Optimisation: strict enumerations

[simonmar]

Strict enumeration types should be implemented by Int#, both in the strictness analyser and for constructor fields annotated with {-# UNPACK #-}.

[simonmar]

We get much of the benefit of this optimisation from pointer-tagging (in 6.8.1), so the payoff will be less. Probably still worth doing, but not as important now.

[simonmar]

Also requested in #4936

[simonpj]

I'd still like to see a test program showing the existing setup, compared what you get with manual unboxing of enumerations. That would give us an idea of whether there's a real perf win here.

Simon

[rl]

Wouldn't this make boxed enumeration values (esp. Bool) larger by a word?

[tibbe]

Replying to rl:

Wouldn't this make boxed enumeration values (esp. Bool) larger by a word?

Yes, unless they are also unpacked.

[rl]

I think there is actually a subtle difference between #4936 and this ticket. #4936 proposes to represent enumerations as Int which would indeed make boxed enumeration values bigger. This ticket proposes to keep the representation of boxed values as it is now but to unbox them to Int# when possible. This seems more efficient.

[tibbe]

Replying to rl:

I think there is actually a subtle difference between #4936 and this ticket. #4936 proposes to represent enumerations as Int which would indeed make boxed enumeration values bigger. This ticket proposes to keep the representation of boxed values as it is now but to unbox them to Int# when possible. This seems more efficient.

Yes. I think only transforming strict enumerations (to an Int#) is enough. No need to change non-strict enumerations to an Int.