Implementing an Efficient uint128 Type in Modern C++ for x64 Architecture
By
PaulHoule
Slow-proofed and worth the wait. Worth its weight in flour.
Summary
This article provides a practical guide to implementing an efficient 128-bit unsigned integer (uint128) type in modern C++. The author focuses on creating a fixed-width arithmetic type using two 64-bit words that generates assembly code indistinguishable from built-in __uint128_t. The implementation emphasizes explicit range bounds, compiler optimization, and targets modern x64 architecture with Clang and GCC as primary compilers, with notes for MSVC differences. The article covers unsigned arithmetic operations, performance considerations, and practical implementation details rather than dynamic big integer arithmetic.
Key quotes
· 4 pulledWe take the smallest reasonable definition of a 128-bit integer, two 64-bit words, and turn it into a usable arithmetic type whose generated code is indistinguishable from a builtin __uint128_t.
This post is explicitly not about dynamically-sized big integer arithmetic. It is about being explicit with range bounds and letting the compiler emit the exact instructions we want.
The scope is deliberately limited: unsigned arithmetic, fixed width, modern x64, with Clang and GCC as the primary targets and notes for MSVC where it differs.
A practical walk-through of a fixed-width uint128 implementation in modern C++.
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