[Haskell-cafe] RFC: "Native -XCPP" Proposal

[Malcolm Wallace]

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