-level module provides routines for searching for 1, 2 or 3 bytes + in the forward or reverse direction. When searching for more than one byte, + positions are considered a match if the byte at that position matches any + of the bytes. +* The `memmem` sub-module provides forward and reverse substring search + routines. + +In all such cases, routines operate on `&[u8]` without regard to encoding. This +is exactly what you want when searching either UTF-8 or arbitrary bytes. + +### Compiling without the standard library + +memchr links to the standard library by default, but you can disable the +`std` feature if you want to use it in a `#![no_std]` crate: + +```toml +[dependencies] +memchr = { version = "2", default-features = false } +``` + +On `x86_64` platforms, when the `std` feature is disabled, the SSE2 accelerated +implementations will be used. When `std` is enabled, AVX2 accelerated +implementations will be used if the CPU is determined to support it at runtime. + +SIMD accelerated routines are also available on the `wasm32` and `aarch64` +targets. The `std` feature is not required to use them. + +When a SIMD version is not available, then this crate falls back to +[SWAR](https://en.wikipedia.org/wiki/SWAR) techniques. + +### Minimum Rust version policy + +This crate's minimum supported `rustc` version is `1.61.0`. + +The current policy is that the minimum Rust version required to use this crate +can be increased in minor version updates. For example, if `crate 1.0` requires +Rust 1.20.0, then `crate 1.0.z` for all values of `z` will also require Rust +1.20.0 or newer. However, `crate 1.y` for `y > 0` may require a newer minimum +version of Rust. + +In general, this crate will be conservative with respect to the minimum +supported version of Rust. + + +### Testing strategy + +Given the complexity of the code in this crate, along with the pervasive use +of `unsafe`, this crate has an extensive testing strategy. It combines multiple +approaches: + +* Hand-written tests. +* Exhaustive-style testing meant to exercise all possible branching and offset + calculations. +* Property based testing through [`quickcheck`](https://github.com/BurntSushi/quickcheck). +* Fuzz testing through [`cargo fuzz`](https://github.com/rust-fuzz/cargo-fuzz). +* A huge suite of benchmarks that are also run as tests. Benchmarks always + confirm that the expected result occurs. + +Improvements to the testing infrastructure are very welcome. + + +### Algorithms used + +At time of writing, this crate's implementation of substring search actually +has a few different algorithms to choose from depending on the situation. + +* For very small haystacks, + [Rabin-Karp](https://en.wikipedia.org/wiki/Rabin%E2%80%93Karp_algorithm) + is used to reduce latency. Rabin-Karp has very small overhead and can often + complete before other searchers have even been constructed. +* For small needles, a variant of the + ["Generic SIMD"](http://0x80.pl/articles/simd-strfind.html#algorithm-1-generic-simd) + algorithm is used. Instead of using the first and last bytes, a heuristic is + used to select bytes based on a background distribution of byte frequencies. +* In all other cases, + [Two-Way](https://en.wikipedia.org/wiki/Two-way_string-matching_algorithm) + is used. If possible, a prefilter based on the "Generic SIMD" algorithm + linked above is used to find candidates quickly. A dynamic heuristic is used + to detect if the prefilter is ineffective, and if so, disables it. + + +### Why is the standard library's substring search so much slower? + +We'll start by establishing what the difference in performance actually +is. There are two relevant benchmark classes to consider: `prebuilt` and +`oneshot`. The `prebuilt` benchmarks are designed to measure---to the extent +possible---search time only. That is, the benchmark first starts by building a +searcher and then only tracking the time for _using_ the searcher: + +``` +$ rebar rank benchmarks/record/x86_64/2023-08-26.csv --intersection -e memchr/memmem/prebuilt -e std/memmem/prebuilt +Engine Version Geometric mean of speed ratios Benchmark count +------ ------- ------------------------------ --------------- +rust/memchr/memmem/prebuilt 2.5.0 1.03 53 +rust/std/memmem/prebuilt 1.73.0-nightly 180dffba1 6.50 53 +``` + +Conversely, the `oneshot` benchmark class measures the time it takes to both +build the searcher _and_ use it: + +``` +$ rebar rank benchmarks/record/x86_64/2023-08-26.csv --intersection -e memchr/memmem/oneshot -e std/memmem/oneshot +Engine Version Geometric mean of speed ratios Benchmark count +------ ------- ------------------------------ --------------- +rust/memchr/memmem/oneshot 2.5.0 1.04 53 +rust/std/memmem/oneshot 1.73.0-nightly 180dffba1 5.26 53 +``` + +**NOTE:** Replace `rebar rank` with `rebar cmp` in the above commands to +explore the specific benchmarks and their differences. + +So in both cases, this crate is quite a bit faster over a broad sampling of +benchmarks regardless of whether you measure only search time or search time +plus construction time. The difference is a little smaller when you include +construction time in your measurements. + +These two different types of benchmark classes make for a nice segue into +one reason why the standard library's substring search can be slower: API +design. In the standard library, the only APIs available to you require +one to re-construct the searcher for every search. While you can benefit +from building a searcher once and iterating over all matches in a single +string, you cannot reuse that searcher to search other strings. This might +come up when, for example, searching a file one line at a time. You'll need +to re-build the searcher for every line searched, and this can [really +matter][burntsushi-bstr-blog]. + +**NOTE:** The `prebuilt` benchmark for the standard library can't actually +avoid measuring searcher construction at some level, because there is no API +for it. Instead, the benchmark consists of building the searcher once and then +finding all matches in a single string via an iterator. This tends to +approximate a benchmark where searcher construction isn't measured, but it +isn't perfect. While this means the comparison is not strictly +apples-to-apples, it does reflect what is maximally possible with the standard +library, and thus reflects the best that one could do in a real world scenario. + +While there is more to the story than just API design here, it's important to +point out that even if the standard library's substring search were a precise +clone of this crate internally, it would still be at a disadvantage in some +workloads because of its API. (The same also applies to C's standard library +`memmem` function. There is no way to amortize construction of the searcher. +You need to pay for it on every call.) + +The other reason for the difference in performance is that +the standard library has trouble using SIMD. In particular, substring search +is implemented in the `core` library, where platform specific code generally +can't exist. That's an issue because in order to utilize SIMD beyond SSE2 +while maintaining portable binaries, one needs to use [dynamic CPU feature +detection][dynamic-cpu], and that in turn requires platform specific code. +While there is [an RFC for enabling target feature detection in +`core`][core-feature], it doesn't yet exist. + +The bottom line here is that `core`'s substring search implementation is +limited to making use of SSE2, but not AVX. + +Still though, this crate does accelerate substring search even when only SSE2 +is available. The standard library could therefore adopt the techniques in this +crate just for SSE2. The reason why that hasn't happened yet isn't totally +clear to me. It likely needs a champion to push it through. The standard +library tends to be more conservative in these things. With that said, the +standard library does use some [SSE2 acceleration on `x86-64`][std-sse2] added +in [this PR][std-sse2-pr]. However, at the time of writing, it is only used +for short needles and doesn't use the frequency based heuristics found in this +crate. + +**NOTE:** Another thing worth mentioning is that the standard library's +substring search routine requires that both the needle and haystack have type +`&str`. Unless you can assume that your data is valid UTF-8, building a `&str` +will come with the overhead of UTF-8 validation. This may in turn result in +overall slower searching depending on your workload. In contrast, the `memchr` +crate permits both the needle and the haystack to have type `&[u8]`, where +`&[u8]` can be created from a `&str` with zero cost. Therefore, the substring +search in this crate is strictly more flexible than what the standard library +provides. + +[burntsushi-bstr-blog]: https://blog.burntsushi.net/bstr/#motivation-based-on-performance +[dynamic-cpu]: https://doc.rust-lang.org/std/arch/index.html#dynamic-cpu-feature-detection +[core-feature]: https://github.com/rust-lang/rfcs/pull/3469 +[std-sse2]: https://github.com/rust-lang/rust/blob/bf9229a2e366b4c311f059014a4aa08af16de5d8/library/core/src/str/pattern.rs#L1719-L1857 +[std-sse2-pr]: https://github.com/rust-lang/rust/pull/103779 |