#5835: Make better use of known dictionaries -------------------------------------+------------------------------------- Reporter: rl | Owner: Type: feature | Status: new request | Milestone: 7.10.1 Priority: normal | Version: 7.5 Component: Compiler | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: Runtime | Related Tickets: performance bug | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Changes (by thomie): * failure: None/Unknown => Runtime performance bug Old description:

Example:

{{{ data T a where T :: Eq a => a -> T a

foo :: a -> T a -> Bool {-# INLINE foo #-} foo x = \(T y) -> x == y

appl :: (a -> b) -> a -> b {-# NOINLINE appl #-} appl f x = f x

bar :: T Int -> Bool bar t = appl (foo 42) t }}}

GHC generates this for `bar`:

{{{ bar2 :: Int bar2 = I# 42

bar1 :: T Int -> Bool bar1 = \ (ds_dhk :: T Int) -> case ds_dhk of _ { T $dEq_agz y_aa4 -> == @ Int $dEq_agz bar2 y_aa4 }

bar :: T Int -> Bool bar = \ (t_aga :: T Int) -> appl @ (T Int) @ Bool bar1 t_aga }}}

Note how it want to get the `Eq` dictionary for `Int` from `T`. But we know the `Eq Int` instance without inspecting `T` and `bar` could be significantly simplified if we used that.

New description: Example: {{{ {-# LANGUAGE GADTs #-} module T5835 where data T a where T :: Eq a => a -> T a foo :: a -> T a -> Bool {-# INLINE foo #-} foo x = \(T y) -> x == y appl :: (a -> b) -> a -> b {-# NOINLINE appl #-} appl f x = f x bar :: T Int -> Bool bar t = appl (foo 42) t }}} GHC generates this for `bar`: {{{ bar2 :: Int bar2 = I# 42 bar1 :: T Int -> Bool bar1 = \ (ds_dhk :: T Int) -> case ds_dhk of _ { T $dEq_agz y_aa4 -> == @ Int $dEq_agz bar2 y_aa4 } bar :: T Int -> Bool bar = \ (t_aga :: T Int) -> appl @ (T Int) @ Bool bar1 t_aga }}} Note how it want to get the `Eq` dictionary for `Int` from `T`. But we know the `Eq Int` instance without inspecting `T` and `bar` could be significantly simplified if we used that. -- Comment: Core unchanged in HEAD vs 7.5. To reproduce, use this command: `ghc -O2 -ddump-simpl -dsuppress-module-prefixes -dsuppress-idinfo T5835.hs` -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/5835#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler