On 21 May 2015, at 15:54, Bardur Arantsson wrote:
fork/exec is almost certainly going to be negligable compared to the overall compile time anyway. It's not like GHC is fast enough for it to matter.
Don't count on it. On our Windows desktop machines, fork/exec costs approximately one third of a second, instead of the expected small number of milliseconds or less. The reasons are unknown, but we suspect a misconfigured anti-virus scanner (and for various company policy reasons we are prohibited from doing the investigation that could confirm or deny this hypothesis). This means that when ghc --make does lots of external things requiring a fork, such as preprocessing, a medium sized project (using many library packages) can take a surprisingly large amount of time (minutes instead of seconds), even for an incremental build where very little code has changed. We think an in-process cpphs could make some of our compilations literally hundreds of times faster. Regards, Malcolm