Potential GSoC proposal: Reduce the speed gap between 'ghc -c' and 'ghc --make'

[Ryan Newton]

>
> <the.dead.shall.rise at gmail.com> wrote:
> > Thanks. I'll look into how to optimise .hi loading by more traditional
> > means, then.
>
> Lennart is working on speeding up the binary package (which I believe
> is used to decode the .hi files.) His work might benefit this effort.


Last time I tested it, mmap still offered better performance than fread on
linux.  In addition to improving the deserialization code it would seem
like a good idea to mmap the whole file at the outset as well.

It seems like readBinMem is the relevant function (readIFace ->
readBinIFace -> readBinMem), which occurs here:


https://github.com/ghc/ghc/blob/08894f96407635781a233145435a78f144accab0/compiler/utils/Binary.hs#L222

Currently it does one big hGetBuf to read the file.  Since the interface
files aren't changing dynamically, I think it's safe to just replace this
code with an mmap.

It's nice to see that we have several wrapped versions of mmap provided on
hackage:

http://hackage.haskell.org/package/vector-mmap
http://hackage.haskell.org/package/bytestring-mmap-0.2.2
http://hackage.haskell.org/package/mmap-0.5.7

Cheers,
  -Ryan
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