#14737: Improve performance of Simplify.simplCast -------------------------------------+------------------------------------- Reporter: tdammers | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 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): | Wiki Page: -------------------------------------+------------------------------------- Splitting off task 3 from #11735. When compiling [https://ghc.haskell.org/trac/ghc/attachment/ticket/14683/Grammar.hs], `simplCast` eats up more execution time than we think it should. From [https://ghc.haskell.org/trac/ghc/ticket/11735#comment:10]:
Something is clearly wrong with `Simplify.simplCast`. I think I know what it is. Given {{{ (fun |> co) @t1 @t2 ... @tn }}} we will call `pushCoTyArg` `n` times, and hence does `n` singleton substitutions, via the `n` calls to `piResultTy`.
Solution: gather up those type arguments (easy) and define {{{ pushCoTyArgs :: Coercion -> [Type] -> Maybe ([Type], Coercion) }}}
OK. I looked at `pushCoTyArg` and friends, and I have a very simple solution: just move the `isReflexiveCo` case in `addCoerce` (a local function within `Simplify.simplCast`) to the top. That should do it. Then `pushCoTyArg` is never called with a reflexive coercion, and so the `piResultTy` case won't happen.
Now, `pushCoArgs` might still call `pushCoTyArg` with a reflexive coercion, but it can be taught not to as well: Have `pushCoArgs` return a `Maybe ([CoreArg], Maybe Coercion)` and `pushCoArg` return a `Maybe (CoreArg, Maybe Coercion)`. If the second return values are `Nothing`,
And [https://ghc.haskell.org/trac/ghc/ticket/11735#comment:41]: that means that there is no cast (i.e., that the cast would have been reflexive). The only client of `pushCoArg(s)` is `exprIsConApp_maybe`, which simply omits a cast if `pushCoArgs` returns `Nothing`. Then, we never have to bother creating the reflexive coercions.
This should be an easy win all around.
-- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14737 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler