Back in 2019, Nick Fitzgerald published always bump downwards, an article making the case that for bump allocators, bumping “down” (towards lower addresses) is better than bumping up. The biggest reasons for this are bumping up requires 3 branches vs 2 for bumping down and rounding down requires fewer instructions than rounding up. This became the method used for the popular bumpalo crate. In this post, I want to go back and revisit that analysis.