On 17/02/2012 00:25, Johan Tibell wrote:
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.
Why only primitive types? I think you probably want to do it for any type that unpacks to a single word (or maybe at most 2 words? another thing to tune).
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.
nofib probably has, to a first approximation, zero strictness annotations. Because most of the programs in there predate the addition of strictness annotations to Haskell. But I think the approach of ignoring UNPACK pragmas first is the right one - otherwise we have no way to get good data about how useful it is to add it automatically. It's just a question of finding some good benchmarks then. GHC itself would be a good benchmark, incidentally... Cheers, Simon
I imagine that ignoring UNPACK pragmas selectively wouldn't be too hard. Where the relevant code?
Cheers, Johan
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