Commit 4291bdda authored by Committed by Marge Bot
Major improvements to the specialiser
This patch is joint work of Alexis King and Simon PJ. It does some significant refactoring of the type-class specialiser. Main highlights: * We can specialise functions with types like f :: Eq a => a -> Ord b => b => blah where the classes aren't all at the front (#16473). Here we can correctly specialise 'f' based on a call like f @Int @Bool dEqInt x dOrdBool This change really happened in an earlier patch commit 2d0cf625 Author: Sandy Maguire <firstname.lastname@example.org> Date: Thu May 16 12:12:10 2019 -0400 work that this new patch builds directly on that work, and refactors it a bit. * We can specialise functions with implicit parameters (#17930) g :: (?foo :: Bool, Show a) => a -> String Previously we could not, but now they behave just like a non-class argument as in 'f' above. * We can specialise under-saturated calls, where some (but not all of the dictionary arguments are provided (#17966). For example, we can specialise the above 'f' based on a call map (f @Int dEqInt) xs even though we don't (and can't) give Ord dictionary. This may sound exotic, but #17966 is a program from the wild, and showed significant perf loss for functions like f, if you need saturation of all dictionaries. * We fix a buglet in which a floated dictionary had a bogus demand (#17810), by using zapIdDemandInfo in the NonRec case of specBind. * A tiny side benefit: we can drop dead arguments to specialised functions; see Note [Drop dead args from specialisations] * Fixed a bug in deciding what dictionaries are "interesting"; see Note [Keep the old dictionaries interesting] This is all achieved by by building on Sandy Macguire's work in defining SpecArg, which mkCallUDs uses to describe the arguments of the call. Main changes: * Main work is in specHeader, which marched down the [InBndr] from the function definition and the [SpecArg] from the call site, together. * specCalls no longer has an arity check; the entire mechanism now handles unders-saturated calls fine. * mkCallUDs decides on an argument-by-argument basis whether to specialise a particular dictionary argument; this is new. See mk_spec_arg in mkCallUDs. It looks as if there are many more lines of code, but I think that all the extra lines are comments!
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