The article presents highly optimized algorithms for computing the day of the week from a day-count, using fast modulus techniques that outperform compiler-generated code. A notable finding is that ISO weekday format [1..7] can be computed with the same instruction sequence as Unix format [0..6] at zero speed penalty. The best x86 implementation uses just 3 instructions plus a constant load, achieving single-multiplication latency.
Background
Computing the day of the week from a date is a common operation in software, typically handled by standard libraries. This article dives into the low-level optimization of this seemingly trivial computation for high-performance scenarios like database engines and compilers.
- Source
- Lobsters
- Published
- Aug 17, 2026 at 04:52 AM
- Score
- 6.0 / 10