Unpack primitive types by default in data

[Johan Tibell]

Hi all,

I've been thinking about this some more and I think we should
definitely unpack primitive types (e.g. Int, Word, Float, Double,
Char) by default.

The worry is that reboxing will cost us, but I realized today that at
least one other language, Java, does this already today and even
though it hurts performance in some cases, it seems to be a win on
average. In Java all primitive fields get auto-boxed/unboxed when
stored in polymorphic fields (e.g. in a HashMap which stores keys and
fields as Object pointers.) This seems analogous to our case, except
we might also unbox when calling lazy functions.

Here's an idea of how to test this hypothesis:

 1. Get a bunch of benchmarks.
 2. Change GHC to make UNPACK a no-op for primitive types (as library
authors have already worked around the lack of unpacking by using this
pragma.)
 3. Run the benchmarks.
 4. Change GHC to always unpack primitive types (regardless of the
presence of an UNPACK pragma.)
 5. Run the benchmarks.
 6. Compare the results.

Number (1) might be what's keeping us back right now, as we feel that
we don't have a good benchmark set. I suggest we try with nofib first
and see if there's a different and then move on to e.g. the shootout
benchmarks.

I imagine that ignoring UNPACK pragmas selectively wouldn't be too
hard. Where the relevant code?

Cheers,
Johan