Fixing ill-formed UTF-16 strings with SIMD instructions

TL;DR

This paper introduces a SIMD-based method to efficiently fix ill-formed UTF-16 strings, improving performance in JavaScript engines like V8 used in major browsers.

Contribution

It presents a novel SIMD-accelerated approach for fixing mismatched surrogates in UTF-16 strings, enhancing speed and reliability.

Findings

Significant performance improvements over traditional methods

Implementation integrated into Google's V8 JavaScript engine

Applicable to web browsers for more robust Unicode handling

Abstract

UTF-16 is a widely used Unicode encoding representing characters with one or two 16-bit code units. The format relies on surrogate pairs to encode characters beyond the Basic Multilingual Plane, requiring a high surrogate followed by a low surrogate. Ill-formed UTF-16 strings -- where surrogates are mismatched -- can arise from data corruption or improper encoding, posing security and reliability risks. Consequently, programming languages such as JavaScript include functions to fix ill-formed UTF-16 strings by replacing mismatched surrogates with the Unicode replacement character (U+FFFD). We propose using Single Instruction, Multiple Data (SIMD) instructions to handle multiple code units in parallel, enabling faster and more efficient execution. Our software is part of the Google JavaScript engine (V8) and thus part of several major Web browsers.