September 2017 - ghc-tickets

[GHC] #14056: Figure out what to do about libffi
by GHC 21 Jan '18

21 Jan '18

#14056: Figure out what to do about libffi -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: task | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- For some platform GHC depends upon `libffi` for foreign call support in the bytecode interpreter. Unfortunately `libffi`'s upstream seems to be unresponsive and there has not been a release in nearly three years. This means that there is currently no release which has, e.g., support for AArch64 and iOS's compact debug information. While we obviously want to avoid shipping our own patched libffi, at this point we are starting to run out of options. Consider options for resolving this. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14056> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 5

[GHC] #14122: Core lint error while compiling GHC.IO.Handle
by GHC 21 Jan '18

21 Jan '18

#14122: Core lint error while compiling GHC.IO.Handle -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- While compiling `GHC.IO.Handle` with `-g3 -dcore-lint -O` I encountered the following, {{{ *** Core Lint errors : in result of Simplifier *** <no location info>: warning: [RHS of w1_s8qY :: Addr#] Recursive or top-level binder has strict demand info: w1_s8qY Binder's demand info: <L,U> }}} The binding in question is, {{{#!hs w1_s8qY :: Addr# [LclId, Unf=Unf{Src=<vanilla>, TopLvl=False, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 20 0}] w1_s8qY = src<libraries/base/GHC/IO/Handle.hs:745:37-50> src<libraries/base/GHC/IO/Handle.hs:739:9-10> "}"# }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14122> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 3

[GHC] #14006: Heap profiling ghc gives hp2ps error
by GHC 21 Jan '18

21 Jan '18

#14006: Heap profiling ghc gives hp2ps error -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Runtime | Version: 8.3 System | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Debugging Unknown/Multiple | information is incorrect Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Running GHC with heap profiling and then running `hp2ps` on the result gives {{{ hp2ps: ghc-stage2.hp, line 17405, samples out of sequence }}} Reproduction (GHC commit 4700baaf8f9bf3c44a53a595d840c7c14cfd6f98) {{{ cabal unpack skylighting-0.1.1.5 cd skylighting-0.1.1.5 cabal configure --with-ghc=$HOME/src/ghc-clean-july12/inplace/bin/ghc- stage2 \ --enable-profiling -fexecutable cabal build touch src/Skylighting/Syntax/Php.hs /home/dfeuer/src/ghc-clean-july12/inplace/bin/ghc-stage2 --make -i \ -isrc -idist/build -O -prof -fprof-auto-exported -osuf p_o -hisuf \ p_hi -optPdist/build/autogen/cabal_macros.h -hide-all-packages \ -package-db dist/package.conf.inplace -package-id aeson-1.2.1.0 \ -package base-4.10.0.0 -package blaze-html-0.9.0.1 -package \ bytestring-0.10.8.2 -package case-insensitive-1.2.0.10 -package \ containers-0.5.10.2 -package directory-1.3.0.2 -package \ filepath-1.4.1.2 -package hxt-9.3.1.16 -package mtl-2.2.1 -package \ regex-pcre-builtin-0.94.4.8.8.35 -package safe-0.3.15 -package \ text-1.2.2.2 -package utf8-string-1.0.1.1 -XHaskell2010 \ Skylighting.Types Skylighting.Regex \ Skylighting.Tokenizer Skylighting.Syntax.Php -j1 +RTS \ -s -h -N1 hp2ps ghc-stage2.hp }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14006> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 8

[GHC] #12825: ghc panic on ppc64el, ghc 8.0.1, agda 2.5.1.1 patched for newer EdisonAPI
by GHC 21 Jan '18

21 Jan '18

#12825: ghc panic on ppc64el, ghc 8.0.1, agda 2.5.1.1 patched for newer EdisonAPI ---------------------------------+--------------------------------- Reporter: clint | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Linux Architecture: powerpc64 | Type of failure: None/Unknown Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: ---------------------------------+--------------------------------- {{{ [331 of 332] Compiling Agda.Interaction.EmacsTop ( src/full/Agda/Interaction/EmacsTop.hs, dist- ghc/build/Agda/Interaction/EmacsTop.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.0.1 for powerpc64le-unknown-linux): applyTypeToArgs Expression: $w$cgmapQl2 stdin LineBuffering s_aJ4 Type: forall r_a2gyr r'_a2gys. (r_a2gyr -> r'_a2gys -> r_a2gyr) -> r_a2gyr -> (forall d_a2gyt. Data d_a2gyt => d_a2gyt -> r'_a2gys) -> [ModulePragma] -> ModuleName -> r_a2gyr Args: [stdin, LineBuffering, s_aJ4] Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12825> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 3

[GHC] #11359: Consider removing RelaxedLayout and friends
by GHC 21 Jan '18

21 Jan '18

#11359: Consider removing RelaxedLayout and friends -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: task | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 7.10.3 (Parser) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Other Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- These three parsing-related language extensions were [[https://ghc.haskell.org/trac/ghc/wiki/LanguagePragmaHistory|introduced]] some time ago yet are likely not used in the wild as they still are not registered in Cabal's language extension list (which Hackage uploads are checked against, see #8176), * `RelaxedLayout` (introduced in 7.2, prompted by #1060) * `AlternativeLayoutRule` (introduced in 7.0, reason unknown) * `AlternativeLayoutRuleTransitional` (introduced in 7.0, reason unknown) As far as [[https://mail.haskell.org/pipermail/ghc- devs/2013-September/002267.html|I can]] [[https://ghc.haskell.org/trac/ghc/ticket/8176#comment:10|tell]] no one knows the status of these, nor are they listed in the users guide. We should ask the community whether they would mind seeing these extensions vanish and remove them if not. If so then we should see to it that they are registered properly with Cabal so they can be used on Hackage. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11359> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 2

[GHC] #11091: Fix MonadFail warnings
by GHC 21 Jan '18

21 Jan '18

#11091: Fix MonadFail warnings -------------------------------------+------------------------------------- Reporter: quchen | Owner: Type: task | Status: new Priority: low | Milestone: 8.2.1 Component: Compiler | Version: 7.11 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- #10751 implements the MonadFail warning mechanism. Using `-fwarn-missing-monadfail-instance`, GHC's internal warnings created by the patch can be shown and fixed. A second pass with `-XMonadFailDesugaring` then provides a hard check on whether any issues remain, by switching on the new desugaring mechanism properly. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11091> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 1

[GHC] #14273: Typed holes are oblivious to type class constraints
by GHC 20 Jan '18

20 Jan '18

#14273: Typed holes are oblivious to type class constraints -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.3 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This example is taken from Chris Allen's blog post [http://bitemyapp.com//posts/2017-09-23-please-stop-using-typed-holes.html here]: {{{#!hs pleaseShow :: Show a => Bool -> a -> Maybe String pleaseShow False _ = Nothing pleaseShow True a = Just (show _a) }}} On a recent GHC HEAD build, compiling this outputs: {{{ Bug.hs:3:32: error: • Found hole: _a :: a0 Where: ‘a0’ is an ambiguous type variable Or perhaps ‘_a’ is mis-spelled, or not in scope • In the first argument of ‘show’, namely ‘_a’ In the first argument of ‘Just’, namely ‘(show _a)’ In the expression: Just (show _a) • Relevant bindings include a :: a (bound at Bug.hs:3:17) pleaseShow :: Bool -> a -> Maybe String (bound at Bug.hs:2:1) Valid substitutions include (++) :: forall a. [a] -> [a] -> [a] (imported from ‘Prelude’ at Bug.hs:1:1 (and originally defined in ‘GHC.Base’)) fail :: forall (m :: * -> *). Monad m => forall a. String -> m a (imported from ‘Prelude’ at Bug.hs:1:1 (and originally defined in ‘GHC.Base’)) return :: forall (m :: * -> *). Monad m => forall a. a -> m a (imported from ‘Prelude’ at Bug.hs:1:1 (and originally defined in ‘GHC.Base’)) errorWithoutStackTrace :: forall (a :: TYPE r). [Char] -> a (imported from ‘Prelude’ at Bug.hs:1:1 (and originally defined in ‘GHC.Err’)) seq :: forall a b. a -> b -> b (imported from ‘Prelude’ at Bug.hs:1:1 (and originally defined in ‘GHC.Prim’)) (<>) :: forall a. Semigroup a => a -> a -> a (imported from ‘Prelude’ at Bug.hs:1:1 (and originally defined in ‘GHC.Base’)) (Some substitutions suppressed; use -fmax-valid-substitutions=N or -fno-max-valid-substitutions) | 3 | pleaseShow True a = Just (show _a) | ^^ }}} There are a couple very unsavory things about this error: 1. GHC makes no attempt to inform me that `a0` is a `Show` instance! This is the primary gripe in the blog post, and it's worth emphasizing, since without the `Show` constraint, `a0` just looks like any other random type variable. Speaking of which... 2. The list of valid substitutions is incorrect! It suggests several things which have function types, such as `(++)` and `fail`, but `(->)` does not have a `Show` instance! This list ought to be pruned based on the current type class constraints in scope. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14273> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 24

[GHC] #13016: SPECIALIZE INLINE doesn't necessarily inline specializations of a recursive function
by GHC 17 Jan '18

17 Jan '18

#13016: SPECIALIZE INLINE doesn't necessarily inline specializations of a recursive function -------------------------------------+------------------------------------- Reporter: nfrisby | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: #13014 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_ext… #specialize-inline The user's guide] for `SPECIALIZE INLINE` states it will do a "type-based unrolling" of a recursive function over GADTs. It gives an example, which I've munged a bit to simplify and listed here. {{{#!hs {-# Language GADTs #-} module C where data Arr e where ArrInt :: !Int -> Arr Int ArrPair :: Arr e1 -> Arr e2 -> Arr (e1, e2) (!:) :: Arr e -> Int -> e {-# SPECIALISE INLINE (!:) :: Arr Int -> Int -> Int #-} {-# SPECIALISE INLINE (!:) :: Arr (a, b) -> Int -> (a, b) #-} (ArrInt ba) !: i = ba * i (ArrPair a1 a2) !: i = (a1 !: i, a2 !: i) }}} {{{#!hs module D where import C example = ArrPair (ArrInt 2) (ArrInt 3) !: 5 }}} The specialize rule for pairs fires, but it does not get inlined. This is because the specialization for pairs is marked as a loopbreaker. This behavior contradicts the text from the users guide, emphasis mine: Here, `(!:)` is a recursive function that indexes arrays of type `Arr e`. Consider a call to `(!:)` at type `(Int,Int)`. The second specialisation will fire, ''and the specialised function will be inlined''. It has two calls to `(!:)`, both at type `Int`. Both these calls fire the first specialisation, whose body is also inlined. The result is a type-based unrolling of the indexing function. If I move the `SPECIALIZE INLINE` pragma to the downstream module, then it is not marked as a loopbreaker and we see the expected type-based unrolling. Two possible ways to resolve this ticket: * `SPECIALIZE INLINE` should always achieve supercompilation even if declared in the defining module; the specialization should not be marked as a loopbreaker. * The docs should be updated to say the pragma must be declared in a separate module. I suggest the former. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13016> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 12

[GHC] #13683: Use epoll rather than poll where appropriate
by GHC 16 Jan '18

16 Jan '18

#13683: Use epoll rather than poll where appropriate ----------------------------------------+--------------------------------- Reporter: dfeuer | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: 8.4.1 Component: Core Libraries | Version: 8.0.1 Keywords: | Operating System: Linux Architecture: Unknown/Multiple | Type of failure: None/Unknown Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: ----------------------------------------+--------------------------------- Linux supports an `epoll` system call that's supposed to scale better to large numbers of watched file descriptors than `poll`. We should probably use it when available and appropriate. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13683> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 4

[GHC] #10972: Add a :binfo (beginner info) GHCi command
by GHC 16 Jan '18

16 Jan '18

#10972: Add a :binfo (beginner info) GHCi command -------------------------------------+------------------------------------- Reporter: kanetw | Owner: Type: feature | Status: new request | Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: #10963 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- From goldfire: > Add a new command to GHCi, :binfo (for "beginner info") (please suggest a better name) that is like :info but presents different information. It suppresses in-scope instances, but attempts to specialize a function's type and presents specializations, using heuristics. So when we say :binfo find, we'll see Foldable t => (a -> Bool) -> t a -> Maybe a but also (a -> Bool) -> [a] -> Maybe a. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10972> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 10