#9522: SPECIALISE pragmas for derived instances -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: | Blocked By: None/Unknown | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Description changed by simonpj: Old description:

In package `ghc-prim`, in `GHC.Classes` we have {{{ instance (Eq a) => Eq [a] where {-# SPECIALISE instance Eq [[Char]] #-} {-# SPECIALISE instance Eq [Char] #-} {-# SPECIALISE instance Eq [Int] #-} [] == [] = True (x:xs) == (y:ys) = x == y && xs == ys _xs == _ys = False }}} The `SPECIALISE instance` pragmas instantiate the code for these commonly-used types.

But for tuples we use `deriving`: {{{ deriving instance (Eq a, Eq b) => Eq (a, b) }}} and many more similar. There is no way to add a `SPECIALISE instance` pragma for a derived instance. This is bad, because they are heavily used.

It should probably be possible to have a top-level {{{ {-# SPECIALISE instance Eq (Int, Bool) #-} }}} even in another module (as we can now do for functions. To do this right, we'd need to make the code for derived methods `INLINEALBE`.

New description: In package `ghc-prim`, in `GHC.Classes` we have {{{ instance (Eq a) => Eq [a] where {-# SPECIALISE instance Eq [[Char]] #-} {-# SPECIALISE instance Eq [Char] #-} {-# SPECIALISE instance Eq [Int] #-} [] == [] = True (x:xs) == (y:ys) = x == y && xs == ys _xs == _ys = False }}} The `SPECIALISE instance` pragmas instantiate the code for these commonly- used types. But for tuples we use `deriving`: {{{ deriving instance (Eq a, Eq b) => Eq (a, b) }}} and many more similar. There is no way to add a `SPECIALISE instance` pragma for a derived instance. This is bad, because they are heavily used. You can see the lossage from messages lie {{{ WARNING: file compiler/specialise/Specialise.lhs, line 673 specImport discarding: GHC.Classes.$w$c== :: forall a b. (Eq a, Eq b) => a -> b -> a -> b -> Bool @ Module.Module @ Module.Module Module.$fEqModule Module.$fEqModule }}} which says that we will end up calling `$w$c==` for pairs of modules, passing dictionaries to compare the modules for equality. These messages show up when compiling the libraries if you build your stage1 compiler with `-DDEBUG`. It should probably be possible to have a top-level {{{ {-# SPECIALISE instance Eq (Int, Bool) #-} }}} even in another module (as we can now do for functions. To do this right, we'd need to make the code for derived methods `INLINEALBE`. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9522#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#9522: SPECIALISE pragmas for derived instances -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by nfrisby): * cc: nfrisby (added) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9522#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler