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A very fast library for validating UTF-8 using AVX2/SSE4 instructions
C Lua Python
Branch: master
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zwegner Improve the early ASCII exit: before, we were only exiting if an inpu…
…t vector was all ASCII and we weren't expecting any continuation bytes, but we can exit either way when a vector is only ASCII. Expecting continuation bytes just means we return invalid early.
Latest commit cf48201 Nov 18, 2019


This library is a very fast UTF-8 validator using AVX2/SSE4 instructions. As far as I am aware, it is the fastest validator in the world on the CPUs that support these instructions (...and not AVX-512). Using AVX2, it can validate random UTF-8 text as fast as .26 cycles/byte, and random ASCII text at .09 cycles/byte. For UTF-8, this is roughly 1.5-1.7x faster than the fastvalidate-utf-8 library.

This repository contains the library (one C file), a build script for the build system, and a Lua test script (which requires LuaJIT due to use of the ffi module).

A detailed description of the algorithm can be found in z_validate.c. This algorithm should map fairly nicely to AVX-512, and should in fact be a bit faster than 2x the speed of AVX2 since a few instructions can be saved. But I don't have an AVX-512 machine, so I haven't tried it yet.


Here's some raw numbers, measured on my 2.4GHz Haswell laptop, using a modified version of the benchmark in the fastvalidate-utf-8 repository. There are four configurations of test input: random UTF-8 bytes or random ASCII bytes, and either 64K bytes or 16M bytes. All measurements are the best of 50 runs, with each run using a different random seed, but each validator tested with the same seeds (and thus the same inputs). All measurements are in cycles per byte. The first two rows are the fastvalidate-utf-8 AVX2 functions, and the second two rows are this library, using AVX2 and SSE4 instruction sets.

Validator 64K UTF-8 64K ASCII 16M UTF-8 16M ASCII
validate_utf8_fast_avx 0.410 0.410 0.496 0.429
validate_utf8_fast_avx_asciipath 0.436 0.074 0.457 0.156
z_validate_utf8_avx2 0.264 0.079 0.290 0.160
z_validate_utf8_sse4 0.568 0.163 0.596 0.202
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