I'm trying to write a rolling window variation of the adler32 hash function (here: https://gist.github.com/1011151), and I got it to a nearly acceptable speed under ghc 6.12.3 (~35MB/s on my work machine), so I thought I'd see how the new ghc 7.0.3 treated it. The answer is "not good". I get about 270KB/s. The key difference that I'm seeing from looking at the ghc-core program is that ghc 7.0.3 isn't using primitive ops for anything (unless they are no longer annotated with "#"). Can anybody shed any light as to what's going on? I'd also welcome any optimization tips that people would like to share. I've been messing with the code so much over the past day or two that I'm not even sure if it computes an adler32 anymore, but I hope that any optimization tops that pertain to that code would also apply to a correct algorithm. Right now, I'd just like to have the code run quickly, and to figure out why I'm seeing such a terrible regression under ghc7.