December 2015 - ghc-tickets

[GHC] #10445: Wrong stack space size when using -Ksize
by GHC 19 Nov '18

19 Nov '18

#10445: Wrong stack space size when using -Ksize -------------------------------------+------------------------------------- Reporter: asr | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.10.2 Component: Compiler | Version: 7.10.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- After compiling the first example in https://wiki.haskell.org/Performance/Accumulating_parameter: {{{ $ cat Test.hs len :: [a] -> Int len [] = 0 len (x:xs) = len xs + 1 main :: IO () main = print $ len [1..1000000] }}} {{{ $ ghc Test.hs -rtsopts }}} and running {{{ ./Test +RTS -K10K }}} GHC 7.6.3 reports {{{ Stack space overflow: current size 10240 bytes. }}} GHC 7.8.4 reports {{{ Stack space overflow: current size 33632 bytes. }}} and GHC 7.10.1 reports {{{ Stack space overflow: current size 99136 bytes. }}} GHC 7.8.3 and 7.10.1 report an incorrect stack space size. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10445> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10347: Spurious "unused constructor" warning with Coercible
by GHC 19 Nov '18

19 Nov '18

#10347: Spurious "unused constructor" warning with Coercible -------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.11 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- When I say {{{ {-# LANGUAGE RoleAnnotations #-} module Bug (N, twiddle) where import Data.Coerce newtype N a = MkN Int type role N representational twiddle :: N a -> N b twiddle = coerce }}} I get {{{ Bug.hs:7:15: warning: Defined but not used: data constructor ‘MkN’ }}} But the constructor ''is'' used, in order to do the coercion. The role annotation is to force GHC to use the constructor to do the coercion -- otherwise, it might not have. This bug exists in 7.10 and in HEAD. I will fix. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10347> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9207: Detect obvious cases of infinite recursion.
by GHC 18 Nov '18

18 Nov '18

#9207: Detect obvious cases of infinite recursion. --------------------------------------+------------------------------------ Reporter: mrugiero | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: infinite recursion | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: --------------------------------------+------------------------------------ Pure functions are guaranteed to return the same value if the same arguments are used. Because of that, a compiler could be able to detect infinite recursion in the form of a function calling itself with the same arguments (case, some distracted programmer forgot to apply the function tail to a list). The way I see it, one of Haskell's main selling points is that it helps the programmer to avoid sources of bugs as much as it can, and infinite recursion is one of the few that stand unchecked. I specifically point to this example because both, I'm a newbie and made that newbie mistake, and think it shouldn't be hard to implement to the parsing stage of the compiler/interpreter. It probably should throw an error when this happens in pure functions, and maybe an opt-in warning if it happens in monads. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9207> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10675: GHC does not check the functional dependency consistency condition correctly
by GHC 16 Nov '18

16 Nov '18

#10675: GHC does not check the functional dependency consistency condition correctly -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Consider {{{ {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, ScopedTypeVariables #-} class C a b | a -> b where op :: a -> b instance C [x] [x] instance {-# OVERLAPS #-} C x y => C [x] [Maybe y] f x = op [x] }}} Should these two instance declarations be accepted? They are simply ''inconsistent'' in the sense of Definition 6 of the [http://research.microsoft.com/en-us/um/people/simonpj/papers/fd- chr/jfp06.pdf FDs through CHRs paper]. Sadly GHC does not currently reject these as inconsistent. As a result it'll use ''both'' instance for improvement. In the definition of `f` for example we get {{{ C [alpha] beta }}} where `x:alpha` and the result type of `f` is `beta`. By using both instances for improvement we get {{{ C [Maybe gamma] [Maybe gamma] }}} Is that what we want? The two instances don't ''contradict'' each other, but neither do they agree as all published work on FDs says they should! Examples in the testsuite that exploit this loophole are {{{ ghci/scripts ghci047 polykinds T9106 typecheck/should_compile FD4 typecheck/should_compile T7875 }}} I'm not sure what the right thing here is. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10675> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10854: Remove recursive uses of `pprParendHsExpr` from `HsExpr.ppr_expr`
by GHC 15 Nov '18

15 Nov '18

#10854: Remove recursive uses of `pprParendHsExpr` from `HsExpr.ppr_expr` -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: newcomer | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- An `HsExpr` should look exactly as a user wrote some Haskell code. Its pretty printer should therefore be quite dumb. Just print out the code! But a few places in GHC create `HsExpr`s and aren't scrupulous with regard to adding `HsPar` nodes where source syntax would need them. Thus, well- meaning souls have added `pprParendExpr` calls to the `HsExpr` pretty- printer to try to cover these cases. Simon and I [https://phabricator.haskell.org/D1219#34199 think] that we should remove all recursive uses of `pprParendExpr` from the `HsExpr` pretty-printer and instead add the `HsPar`s where necessary. It was recently reported that the following test cases break when simply removing `pprParendExpr`: {{{ Unexpected failures: deriving/should_fail T4846 [stderr mismatch] (normal) generics GenDerivOutput [stderr mismatch] (normal) generics GenDerivOutput1_0 [stderr mismatch] (normal) generics GenDerivOutput1_1 [stderr mismatch] (normal) ghci/scripts T10248 [bad stderr] (ghci) indexed-types/should_compile PushedInAsGivens [stderr mismatch] (normal) safeHaskell/safeLanguage SafeLang10 [stderr mismatch] (normal) safeHaskell/safeLanguage SafeLang17 [stderr mismatch] (normal) typecheck/should_fail T8603 [stderr mismatch] (normal) typecheck/should_fail tcfail177 [stderr mismatch] (normal) }}} See also considerable commentary on Phab:D1114. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10854> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9692: Template Haskell reification for Data Families is buggy?
by GHC 08 Nov '18

08 Nov '18

#9692: Template Haskell reification for Data Families is buggy? -------------------------------------+------------------------------------- Reporter: hpacheco | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: TemplateHaskell | Operating System: MacOS X Architecture: Unknown/Multiple | Type of failure: Difficulty: Unknown | None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- I have found an example where I think that the TH reification for data/newtype families is incorrect, in that if forgets non-instantiated type variables in instance declarations. For the attached example, it will infer the Info: {{{#!hs FamilyI (FamilyD DataFam TyFamReifyDefs.F [PlainTV a_1627399879,PlainTV b_1627399880] (Just StarT)) [DataInstD [] TyFamReifyDefs.F [ConT GHC.Types.Int] [NormalC TyFamReifyDefs.FInt [(NotStrict,VarT x_1627399881)]] []] }}} Note how the variable x is not declared in the body of the DataInstD. Since it does not need to have the same name as the family-declared variable, substituing is non-trivial this way... -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9692> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11008: GND with Type Families
by GHC 07 Nov '18

07 Nov '18

#11008: GND with Type Families -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: 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: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- In the following code: {{{ {-# LANGUAGE TypeFamilies, FlexibleContexts, UndecidableInstances #-} class C r type family F r newtype Foo r = Foo r instance (C (F r), Eq r) => Eq (Foo r) newtype Bar r = Bar (Foo r) deriving (Eq) }}} GHC produces the error {{{ No instance for (C (F r)) arising from the 'deriving' clause of a data type declaration Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (Eq (Bar r)) }}} If I enable `StandaloneDeriving`, I am able to derive the instance: `deriving instance (Eq (Foo r)) => Eq (Bar r)` [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/deriving.ht… #newtype-deriving The docs] indicate that this is exactly the instance that should be generated by GND for `newtype T v1..vn = MkT (t vk+1..vn) deriving (C t1..tj)`, where my code uses the following parameters: {{{ T = Bar v1 = r n = 1 MkT = Bar t = Foo r k= 1 C = Eq j = 0 }}} and all of the conditions enumerated in the docs seem to be met. Furthermore, `Bar` seems to fit squarely into the format of `data T1` in section 7.5.1 of the same documentation, which indicates it should not need `StandaloneDeriving`. It seems that the type family constraint on the `Eq` instance for `Foo` is causing the problem, but it's not clear why I'm forced to write a standalone instance that is identical to the one that should be generated by GND. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11008> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9545: Evaluate Takano Akio's foldrW/buildW fusion framework as a possible replacement for foldr/build
by GHC 29 Oct '18

29 Oct '18

#9545: Evaluate Takano Akio's foldrW/buildW fusion framework as a possible replacement for foldr/build -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: task | Status: new Priority: normal | Milestone: Component: libraries/base | Version: 7.9 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Project (more | Type of failure: than a week) | None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Takano Akio's [https://github.com/takano-akio/ww-fusion worker-wrapper fusion] modifies `foldr/build` fairly conservatively to allow safe fusion of `foldl` and friends without relying on arity analysis. This advantage is important for two reasons that I know of: 1. As discussed in Joachim Breitner's original paper, the current arity analysis is unable to infer arity correctly when facing certain forms of recursion. 2. The current arity analysis, and probably ''any'' practical arity analysis, depends on inlining to a degree that can sometimes be unhealthy. I would love to make functions like `hPutStr` fuse, but I suspect doing so safely at present would likely cause a code explosion—the function being folded is too large to want to inline to make it available for arity analysis following fusion. I don't understand it completely yet, but it looks like `foldrW/buildW` can eliminate a lot of this uncertainty. Furthermore, it appears that functions currently written using `foldr` in a "right-handed" way can remain unchanged, using an implementation of `foldr` in terms of `foldrW`. Only "left-handed" folds will need to be rewritten to take advantage of this framework, along with all the `build`s. That said, `foldrW/buildW` probably has its weaknesses too. The basic fusion rule looks like this. {{{#!hs {-# RULES "foldrW/buildW" forall f z (i :: Wrap f b) (g :: forall c g . (Wrap g c) -> (a -> c -> c) -> c -> c) . foldrW i f z (buildW g) = g i f z #-} }}} If `g` and `i` are not inlined sufficiently to merge with each other, I imagine this could produce worse results than `foldr/build` when the latter works properly. The only way to really get a feel for performance would be to carefully modify everything to work as well as it can with this framework and see how the results compare. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9545> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9374: Investigate Static Argument Transformation
by GHC 29 Oct '18

29 Oct '18

#9374: Investigate Static Argument Transformation -------------------------------------+------------------------------------- Reporter: jstolarek | Owner: Type: task | Status: new Priority: lowest | Milestone: Component: Compiler | Version: 7.9 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- At the moment the Static Argument Transformation (SAT) optimisation is not run unless explicitly enabled with `-fstatic-argument-transformation`. The comment in DynFlags says: {{{ Max writes: I think it's probably best not to enable SAT with -O2 for the 6.10 release. The version of SAT in HEAD at the moment doesn't incorporate several improvements to the heuristics, and I'm concerned that without those changes SAT will interfere with some attempts to write "high performance Haskell", as we saw in some posts on Haskell-Cafe earlier this year. In particular, the version in HEAD lacks the tail call criterion, so many things that look like reasonable loops will be turned into functions with extra (unneccesary) thunk creation. }}} 6.10 was a long time ago. Has anything changed since then? Does it make sense to enable that optimisation now? What are the mentioned heuristics and were they finally implemented? Does anyone know what Haskell-cafe posts does Max refer to? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9374> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9163: Ptr should have a phantom role
by GHC 28 Oct '18

28 Oct '18

#9163: Ptr should have a phantom role ------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | 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: | ------------------------------------+------------------------------------- In `GHC.Ptr` we see {{{ type role Ptr representational data Ptr a = Ptr Addr# deriving (Eq, Ord) }}} with no comments. Why is `Ptr` representational? In the same module we have `castPtr`: {{{ castPtr :: Ptr a -> Ptr b castPtr (Ptr addr) = Ptr addr }}} which unpacks and repacks a `Ptr`. If `Ptr` was phantom, we could use `coerce`. And that in turn would actually make a lot of code more efficient – there are lots of calls to `castPtr`. Specifically, in `nofib`, I tried implementing `castPtr` with `unsafeCoerce`. Then I found: * 12% less allocation in `reverse-complem` * 7.3% less allocation in `fasta`. * Binary sizes fell 0.1%. Both these benchmarks are ones that do a ''lot'' of I/O, and it turns out that `GHC.IO.Handle.Text.$wa1` has a `castPtr` that (all by itself) accounts for 12% of `reverse-complem`'s total allocation! So making `castPtr` free is good. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9163> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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