January 2015 - ghc-tickets

[GHC] #9557: Deriving instances is slow
by GHC 06 May '17

06 May '17

#9557: Deriving instances is slow -------------------------------------+------------------------------------- Reporter: Feuerbach | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Let's take [https://raw.githubusercontent.com/haskell-suite/haskell-src- exts/8e0153d33b66add96fd5606614ce7938d2029510/src/Language/Haskell/Exts/Annotated/Syntax.hs this file] (from haskell-src-exts) and build it with -O0. On my machine it takes 55s. If we remove deriving of all classes except Functor (which is the only one used by the module itself), the time will drop down to 4s. Different classes vary in their compile-time cost, but there's no single culprit. Among the costly ones are Generic, Data, Show, Ord. haskell-src-exts users are very annoyed by its long compile time, about 10 minutes, especially because we don't compile with -O0 by default. It would be nice to get this fixed. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9557> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8443: cannot find normal object file when compiling TH code
by GHC 01 May '17

01 May '17

#8443: cannot find normal object file when compiling TH code ----------------------------+---------------------------------------------- Reporter: snoyberg | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: GHC rejects valid program (amd64) | Test Case: Difficulty: Unknown | Blocking: Blocked By: | Related Tickets: | ----------------------------+---------------------------------------------- Unfortunately, I haven't been able to whittle this down to a smaller test case. In order to reproduce, just run `cabal install yesod-core-1.2.4.5 --enable-tests`. The result is: {{{ test/YesodCoreTest/NoOverloadedStrings.hs:16:26: cannot find normal object file ‛dist/build/tests/tests- tmp/YesodCoreTest/NoOverloadedStringsSub.dyn_o’ while linking an interpreted expression }}} Tested using: The Glorious Glasgow Haskell Compilation System, version 7.7.20131012 cabal-install version 1.18.0.2 using version 1.18.0 of the Cabal library I'm on Ubuntu 12.04 64-bit. Compiling with GHC 7.6.3 and 7.4.2 works correctly. Let me know if there's any other information I can provide. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8443> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8996: mark more things as const in C codegen
by GHC 24 Apr '17

24 Apr '17

#8996: mark more things as const in C codegen -------------------------------------------+------------------------------- Reporter: slyfox | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (CodeGen) | Version: 7.8.2 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Easy (less than 1 hour) | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: -------------------------------------------+------------------------------- Recently i've tried to build ghc-7.8.2 on ia64 and it overflows so called "short data" section. (it's where global 'static' variables live). I've looked at low-hanging fruits of squashing such global "variables". One source of them seems to be a string literal. Consider one-line module: {{{ module B (v) where v = "hello" }}} in '''-fvia-C''' (or unreg) mode it generates code like {{{ static char gibberish_str[] = "hello"; }}} It uselessly eats data section. The patch switches generator to emit: {{{ static const char gibberish_str[] = "hello"; }}} Some notes: 1. as far as I see native codegens already put ot to .rodata, thus it should be safe. 2. I likely didn't cover more cases, like - RelocatableReadOnlyData - and mysterious '''pprTop (CmmData _section (Statics lbl [CmmUninitialised size]))''' Size of unreg-stage2 on amd64 before and after the patch: {{{ $ size inplace/lib/bin/ghc-stage2 (unpatched) text data bss dec hex filename 81986382 20776344 44096 102806822 620b526 inplace/lib/bin/ghc-stage2 $ size inplace/lib/bin/ghc-stage2 (patched) text data bss dec hex filename 83648494 19062936 44096 102755526 61fecc6 inplace/lib/bin/ghc-stage2 }}} Text section increased for 1.6MBs (consts moved here, likely .rodata actually), data section decreased for 1.7MBs. I think with minor fiddling with linker flags we can merge equal constants later on. Thanks! -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8996> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8808: ImpredicativeTypes type checking fails depending on syntax of arguments
by GHC 24 Apr '17

24 Apr '17

#8808: ImpredicativeTypes type checking fails depending on syntax of arguments -------------------------------------+------------------------------------- Reporter: guest | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.8.1-rc1 checker) | Operating System: Unknown/Multiple Keywords: | Type of failure: GHC rejects Architecture: Unknown/Multiple | valid program Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | -------------------------------------+------------------------------------- g1 and g2 below type check, but g1', g2', and g2'' don't even though the types are exactly the same. {{{ {-# LANGUAGE ImpredicativeTypes, NoMonomorphismRestriction #-} module Test where f1 :: Maybe (forall a. [a] -> [a]) -> Maybe ([Int], [Char]) f1 (Just g) = Just (g [3], g "hello") f1 Nothing = Nothing f2 :: [forall a. [a] -> [a]] -> Maybe ([Int], [Char]) f2 [g] = Just (g [3], g "hello") f2 [] = Nothing g1 = (f1 . Just) reverse g1' = f1 (Just reverse) g2 = f2 [reverse] g2' = f2 ((:[]) reverse) g2'' = f2 (reverse : []) }}} Compiling it with HEAD gives these errors: {{{ [1 of 1] Compiling Test ( test.hs, test.o ) test.hs:12:16: Couldn't match expected type ‛forall a. [a] -> [a]’ with actual type ‛[a2] -> [a2]’ In the first argument of ‛Just’, namely ‛reverse’ In the first argument of ‛f1’, namely ‛(Just reverse)’ test.hs:15:17: Couldn't match expected type ‛forall a. [a] -> [a]’ with actual type ‛[a0] -> [a0]’ In the first argument of ‛: []’, namely ‛reverse’ In the first argument of ‛f2’, namely ‛((: []) reverse)’ test.hs:16:12: Couldn't match expected type ‛forall a. [a] -> [a]’ with actual type ‛[a1] -> [a1]’ In the first argument of ‛(:)’, namely ‛reverse’ In the first argument of ‛f2’, namely ‛(reverse : [])’ }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8808> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9210: "overlapping instances" through FunctionalDependencies
by GHC 21 Apr '17

21 Apr '17

#9210: "overlapping instances" through FunctionalDependencies -------------------------------------------+------------------------------- Reporter: rwbarton | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type checker) | Version: 7.8.1 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: -------------------------------------------+------------------------------- This program prints `("1",2)`, but if you reverse the order of the two instances, it prints `(1,"2")`. {{{ {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FunctionalDependencies #-} -- extracted from http://lpaste.net/105656 import Control.Applicative import Data.Functor.Identity modify :: ((a -> Identity b) -> s -> Identity t) -> (a -> b) -> (s -> t) modify l f s = runIdentity (l (Identity . f) s) class Foo s t a b | a b s -> t where foo :: Applicative f => (a -> f b) -> s -> f t instance Foo (x, a) (y, a) x y where foo f (a,b) = (\fa -> (fa,b)) <$> f a instance Foo (a, x) (a, y) x y where foo f (a,b) = (\fb -> (a,fb)) <$> f b main = print $ modify foo (show :: Int -> String) (1 :: Int, 2 :: Int) }}} Note that the two instances involved `Foo (Int, Int) (String, Int) Int String` and `Foo (Int, Int) (Int, String) Int String` are not actually overlapping. But, they have the same `a`, `b`, and `s` fields and it seems that this makes GHC think that either one is equally valid, thanks to the fundep. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9210> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9456: Weird behavior with polymorphic function involving existential quantification and GADTs
by GHC 19 Apr '17

19 Apr '17

#9456: Weird behavior with polymorphic function involving existential quantification and GADTs -------------------------------------+------------------------------------- Reporter: haasn | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.8.2 checker) | Operating System: Keywords: | Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: GHC Difficulty: Unknown | rejects valid program Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- This program is rejected by GHC 7.8.2: {{{ {-# LANGUAGE GADTs, ExistentialQuantification #-} module Foo where data Box = forall a. Box a foo :: Monad m => m () foo = do box <- return (Box ()) let f x = let _ = Box x `asTypeOf` box in x g :: a -> a g = f return () }}} With the following type error: {{{ foo.hs:15:11: Couldn't match type ‘a0’ with ‘a’ because type variable ‘a’ would escape its scope This (rigid, skolem) type variable is bound by the type signature for g :: a -> a at foo.hs:14:12-17 Expected type: a -> a Actual type: a0 -> a0 Relevant bindings include g :: a -> a (bound at foo.hs:15:7) f :: a0 -> a0 (bound at foo.hs:10:7) In the expression: f In an equation for ‘g’: g = f }}} Perplexingly, however, you can get it to compile either by adding a type signature to f: {{{ {-# LANGUAGE GADTs, ExistentialQuantification #-} module Foo where data Box = forall a. Box a foo :: Monad m => m () foo = do box <- return (Box ()) let f :: x -> x f x = let _ = Box x `asTypeOf` box in x g :: a -> a g = f return () }}} Or by disabling the GADTs extension: {{{ {-# LANGUAGE ExistentialQuantification #-} module Foo where data Box = forall a. Box a foo :: Monad m => m () foo = do box <- return (Box ()) let f x = let _ = Box x `asTypeOf` box in x g :: a -> a g = f return () }}} Or by getting rid of the asTypeOf: {{{ {-# LANGUAGE GADTs, ExistentialQuantification #-} module Foo where data Box = forall a. Box a foo :: Monad m => m () foo = do box <- return (Box ()) let f x = let _ = Box x in x g :: a -> a g = f return () }}} Or by defining ‘box’ differently: {{{ {-# LANGUAGE GADTs, ExistentialQuantification #-} module Foo where data Box = forall a. Box a foo :: Monad m => m () foo = do let box = Box () let f x = let _ = Box x `asTypeOf` box in x g :: a -> a g = f return () }}} And perhaps some other factors that I haven't tested yet. It appears to me that all of these should be valid programs. Real world source: https://github.com/andygill/io- reactive/blob/master/Control/Concurrent/Reactive.hs It happens here due to the usage of (putMVar box ret), which makes the function requestit in line 77 not as polymorphic as it should be, unless you add a type signature. The putMVar serves the same role as the `asTypeOf` in my smaller example. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9456> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9605: Misleading error message with forgotten "do"
by GHC 19 Apr '17

19 Apr '17

#9605: Misleading error message with forgotten "do" -------------------------------------+------------------------------------- Reporter: owst | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- In the following (simplified) code, I forgot to add the do keyword, when doing some refactoring. This caused GHC to give a strange error message that said approximately "The function F is applied to two arguments, but its type T has only two"... which is odd! {{{#!hs import Control.Monad.RWS ( evalRWS, RWS ) import Control.Monad.Writer.Class ( tell ) main = print $ evalRWS (go 1) () () go :: Int -> RWS () [String] () () go i = --Oops, forgot a do here return (show i) >>= (\l -> tell [l]) -- go $ i + 1 go i }}} Strangely, replacing the "go i" line with the commented-out line above it gives a sensible error message, the "Possible cause: `\l -> tell [l]' is applied to too many arguments" message, which is correct, and would've pointed me straight to the problem. The invalid error message, which appears to be the same using GHC 7.6.3 and 7.8.3, is: {{{ NFA.hs:8:25: Couldn't match type ‘String -> Control.Monad.Trans.RWS.Lazy.RWST () [String] () Data.Functor.Identity.Identity ()’ with ‘()’ Expected type: Int -> String -> Control.Monad.Trans.RWS.Lazy.RWST () [String] () Data.Functor.Identity.Identity () Actual type: Int -> () The function ‘\ l -> tell [l]’ is applied to two arguments, but its type ‘(Int -> RWS () [String] () ()) -> Int -> ()’ has only two In the second argument of ‘(>>=)’, namely ‘(\ l -> tell [l]) go i’ In the expression: return (show i) >>= (\ l -> tell [l]) go i }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9605> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8426: one-shot compilation + TH doesn't see instances that is seen in batch mode
by GHC 15 Apr '17

15 Apr '17

#8426: one-shot compilation + TH doesn't see instances that is seen in batch mode ----------------------------------------------+---------------------------- Reporter: errge | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Template Haskell | Version: 7.6.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Moderate (less than a day) | Type of failure: Other Blocked By: | Test Case: Related Tickets: #7867 | Blocking: ----------------------------------------------+---------------------------- Haskell prime and Haskell98 both states that instances are always seen if there is any chain of imports leading from the current module to the instance. http://darcs.haskell.org/haskell-prime-report/report/haskell-report- html/modules.html#import-instances The ghc manual contains a discussion why this specification is too performance heavy to näively implement and contains the definition of orphanness and an algorithm to reproduce the specification with better performance. http://www.haskell.org/ghc/docs/7.6.3/html/users_guide/separate- compilation.html#orphan-modules Unfortunately this cleverness breaks template haskell. Namely, if: - `Class.hs` contains a class, - `NonOrphan.hs` contains a data that implements the class, - `Importer.hs` just imports `NonOrphan.hs`, - `Main.hs` imports `Importer.hs`, - `Main.hs` reifies the class; then: - one-shot compilation's reify doesn't see the instance, - batch compilation's reify sees the instance. This ambiguity is shown in the attached tgz. Furthermore if `NonOrphan.hs` is in a separate package, then of course even batch compilation mode wouldn't reify the instance correctly. In that case there is no disambiguity between one-shot and batch, simply both is missing the info. I propose to solve this by enforcing a loading of all the interface files for every import transitively for the current module, when we meet an open type family or class reification request in template haskell's reify handler in `TcSplice.hs`. So I propose to keep the optimization of orphan instances and use it 99.9% of the time. We would only ever go on the quest of reading interface files for every import transitively when the user tries to reify something where we have to return an instance list. This means that TH compilations that reify types will get a little bit slower in exchange of being correct and unambiguous. Compilation time in cases when TH reification is not used will not change. I volunteer to prepare the patch, but would like to hear others first. So, any opinions, alternative ideas? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8426> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9066: Template Haskell cannot splice an infix declaration for a data constructor
by GHC 13 Apr '17

13 Apr '17

#9066: Template Haskell cannot splice an infix declaration for a data constructor ------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Template Haskell | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- When I say {{{ $([d| data Blargh = (:<=>) Int Int infix 4 :<=> |]) }}} I get {{{ Illegal variable name: ‘:<=>’ When splicing a TH declaration: infix 4 :<=>_0 }}} The code inside the TH quote works when not used with TH. I will fix in due course. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9066> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8585: Loopification should omit stack check
by GHC 11 Apr '17

11 Apr '17

#8585: Loopification should omit stack check ------------------------------+-------------------------------------------- Reporter: jstolarek | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime performance bug Unknown/Multiple | Test Case: Difficulty: Unknown | Blocking: Blocked By: | Related Tickets: | ------------------------------+-------------------------------------------- When we optimize a self-recursive tail call using loopification we don't need to perform a stack check in every loop. This is because a tail recursive function uses constant amount of stack. We should therefore place the loop header after the stack check, but before the heap check (because heap may grow in every call). parcs proposed a patch [https://ghc.haskell.org/trac/ghc/ticket/8275#comment:34 here]. We just need to make sure why it works and add some documentation. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8585> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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