gecko-dev/third_party/rust/encoding_rs

encoding_rs

encoding_rs an implementation of the (non-JavaScript parts of) the Encoding Standard written in Rust and used in Gecko (starting with Firefox 56).

Additionally, the mem module provides various operations for dealing with in-RAM text (as opposed to data that's coming from or going to an IO boundary). The mem module is a module instead of a separate crate due to internal implementation detail efficiencies.

Functionality

Due to the Gecko use case, encoding_rs supports decoding to and encoding from UTF-16 in addition to supporting the usual Rust use case of decoding to and encoding from UTF-8. Additionally, the API has been designed to be FFI-friendly to accommodate the C++ side of Gecko.

Specifically, encoding_rs does the following:

• Decodes a stream of bytes in an Encoding Standard-defined character encoding into valid aligned native-endian in-RAM UTF-16 (units of u16 / char16_t).
• Encodes a stream of potentially-invalid aligned native-endian in-RAM UTF-16 (units of u16 / char16_t) into a sequence of bytes in an Encoding Standard-defined character encoding as if the lone surrogates had been replaced with the REPLACEMENT CHARACTER before performing the encode. (Gecko's UTF-16 is potentially invalid.)
• Decodes a stream of bytes in an Encoding Standard-defined character encoding into valid UTF-8.
• Encodes a stream of valid UTF-8 into a sequence of bytes in an Encoding Standard-defined character encoding. (Rust's UTF-8 is guaranteed-valid.)
• Does the above in streaming (input and output split across multiple buffers) and non-streaming (whole input in a single buffer and whole output in a single buffer) variants.
• Avoids copying (borrows) when possible in the non-streaming cases when decoding to or encoding from UTF-8.
• Resolves textual labels that identify character encodings in protocol text into type-safe objects representing the those encodings conceptually.
• Maps the type-safe encoding objects onto strings suitable for returning from document.characterSet.
• Validates UTF-8 (in common instruction set scenarios a bit faster for Web workloads than the standard library; hopefully will get upstreamed some day) and ASCII.

Additionally, encoding_rs::mem does the following:

• Checks if a byte buffer contains only ASCII.
• Checks if a potentially-invalid UTF-16 buffer contains only Basic Latin (ASCII).
• Checks if a valid UTF-8, potentially-invalid UTF-8 or potentially-invalid UTF-16 buffer contains only Latin1 code points (below U+0100).
• Checks if a valid UTF-8, potentially-invalid UTF-8 or potentially-invalid UTF-16 buffer or a code point or a UTF-16 code unit can trigger right-to-left behavior (suitable for checking if the Unicode Bidirectional Algorithm can be optimized out).
• Combined versions of the above two checks.
• Converts valid UTF-8, potentially-invalid UTF-8 and Latin1 to UTF-16.
• Converts potentially-invalid UTF-16 and Latin1 to UTF-8.
• Converts UTF-8 and UTF-16 to Latin1 (if in range).
• Finds the first invalid code unit in a buffer of potentially-invalid UTF-16.
• Makes a mutable buffer of potential-invalid UTF-16 contain valid UTF-16.
• Copies ASCII from one buffer to another up to the first non-ASCII byte.
• Converts ASCII to UTF-16 up to the first non-ASCII byte.
• Converts UTF-16 to ASCII up to the first non-Basic Latin code unit.

Integration with std::io

Notably, the above feature list doesn't include the capability to wrap a std::io::Read, decode it into UTF-8 and presenting the result via std::io::Read. The encoding_rs_io crate provides that capability.

Decoding Email

For decoding character encodings that occur in email, use the charset crate instead of using this one directly. (It wraps this crate and adds UTF-7 decoding.)

Windows Code Page Identifier Mappings

For mappings to and from Windows code page identifiers, use the codepage crate.

Preparing Text for the Encoders

Normalizing text into Unicode Normalization Form C prior to encoding text into a legacy encoding minimizes unmappable characters. Text can be normalized to Unicode Normalization Form C using the unic-normal crate.

The exception is windows-1258, which after normalizing to Unicode Normalization Form C requires tone marks to be decomposed in order to minimize unmappable characters. Vietnamese tone marks can be decomposed using the detone crate.

Licensing

Please see the file named COPYRIGHT.

Documentation

Generated API documentation is available online.

There is a long-form write-up about the design and internals of the crate.

C and C++ bindings

An FFI layer for encoding_rs is available as a separate crate. The crate comes with a demo C++ wrapper using the C++ standard library and GSL types.

The bindings for the mem module are in the encoding_c_mem crate.

For the Gecko context, there's a C++ wrapper using the MFBT/XPCOM types.

There's a write-up about the C++ wrappers.

Sample programs

• Rust
• C
• C++

Optional features

There are currently these optional cargo features:

simd-accel

Enables SIMD acceleration using the nightly-dependent packed_simd crate.

This is an opt-in feature, because enabling this feature opts out of Rust's guarantees of future compilers compiling old code (aka. "stability story").

Currently, this has not been tested to be an improvement except for these targets:

• x86_64
• i686
• aarch64
• thumbv7neon

If you use nightly Rust, you use targets whose first component is one of the above, and you are prepared to have to revise your configuration when updating Rust, you should enable this feature. Otherwise, please do not enable this feature.

Note! If you are compiling for a target that does not have 128-bit SIMD enabled as part of the target definition and you are enabling 128-bit SIMD using -C target_feature, you need to enable the core_arch Cargo feature for packed_simd to compile a crates.io snapshot of core_arch instead of using the standard-library copy of core::arch, because the core::arch module of the pre-compiled standard library has been compiled with the assumption that the CPU doesn't have 128-bit SIMD. At present this applies mainly to 32-bit ARM targets whose first component does not include the substring neon.

The encoding_rs side of things has not been properly set up for POWER, PowerPC, MIPS, etc., SIMD at this time, so even if you were to follow the advice from the previous paragraph, you probably shouldn't use the simd-accel option on the less mainstream architectures at this time.

Used by Firefox.

serde

Enables support for serializing and deserializing &'static Encoding-typed struct fields using Serde.

Not used by Firefox.

fast-legacy-encode

A catch-all option for enabling the fastest legacy encode options. Does not affect decode speed or UTF-8 encode speed.

At present, this option is equivalent to enabling the following options:

• fast-hangul-encode
• fast-hanja-encode
• fast-kanji-encode
• fast-gb-hanzi-encode
• fast-big5-hanzi-encode

Adds 176 KB to the binary size.

Not used by Firefox.

fast-hangul-encode

Changes encoding precomposed Hangul syllables into EUC-KR from binary search over the decode-optimized tables to lookup by index making Korean plain-text encode about 4 times as fast as without this option.

Adds 20 KB to the binary size.

Does not affect decode speed.

Not used by Firefox.

fast-hanja-encode

Changes encoding of Hanja into EUC-KR from linear search over the decode-optimized table to lookup by index. Since Hanja is practically absent in modern Korean text, this option doesn't affect perfomance in the common case and mainly makes sense if you want to make your application resilient agaist denial of service by someone intentionally feeding it a lot of Hanja to encode into EUC-KR.

Adds 40 KB to the binary size.

Does not affect decode speed.

Not used by Firefox.

fast-kanji-encode

Changes encoding of Kanji into Shift_JIS, EUC-JP and ISO-2022-JP from linear search over the decode-optimized tables to lookup by index making Japanese plain-text encode to legacy encodings 30 to 50 times as fast as without this option (about 2 times as fast as with less-slow-kanji-encode).

Takes precedence over less-slow-kanji-encode.

Adds 36 KB to the binary size (24 KB compared to less-slow-kanji-encode).

Does not affect decode speed.

Not used by Firefox.

less-slow-kanji-encode

Makes JIS X 0208 Level 1 Kanji (the most common Kanji in Shift_JIS, EUC-JP and ISO-2022-JP) encode less slow (binary search instead of linear search) making Japanese plain-text encode to legacy encodings 14 to 23 times as fast as without this option.

Adds 12 KB to the binary size.

Does not affect decode speed.

Not used by Firefox.

fast-gb-hanzi-encode

Changes encoding of Hanzi in the CJK Unified Ideographs block into GBK and gb18030 from linear search over a part the decode-optimized tables followed by a binary search over another part of the decode-optimized tables to lookup by index making Simplified Chinese plain-text encode to the legacy encodings 100 to 110 times as fast as without this option (about 2.5 times as fast as with less-slow-gb-hanzi-encode).

Takes precedence over less-slow-gb-hanzi-encode.

Adds 36 KB to the binary size (24 KB compared to less-slow-gb-hanzi-encode).

Does not affect decode speed.

Not used by Firefox.

less-slow-gb-hanzi-encode

Makes GB2312 Level 1 Hanzi (the most common Hanzi in gb18030 and GBK) encode less slow (binary search instead of linear search) making Simplified Chinese plain-text encode to the legacy encodings about 40 times as fast as without this option.

Adds 12 KB to the binary size.

Does not affect decode speed.

Not used by Firefox.

fast-big5-hanzi-encode

Changes encoding of Hanzi in the CJK Unified Ideographs block into Big5 from linear search over a part the decode-optimized tables to lookup by index making Traditional Chinese plain-text encode to Big5 105 to 125 times as fast as without this option (about 3 times as fast as with less-slow-big5-hanzi-encode).

Takes precedence over less-slow-big5-hanzi-encode.

Adds 40 KB to the binary size (20 KB compared to less-slow-big5-hanzi-encode).

Does not affect decode speed.

Not used by Firefox.

less-slow-big5-hanzi-encode

Makes Big5 Level 1 Hanzi (the most common Hanzi in Big5) encode less slow (binary search instead of linear search) making Traditional Chinese plain-text encode to Big5 about 36 times as fast as without this option.

Adds 20 KB to the binary size.

Does not affect decode speed.

Not used by Firefox.

Performance goals

For decoding to UTF-16, the goal is to perform at least as well as Gecko's old uconv. For decoding to UTF-8, the goal is to perform at least as well as rust-encoding. These goals have been achieved.

Encoding to UTF-8 should be fast. (UTF-8 to UTF-8 encode should be equivalent to memcpy and UTF-16 to UTF-8 should be fast.)

Speed is a non-goal when encoding to legacy encodings. By default, encoding to legacy encodings should not be optimized for speed at the expense of code size as long as form submission and URL parsing in Gecko don't become noticeably too slow in real-world use.

In the interest of binary size, by default, encoding_rs does not have encode-specific data tables beyond 32 bits of encode-specific data for each single-byte encoding. Therefore, encoders search the decode-optimized data tables. This is a linear search in most cases. As a result, by default, encode to legacy encodings varies from slow to extremely slow relative to other libraries. Still, with realistic work loads, this seemed fast enough not to be user-visibly slow on Raspberry Pi 3 (which stood in for a phone for testing) in the Web-exposed encoder use cases.

See the cargo features above for optionally making CJK legacy encode fast.

A framework for measuring performance is available separately.

Rust Version Compatibility

It is a goal to support the latest stable Rust, the latest nightly Rust and the version of Rust that's used for Firefox Nightly (currently 1.29.0). These are tested on Travis.

Additionally, beta and the oldest known to work Rust version (currently 1.29.0) are tested on Travis. The oldest Rust known to work is tested as a canary so that when the oldest known to work no longer works, the change can be documented here. At this time, there is no firm commitment to support a version older than what's required by Firefox. The oldest supported Rust is expected to move forward rapidly when packed_simd can replace the simd crate without performance regression.

Compatibility with rust-encoding

A compatibility layer that implements the rust-encoding API on top of encoding_rs is provided as a separate crate (cannot be uploaded to crates.io). The compatibility layer was originally written with the assuption that Firefox would need it, but it is not currently used in Firefox.

Regenerating Generated Code

To regenerate the generated code:

• Have Python 2 installed.
• Clone https://github.com/hsivonen/encoding_c next to the encoding_rs directory.
• Clone https://github.com/hsivonen/codepage next to the encoding_rs directory.
• Clone https://github.com/whatwg/encoding next to the encoding_rs directory.
• Checkout revision f381389 of the encoding repo.
• With the encoding_rs directory as the working directory, run python generate-encoding-data.py.

Roadmap

• Design the low-level API.
• Provide Rust-only convenience features.
• Provide an stl/gsl-flavored C++ API.
• Implement all decoders and encoders.
• Add unit tests for all decoders and encoders.
• Finish BOM sniffing variants in Rust-only convenience features.
• Document the API.
• Publish the crate on crates.io.
• Create a solution for measuring performance.
• Accelerate ASCII conversions using SSE2 on x86.
• Accelerate ASCII conversions using ALU register-sized operations on non-x86 architectures (process an usize instead of u8 at a time).
• Split FFI into a separate crate so that the FFI doesn't interfere with LTO in pure-Rust usage.
• Compress CJK indices by making use of sequential code points as well as Unicode-ordered parts of indices.
• Make lookups by label or name use binary search that searches from the end of the label/name to the start.
• Make labels with non-ASCII bytes fail fast.
• ~Parallelize UTF-8 validation using Rayon.~ (This turned out to be a pessimization in the ASCII case due to memory bandwidth reasons.)
• Provide an XPCOM/MFBT-flavored C++ API.
• Investigate accelerating single-byte encode with a single fast-tracked range per encoding.
• Replace uconv with encoding_rs in Gecko.
• Implement the rust-encoding API in terms of encoding_rs.
• Add SIMD acceleration for Aarch64.
• Investigate the use of NEON on 32-bit ARM.
• ~Investigate Björn Höhrmann's lookup table acceleration for UTF-8 as adapted to Rust in rust-encoding.~
• Add actually fast CJK encode options.
• ~Investigate Bob Steagall's lookup table acceleration for UTF-8.~

Release Notes

0.8.20

• Make Decoder::latin1_byte_compatible_up_to return None in more cases to make the method actually useful. While this could be argued to be a breaking change due to the bug fix changing semantics, it does not break callers that had to handle the None case in a reasonable way anyway.

0.8.19

• Removed a bunch of bound checks in convert_str_to_utf16.
• Added mem::convert_utf8_to_utf16_without_replacement.

0.8.18

• Added mem::utf8_latin1_up_to and mem::str_latin1_up_to.
• Added Decoder::latin1_byte_compatible_up_to.

0.8.17

• Update bincode (dev dependency) version requirement to 1.0.

0.8.16

• Switch from the simd crate to packed_simd.

0.8.15

• Adjust documentation for simd-accel (README-only release).

0.8.14

• Made UTF-16 to UTF-8 encode conversion fill the output buffer as closely as possible.

0.8.13

• Made the UTF-8 to UTF-16 decoder compare the number of code units written with the length of the right slice (the output slice) to fix a panic introduced in 0.8.11.

0.8.12

• Removed the clippy:: prefix from clippy lint names.

0.8.11

• Changed minimum Rust requirement to 1.29.0 (for the ability to refer to the interior of a static when defining another static).
• Explicitly aligned the lookup tables for single-byte encodings and UTF-8 to cache lines in the hope of freeing up one cache line for other data. (Perhaps the tables were already aligned and this is placebo.)
• Added 32 bits of encode-oriented data for each single-byte encoding. The change was performance-neutral for non-Latin1-ish Latin legacy encodings, improved Latin1-ish and Arabic legacy encode speed somewhat (new speed is 2.4x the old speed for German, 2.3x for Arabic, 1.7x for Portuguese and 1.4x for French) and improved non-Latin1, non-Arabic legacy single-byte encode a lot (7.2x for Thai, 6x for Greek, 5x for Russian, 4x for Hebrew).
• Added compile-time options for fast CJK legacy encode options (at the cost of binary size (up to 176 KB) and run-time memory usage). These options still retain the overall code structure instead of rewriting the CJK encoders totally, so the speed isn't as good as what could be achieved by using even more memory / making the binary even langer.
• Made UTF-8 decode and validation faster.
• Added method is_single_byte() on Encoding.
• Added mem::decode_latin1() and mem::encode_latin1_lossy().

0.8.10

• Disabled a unit test that tests a panic condition when the assertion being tested is disabled.

0.8.9

• Made --features simd-accel work with stable-channel compiler to simplify the Firefox build system.

0.8.8

• Made the is_foo_bidi() not treat U+FEFF (ZERO WIDTH NO-BREAK SPACE aka. BYTE ORDER MARK) as right-to-left.
• Made the is_foo_bidi() functions report true if the input contains Hebrew presentations forms (which are right-to-left but not in a right-to-left-roadmapped block).

0.8.7

• Fixed a panic in the UTF-16LE/UTF-16BE decoder when decoding to UTF-8.

0.8.6

• Temporarily removed the debug assertion added in version 0.8.5 from convert_utf16_to_latin1_lossy.

0.8.5

• If debug assertions are enabled but fuzzing isn't enabled, lossy conversions to Latin1 in the mem module assert that the input is in the range U+0000...U+00FF (inclusive).
• In the mem module provide conversions from Latin1 and UTF-16 to UTF-8 that can deal with insufficient output space. The idea is to use them first with an allocation rounded up to jemalloc bucket size and do the worst-case allocation only if the jemalloc rounding up was insufficient as the first guess.

0.8.4

• Fix SSE2-specific, simd-accel-specific memory corruption introduced in version 0.8.1 in conversions between UTF-16 and Latin1 in the mem module.

0.8.3

• Removed an #[inline(never)] annotation that was not meant for release.

0.8.2

• Made non-ASCII UTF-16 to UTF-8 encode faster by manually omitting bound checks and manually adding branch prediction annotations.

0.8.1

• Tweaked loop unrolling and memory alignment for SSE2 conversions between UTF-16 and Latin1 in the mem module to increase the performance when converting long buffers.

0.8.0

• Changed the minimum supported version of Rust to 1.21.0 (semver breaking change).
• Flipped around the defaults vs. optional features for controlling the size vs. speed trade-off for Kanji and Hanzi legacy encode (semver breaking change).
• Added NEON support on ARMv7.
• SIMD-accelerated x-user-defined to UTF-16 decode.
• Made UTF-16LE and UTF-16BE decode a lot faster (including SIMD acceleration).

0.7.2

• Add the mem module.
• Refactor SIMD code which can affect performance outside the mem module.

0.7.1

• When encoding from invalid UTF-16, correctly handle U+DC00 followed by another low surrogate.

0.7.0

• Make replacement a label of the replacement encoding. (Spec change.)
• Remove Encoding::for_name(). (Encoding::for_label(foo).unwrap() is now close enough after the above label change.)
• Remove the parallel-utf8 cargo feature.
• Add optional Serde support for &'static Encoding.
• Performance tweaks for ASCII handling.
• Performance tweaks for UTF-8 validation.
• SIMD support on aarch64.

0.6.11

• Make Encoder::has_pending_state() public.
• Update the simd crate dependency to 0.2.0.

0.6.10

• Reserve enough space for NCRs when encoding to ISO-2022-JP.
• Correct max length calculations for multibyte decoders.
• Correct max length calculations before BOM sniffing has been performed.
• Correctly calculate max length when encoding from UTF-16 to GBK.

0.6.9

• Don't prepend anything when gb18030 range decode fails. (Spec change.)

0.6.8

• Correcly handle the case where the first buffer contains potentially partial BOM and the next buffer is the last buffer.
• Decode byte 7F correctly in ISO-2022-JP.
• Make UTF-16 to UTF-8 encode write closer to the end of the buffer.
• Implement Hash for Encoding.

0.6.7

• Map half-width katakana to full-width katana in ISO-2022-JP encoder. (Spec change.)
• Give InputEmpty correct precedence over OutputFull when encoding with replacement and the output buffer passed in is too short or the remaining space in the output buffer is too small after a replacement.

0.6.6

• Correct max length calculation when a partial BOM prefix is part of the decoder's state.

0.6.5

• Correct max length calculation in various encoders.
• Correct max length calculation in the UTF-16 decoder.
• Derive PartialEq and Eq for the CoderResult, DecoderResult and EncoderResult types.

0.6.4

• Avoid panic when encoding with replacement and the destination buffer is too short to hold one numeric character reference.

0.6.3

• Add support for 32-bit big-endian hosts. (For real this time.)

0.6.2

• Fix a panic from subslicing with bad indices in Encoder::encode_from_utf16. (Due to an oversight, it lacked the fix that Encoder::encode_from_utf8 already had.)
• Micro-optimize error status accumulation in non-streaming case.

0.6.1

• Avoid panic near integer overflow in a case that's unlikely to actually happen.
• Address Clippy lints.

0.6.0

• Make the methods for computing worst-case buffer size requirements check for integer overflow.
• Upgrade rayon to 0.7.0.

0.5.1

• Reorder methods for better documentation readability.
• Add support for big-endian hosts. (Only 64-bit case actually tested.)
• Optimize the ALU (non-SIMD) case for 32-bit ARM instead of x86_64.

0.5.0

• Avoid allocating an excessively long buffers in non-streaming decode.
• Fix the behavior of ISO-2022-JP and replacement decoders near the end of the output buffer.
• Annotate the result structs with #[must_use].

0.4.0

• Split FFI into a separate crate.
• Performance tweaks.
• CJK binary size and encoding performance changes.
• Parallelize UTF-8 validation in the case of long buffers (with optional feature parallel-utf8).
• Borrow even with ISO-2022-JP when possible.

0.3.2

• Fix moving pointers to alignment in ALU-based ASCII acceleration.
• Fix errors in documentation and improve documentation.

0.3.1

• Fix UTF-8 to UTF-16 decode for byte sequences beginning with 0xEE.
• Make UTF-8 to UTF-8 decode SSE2-accelerated when feature simd-accel is used.
• When decoding and encoding ASCII-only input from or to an ASCII-compatible encoding using the non-streaming API, return a borrow of the input.
• Make encode from UTF-16 to UTF-8 faster.

0.3

• Change the references to the instances of Encoding from const to static to make the referents unique across crates that use the refernces.
• Introduce non-reference-typed FOO_INIT instances of Encoding to allow foreign crates to initialize static arrays with references to Encoding instances even under Rust's constraints that prohibit the initialization of &'static Encoding-typed array items with &'static Encoding-typed statics.
• Document that the above two points will be reverted if Rust changes const to work so that cross-crate usage keeps the referents unique.
• Return Cows from Rust-only non-streaming methods for encode and decode.
• Encoding::for_bom() returns the length of the BOM.
• ASCII-accelerated conversions for encodings other than UTF-16LE, UTF-16BE, ISO-2022-JP and x-user-defined.
• Add SSE2 acceleration behind the simd-accel feature flag. (Requires nightly Rust.)
• Fix panic with long bogus labels.
• Map 0xCA to U+05BA in windows-1255. (Spec change.)
• Correct the end of the Shift_JIS EUDC range. (Spec change.)

0.2.4

• Polish FFI documentation.

0.2.3

• Fix UTF-16 to UTF-8 encode.

0.2.2

• Add Encoder.encode_from_utf8_to_vec_without_replacement().

0.2.1

• Add Encoding.is_ascii_compatible().

• Add Encoding::for_bom().

• Make == for Encoding use name comparison instead of pointer comparison, because uses of the encoding constants in different crates result in different addresses and the constant cannot be turned into statics without breaking other things.

0.2.0

The initial release.