January 2019 - ghc-tickets

[GHC] #16036: expDouble## 0.0## doesn't get complied into 1.0##
by GHC 20 Feb '19

20 Feb '19

#16036: expDouble## 0.0## doesn't get complied into 1.0## -------------------------------------+------------------------------------- Reporter: Fuuzetsu | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 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: -------------------------------------+------------------------------------- {{{ [nix-shell:/tmp]$ cat T.hs {-# LANGUAGE MagicHash #-} {-# OPTIONS_GHC -O2 -ddump-simpl -ddump-to-file #-} module T (f) where import GHC.Float import GHC.Prim f :: Double f = D# (expDouble# 0.0## -## 1.0##) [nix-shell:/tmp]$ ghc -fforce-recomp T.hs [1 of 1] Compiling T ( T.hs, T.o ) [nix-shell:/tmp]$ cat T.dump-simpl ==================== Tidy Core ==================== 2018-12-12 02:23:40.288094743 UTC Result size of Tidy Core = {terms: 20, types: 6, coercions: 0, joins: 0/0} -- RHS size: {terms: 5, types: 0, coercions: 0, joins: 0/0} f :: Double [GblId, Str=m, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=False, Expandable=False, Guidance=IF_ARGS [] 15 20}] f = GHC.Types.D# (-## (expDouble# 0.0##) 1.0##) … }}} This was spotted in real code where we had GHC generate something like {{{#!haskell 1# -> jump $j9_s1IVU (GHC.Prim.+## sc3_s2pVI (GHC.Prim.-## (GHC.Prim.expDouble# 0.0##) 1.0##)) }}} I would expect this (I think reasonably) to be just `{{{jump $j9_s1IVU sc3_s2pVI}}}`. Maybe {{{expDouble## <literal>##}}} should always evaluate at compile time to not block further constant folding? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16036> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13971: Misleading "Kind mis-match on LHS of default declaration" error
by GHC 19 Feb '19

19 Feb '19

#13971: Misleading "Kind mis-match on LHS of default declaration" error -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: TypeFamilies | 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 program fails to typecheck, unsurprisingly: {{{#!hs {-# LANGUAGE PolyKinds #-} {-# LANGUAGE TypeFamilies #-} module Bug where class C a where type T a :: k type T a = Int }}} But the error message it gives threw me for a loop initially: {{{ $ /opt/ghc/8.2.1/bin/ghci Bug.hs GHCi, version 8.2.0.20170704: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:7:3: error: • Kind mis-match on LHS of default declaration for ‘T’ • In the default type instance declaration for ‘T’ In the class declaration for ‘C’ | 7 | type T a = Int | ^^^^^^^^^^^^^^ Failed, 0 modules loaded. }}} The LHS of the default declaration is perfectly fine - the real source of the error is the RHS! -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13971> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #16168: Prelude docs for Integer mention J#
by GHC 19 Feb '19

19 Feb '19

#16168: Prelude docs for Integer mention J# -------------------------------------+------------------------------------- Reporter: nomeata | Owner: (none) Type: bug | Status: new Priority: low | Milestone: Component: Core | Version: 8.6.3 Libraries | Keywords: newcomer | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I was just looking through http://hackage.haskell.org/package/base-4.12.0.0/docs/Prelude.html#t:Integer and noticed that it mentions `J#` and `I#`. This is certainly an implementation detail that should not be exposed to readers of the Prelude documentation. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16168> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13706: ghci messages like "Ok, modules loaded: ..." are impractical for large sets of modules
by GHC 18 Feb '19

18 Feb '19

#13706: ghci messages like "Ok, modules loaded: ..." are impractical for large sets of modules -------------------------------------+------------------------------------- Reporter: j.waldmann | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: GHCi | Version: 8.0.1 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: -------------------------------------+------------------------------------- Ghci (in the default config) wants to show me all the modules that are currently loaded. If I have a great many of these (think: hundreds) then this is impractical, because I have to scroll back to see what actually interests me (output of last command). I can change the prompt (`:set prompt "> "`), but I don't see how to switch off the display of all module names in `:r` command. I have to scroll back (several screenfulls) to even find out whether the reload was successful, since the status (OK/Failed) is at the very beginning - and the reason for the Failure is before that. Cf. https://mail.haskell.org/pipermail/haskell-cafe/2017-May/126983.html (but there was no reaction) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13706> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14040: Typed holes regression in GHC 8.0.2: No skolem info: z_a1sY[sk:2]
by GHC 18 Feb '19

18 Feb '19

#14040: Typed holes regression in GHC 8.0.2: No skolem info: z_a1sY[sk:2] -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 (Type checker) | Keywords: TypeInType, | Operating System: Unknown/Multiple TypeFamilies, | PartialTypeSignatures | Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: #13877 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- (Originally spun off from #13877.) The following program gives a somewhat decent error message in GHC 8.0.1: {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeApplications #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeInType #-} module Bug where import Data.Kind data family Sing (a :: k) data WeirdList :: Type -> Type where WeirdNil :: WeirdList a WeirdCons :: a -> WeirdList (WeirdList a) -> WeirdList a data instance Sing (z :: WeirdList a) where SWeirdNil :: Sing WeirdNil SWeirdCons :: Sing w -> Sing wws -> Sing (WeirdCons w wws) elimWeirdList :: forall (a :: Type) (wl :: WeirdList a) (p :: forall (x :: Type). x -> WeirdList x -> Type). Sing wl -> (forall (y :: Type). p _ WeirdNil) -> (forall (z :: Type) (x :: z) (xs :: WeirdList (WeirdList z)). Sing x -> Sing xs -> p _ xs -> p _ (WeirdCons x xs)) -> p _ wl elimWeirdList SWeirdNil pWeirdNil _ = pWeirdNil elimWeirdList (SWeirdCons (x :: Sing (x :: z)) (xs :: Sing (xs :: WeirdList (WeirdList z)))) pWeirdNil pWeirdCons = pWeirdCons @z @x @xs x xs (elimWeirdList @(WeirdList z) @xs @p xs pWeirdNil pWeirdCons) }}} {{{ $ /opt/ghc/8.0.1/bin/ghci Foo.hs GHCi, version 8.0.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Foo.hs, interpreted ) Foo.hs:34:8: error: • Cannot apply expression of type ‘Sing wl -> (forall y. p x0 t3 'WeirdNil) -> (forall z (x :: z) (xs :: WeirdList (WeirdList z)). Sing x -> Sing xs -> p (WeirdList z) t2 xs -> p z t1 ('WeirdCons x xs)) -> p a t0 wl’ to a visible type argument ‘WeirdList z’ • In the sixth argument of ‘pWeirdCons’, namely ‘(elimWeirdList @(WeirdList z) @xs @p xs pWeirdNil pWeirdCons)’ In the expression: pWeirdCons @z @x @xs x xs (elimWeirdList @(WeirdList z) @xs @p xs pWeirdNil pWeirdCons) In an equation for ‘elimWeirdList’: elimWeirdList (SWeirdCons (x :: Sing (x :: z)) (xs :: Sing (xs :: WeirdList (WeirdList z)))) pWeirdNil pWeirdCons = pWeirdCons @z @x @xs x xs (elimWeirdList @(WeirdList z) @xs @p xs pWeirdNil pWeirdCons) }}} But in GHC 8.0.2, 8.2.1, and HEAD, it panics to varying degrees: {{{ $ /opt/ghc/8.0.2/bin/ghci Foo.hs GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Foo.hs, interpreted ) Foo.hs:24:41: error: • Found type wildcard ‘_’ standing for ‘t0’ Where: ‘t0’ is an ambiguous type variable ‘x0’ is an ambiguous type variable To use the inferred type, enable PartialTypeSignatures • In the type signature: elimWeirdList :: forall (a :: Type) (wl :: WeirdList a) (p :: forall (x :: Type). x -> WeirdList x -> Type). Sing wl -> (forall (y :: Type). p _ WeirdNil) -> (forall (z :: Type) (x :: z) (xs :: WeirdList (WeirdList z)). Sing x -> Sing xs -> p _ xs -> p _ (WeirdCons x xs)) -> p _ wl • Relevant bindings include elimWeirdList :: Sing wl -> (forall y. p x0 t0 'WeirdNil) -> (forall z (x :: z) (xs :: WeirdList (WeirdList z)). Sing x -> Sing xs -> p (WeirdList z) t1 xs -> p z t2 ('WeirdCons x xs)) -> p a t3 wl (bound at Foo.hs:29:1) Foo.hs:26:44: error:ghc: panic! (the 'impossible' happened) (GHC version 8.0.2 for x86_64-unknown-linux): No skolem info: z_a13X[sk] }}} {{{ $ /opt/ghc/8.2.1/bin/ghci Foo.hs GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Foo.hs, interpreted ) Foo.hs:21:18: error: • The kind of variable ‘wl1’, namely ‘WeirdList a1’, depends on variable ‘a1’ from an inner scope Perhaps bind ‘wl1’ sometime after binding ‘a1’ • In the type signature: elimWeirdList :: forall (a :: Type) (wl :: WeirdList a) (p :: forall (x :: Type). x -> WeirdList x -> Type). Sing wl -> (forall (y :: Type). p _ WeirdNil) -> (forall (z :: Type) (x :: z) (xs :: WeirdList (WeirdList z)). Sing x -> Sing xs -> p _ xs -> p _ (WeirdCons x xs)) -> p _ wl | 21 | elimWeirdList :: forall (a :: Type) (wl :: WeirdList a) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^... Foo.hs:24:41: error: • Found type wildcard ‘_’ standing for ‘w0’ Where: ‘w0’ is an ambiguous type variable ‘x0’ is an ambiguous type variable To use the inferred type, enable PartialTypeSignatures • In the type signature: elimWeirdList :: forall (a :: Type) (wl :: WeirdList a) (p :: forall (x :: Type). x -> WeirdList x -> Type). Sing wl -> (forall (y :: Type). p _ WeirdNil) -> (forall (z :: Type) (x :: z) (xs :: WeirdList (WeirdList z)). Sing x -> Sing xs -> p _ xs -> p _ (WeirdCons x xs)) -> p _ wl | 24 | -> (forall (y :: Type). p _ WeirdNil) | ^ Foo.hs:26:44: error:ghc: panic! (the 'impossible' happened) (GHC version 8.2.1 for x86_64-unknown-linux): No skolem info: z_a1sY[sk:2] Call stack: CallStack (from HasCallStack): prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable pprPanic, called at compiler/typecheck/TcErrors.hs:2653:5 in ghc:TcErrors }}} (The error messages from HEAD, at commit 791947db6db32ef7d4772a821a0823e558e3c05b, are the same as in GHC 8.2.1.) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14040> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #16243: Improve fregs-graph.
by GHC 18 Feb '19

18 Feb '19

#16243: Improve fregs-graph. -------------------------------------+------------------------------------- Reporter: AndreasK | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 (CodeGen) | Keywords: fregs-graph | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I'm creating this as a way to keep track of shortcomings of the current implementation. Some of these are directly taken from TODOS, other are my own opinions. = Things that could be looked at. == Compress spill slots. Currently we assign each vreg a fixed spill slot. This is terrible for cache performance. It's possible to end up with sequences like this: {{{#!asm block_ca01_info: _ca01: movq 16(%rbp),%r14 movq 24(%rbp),%rax movq %rax,144(%rsp) movq 32(%rbp),%rsi movq 40(%rbp),%rdi movq 64(%rbp),%rbx movq 56(%rbp),%rax movq %rax,208(%rsp) movq 48(%rbp),%rax movq %rax,168(%rsp) movq 8(%rbp),%rax movq %rax,216(%rsp) addq $8,%rbp }}} Which splits spill slots over multiple cache lines increasing cache pressure. Things possibly worth trying: * Assign spill slots in the order they appear - hopefully reducing the average spread within code blocks. * Dynamically assign vregs to spill slots. For the later if we have large independent subgraphs we will sometimes get away with using far fewer slots. The reason simply being that non overlapping vregs can share spill slots. Doing this in an efficient manner however isn't easy. == We sometimes load + spill vregs when we could use memory addressing. One example I've seen a few times in nofib is this: {{{ movq 208(%rsp),%rax incq %rax movq %rax,208(%rsp) }}} Which we should replace with a single `inc` operating on memory. == Lift the stack size limit. There is already a patch [https://gitlab.haskell.org/ghc/ghc/merge_requests/219 in flight]. == Revisit spill costs. Currently the allocator always spills the longest live range. Which often works but is certainly not a great metric. Ideas: === Consider loop membership. At least for x86/64 this would be easy to achieve on basic block granularity using the CFG that is also used for block layout. === Revisit chaitins cost model. There have been major changes to cpus (amd64 becoming the norm) and the backend so at a minimum this cost model should be reevaluated. == Try splitting long live ranges instead of simply spilling the complete live range. After all long live ranges were the original reason to switch to a life range only based cost heuristic. == Use a different spill algorithm altogether. Maybe Chows live range splitting based approach would be an option. See "The priority based coloring approach to register allocation" for details. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16243> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12612: Allow kinds of associated types to depend on earlier associated types
by GHC 18 Feb '19

18 Feb '19

#12612: Allow kinds of associated types to depend on earlier associated types -------------------------------------+------------------------------------- Reporter: davemenendez | Owner: Type: bug | Status: new Priority: normal | Milestone: 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: -------------------------------------+------------------------------------- GHC rejects the following code: {{{#!hs class C t where type K t :: Type type T t :: K t -> Type m :: t -> T t a }}} with this error message {{{ • Type constructor ‘K’ cannot be used here (it is defined and used in the same recursive group) • In the kind ‘K t -> Type’ }}} See [https://mail.haskell.org/pipermail/glasgow-haskell- users/2016-September/026402.html e-mail discussion]. This is connected to #12088, at least as far as defining instances of C. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12612> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12218: Implement -fexternal-interpreter via static linking
by GHC 18 Feb '19

18 Feb '19

#12218: Implement -fexternal-interpreter via static linking -------------------------------------+------------------------------------- Reporter: simonmar | Owner: Type: task | Status: new Priority: normal | Milestone: 8.2.1 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: -------------------------------------+------------------------------------- An interesting idea from @hvr. When using `-fexternal-interpreter`, we can statically link the required packages and object files into a new iconv binary, and use that as our external interpreter. This would mean we could support TH and even limited GHCi without needing a dynamic linker of any kind, which is extremely cool. Apparently this would be useful for AIX where doing dynamic linking is hard. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12218> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #16206: Run Haddock's test suite in CI
by GHC 18 Feb '19

18 Feb '19

#16206: Run Haddock's test suite in CI -------------------------------------+------------------------------------- Reporter: sjakobi | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Test Suite | Version: 8.6.3 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: -------------------------------------+------------------------------------- Many changes in GHC affect Haddock's output, breaking its test suite. Recent examples are [https://gitlab.haskell.org/ghc/ghc/commit/53874834b779ad0dfbcde6650069c3792… 5387483] and [https://gitlab.haskell.org/ghc/ghc/commit/7cba71fc25af8287db61f6f6aa80d45ce… 7cba71f]. If cases like these were caught during CI, they could be fixed immediately, avoiding follow-up cleanup work by the Haddock maintainer. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16206> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14860: QuantifiedConstraints: Can't quantify constraint involving type family
by GHC 16 Feb '19

16 Feb '19

#14860: QuantifiedConstraints: Can't quantify constraint involving type family -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 (Type checker) | Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints wipT2893 | Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This program fails to typecheck using the `wip/T2893` branch, to my surprise: {{{#!hs {-# LANGUAGE AllowAmbiguousTypes #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE QuantifiedConstraints #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE UndecidableInstances #-} module Bug where import Data.Proxy type family Apply (f :: * -> *) a type instance Apply Proxy a = Proxy a data ExTyFam f where MkExTyFam :: Apply f a -> ExTyFam f instance (forall a. Show (Apply f a)) => Show (ExTyFam f) where show (MkExTyFam x) = show x showsPrec = undefined -- Not defining these leads to the oddities observed in showList = undefined -- https://ghc.haskell.org/trac/ghc/ticket/5927#comment:32 test :: ExTyFam Proxy -> String test = show }}} This fails with: {{{ $ ghc-cq/inplace/bin/ghc-stage2 --interactive Bug.hs GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:17:24: error: • Could not deduce (Show (Apply f a)) arising from a use of ‘show’ from the context: forall a. Show (Apply f a) bound by the instance declaration at Bug.hs:16:10-57 • In the expression: show x In an equation for ‘show’: show (MkExTyFam x) = show x In the instance declaration for ‘Show (ExTyFam f)’ | 17 | show (MkExTyFam x) = show x | ^^^^^^ }}} I would have expected the `(forall a. Show (Apply f a))` quantified constraint to kick in, but it didn't. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14860> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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