#14737: Improve performance of Simplify.simplCast -------------------------------------+------------------------------------- Reporter: tdammers | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: #11735 #14683 | Differential Rev(s): Phab:D4385 Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): Replying to [comment:16 simonpj]:
If `arg` is `Refl` then `kind_co` is also `Refl`,
Why do you say this? `kind_co` must be reflexive, but it needn't be `Refl`. Especially because it hasn't been optimized yet!
But perhaps spotting a `Refl` argument would be a bit more direct?
I think it would.
Meanwhile * I agree that `kind_co` should be substituted. But how? By calling
`ope_co4` on it? Or `opt_co3`? I don't understand the hierarchy of `opt_co` functions. You call the highest-numbered one for which you can meet the preconditions. In this case, that's `opt_co4` because we know `kind_co`'s role (nominal), and it should keep that role after optimization.
* I find that code for `extendLiftingContext` hard to grok. In the `Refl` case we extend the `TvSubst`; otherwise we extend the `LiftCoEnv`. Very mysterious. I looked at it a bit, but got lost in the deeply cryptic `liftEnvSubst`.
You're right. This needs more documentation -- but a quick glance at this all suggests the code is currently correct.
What we need, as usual, is to accumulate those arguments in a list;
Yes, of course.
You mean that `Refl (forall a. ty)` can be regarded as a form of `ForAllCo`? Especially since `mkForAllCo` goes to some trouble to build a `Refl` if it can. So surely yes, `splitForAllCo_maybe` should split a `Refl (forall a.ty)`.
Yes, that's exactly what I meant.
The only way InstCos can come into being is in the coercion optimizer.
I was searching over a very old checkout of the codebase, and I wrongly assumed this hadn't changed. You're right of course.
Things to do * Fix the missing substitution in `opt_co4` * Fix `splitForAllCo_maybe` * Fix `opt_co4` to behave well on deeply nsted `InstCos` Might you do these -- you are more likely to get them right than me.
OK. Hopefully can do in the next two weeks -- but I can't promise sooner. :( -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14737#comment:17 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler