August 2016 - ghc-tickets

[GHC] #11514: Impredicativity is still sneaking in
by GHC 21 Mar '19

21 Mar '19

#11514: Impredicativity is still sneaking in -------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 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: -------------------------------------+------------------------------------- This is erroneously accepted: {{{ {-# LANGUAGE RankNTypes #-} module Bug where foo :: forall a. (Show a => a -> a) -> () foo = undefined }}} `undefined` is being instantiated at `(Show a => a -> a) -> ()`, which requires impredicativity. GHC does the right thing (reject!) if we try to instantiate with a type involving proper foralls. I imagine this is a simple missing check. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11514> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11371: Bogus in-scope set in substitutions
by GHC 12 Mar '19

12 Mar '19

#11371: Bogus in-scope set in substitutions -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: high | Milestone: 8.2.1 Component: Compiler | Version: 7.10.3 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: -------------------------------------+------------------------------------- Bartosz [https://mail.haskell.org/pipermail/ghc- devs/2016-January/010892.html reports] that substitution isn't working properly for him. He cites this Note in `TyCoRep`: {{{ -- Note [Generating the in-scope set for a substitution] -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -- If we want to substitute [a -> ty1, b -> ty2] I used to -- think it was enough to generate an in-scope set that includes -- fv(ty1,ty2). But that's not enough; we really should also take the -- free vars of the type we are substituting into! Example: -- (forall b. (a,b,x)) [a -> List b] -- Then if we use the in-scope set {b}, there is a danger we will rename -- the forall'd variable to 'x' by mistake, getting this: -- (forall x. (List b, x, x)) -- Urk! This means looking at all the calls to mkOpenTCvSubst.... }}} I think this is an outright bug that has been lurking for ages, and which Bartosz has just managed to tickle. Things to do: * We should add an ASSERT to substTy (and similar functions): **when calling `substTy subst ty` it should be the case that the in-scope set in the substitution is a superset of** * **The free vars of the range of the substitution** * **The free vars of ty minus the domain of the substitution** That ASSERT could be added to the immediate callers of `subst_ty` in `TyCoRep`. (Bartosz: if you add that you should find that it trips before the bug you are actually getting.) * How to fix it properly? The places to look are: everywhere that uses (transitively) one of the `mkOpenTCvSubst` or `zipOpenTCvSubst` functions. * Many calls to `mkOpenTCvSubst` produce a substitution that is only applied to closed types; or at least to types whose only free variables are the ones in the domain of the substitution. Example: the call to `zipOpenTCvSubst` in `DataCon.dataConInstSig`. These ones will all satisfy the ASSERT[[BR]][[BR]] * In other cases, we already have the in-scope set in hand. Example: in `CoreLint.lintTyApp` we find a call to `substTyWith`. But Lint carries an in-scope set, so it would be easy to pass it to `substTyWith`.[[BR]][[BR]] * Finally there may be cases where we really have to take the free vars of the type we are substituting into, and add them to the in-scope set. Doubtless all this applies to substituting in coercions too, and indeed into expressions. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11371> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10069: CPR related performance issue
by GHC 09 Mar '19

09 Mar '19

#10069: CPR related performance issue -------------------------------------+------------------------------------- Reporter: pacak | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1-rc2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- By default CRP analysis can be too aggressive in trying to pass as much as possible in unboxed tuples, in general it's not a problem but when one big datatype is passed to several consumers it might end up pushed to stack several times instead of once - to heap, things are getting worse when there are sufficient fields to cause stack overflow which otherwise is possible to avoid - in our codebase adding one field with ExistentialQuantification (unused, but that prevents ghc from doing CRP transformation) reduces number of stack overflow by a factor of 1000 and increases overall performance by 10%. In provided example performance for both A and B should be identical and yet B is consistently faster by 3-5% It's possible to increase this performance gap by adding more and more fields. I was able to replicate this issue in ghc 7.8.3 and 7.10,1rc2 {{{#!hs {-# LANGUAGE ExistentialQuantification #-} module Blah where import Criterion import Criterion.Main import Data.Typeable data A = A () !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int data B = forall rep. (Typeable rep) => B rep !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int a :: A a = A () 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 b :: B b = B () 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 {-# NOINLINE a1 #-} a1 :: A -> Int a1 (A _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) = f1 {-# NOINLINE a2 #-} a2 :: A -> Int a2 (A _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) = f2 {-# NOINLINE a3 #-} a3 :: A -> Int a3 (A _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) = f3 {-# NOINLINE a4 #-} a4 :: A -> Int a4 (A _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) = f4 {-# NOINLINE b1 #-} b1 :: B -> Int b1 (B _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) = f1 {-# NOINLINE b2 #-} b2 :: B -> Int b2 (B _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) = f2 {-# NOINLINE b3 #-} b3 :: B -> Int b3 (B _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) = f3 {-# NOINLINE b4 #-} b4 :: B -> Int b4 (B _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) = f4 {-# NOINLINE fa #-} fa :: A -> Int fa a = a1 a + a2 a + a3 a + a4 a {-# NOINLINE fb #-} fb :: B -> Int fb b = b1 b + b2 b + b3 b + b4 b main :: IO () main = defaultMain [ bgroup "single call" [ bench "A" $ whnf fa a , bench "B" $ whnf fb b ] ] }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10069> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11441: RFC: Inline intermediate languages (Core, STG, Cmm, even StrictCore)
by GHC 08 Mar '19

08 Mar '19

#11441: RFC: Inline intermediate languages (Core, STG, Cmm, even StrictCore) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 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: -------------------------------------+------------------------------------- Allow intermediate code inline. == Inline ''Core'' == Typeable/WithTypeable: > {{{#!hs > withTypeable :: TTypeRep a -> (Typeable a => r) -> r > }}} > > Currently, `withTypeable` cannot be written in Haskell, but it is straightforward to write in Core. Can we fix this? [http://www.seas.upenn.edu/~sweirich/papers/wadlerfest2016.pdf A reflection on types] (''2016''): > We cannot implement ''withTypeable'' in Haskell source. But we ''can'' implement it in GHC's statically-typed intermediate language, System FC. The definition is simple, roughly like this: > > {{{#!hs > withTypeable tr k = k tr -- Not quite right > }}} > > Its second argument ''k'' expects a ''Typeable'' dictionary as its value argument. But since > a ''Typeable'' dictionary is represented by a ''TypeRep'', we can simply pass ''tr'' to ''k''. > When written in System FC there is a type-safe coercion to move from ''TypeRep a'' to ''Typeable a'', > but that coercion is erased at runtime. Since the definition can be statically type checked, > ''withTypeable'' does not form part of the trusted code base. == Inline ''Cmm'' == Edkward Kmett writes his own `.cmm` [https://github.com/ekmett/concurrent/blob/master/cbits/hashmap.cmm hither], [https://github.com/ekmett/concurrent/blob/master/cbits/array.cmm yon]. == Inline ''Strict Core'' == Should [http://research.microsoft.com/en-us/um/people/simonpj/papers /strict-core/tacc-hs09.pdf Types are calling conventions] (''2009'') be implemented: > === '''4.4 The ''`seq`'' function''' === > > A nice feature of Strict Core,,ANF,, is that it is possible to give a straightforward definition of the primitive ''seq'' function of Haskell: > {{{#!hs > seq : {a:* -> b:* -> {a} -> {b} -> b} > = { \a:*. \b:*. \x:{a}. \y:{b}. let _:a = x <> in y <> } > }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11441> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11315: GHC doesn't restore split objects
by GHC 08 Mar '19

08 Mar '19

#11315: GHC doesn't restore split objects -------------------------------------+------------------------------------- Reporter: snowleopard | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 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: -------------------------------------+------------------------------------- While working on the new Shake-based build system I came across an annoying problem related to missing split objects ([#note see note]). I still don't fully understand the reasons behind the problem, but I found that GHC doesn't restore missing split objects, as explained in the example below. I think this is a bug, but maybe this is actually a missing feature. I think it shouldn't be difficult to fix. {{{#!hs module Test (test) where test :: Int test = 0 }}} I compile the above with `ghc -split-objs Test.hs` and get two split object files in `Test_o_split`. Then if I delete any of these files, or even the whole folder, and rerun `ghc -split-objs Test.hs` the compiler doesn't bother to restore split objects, pretending that everything is up- to-date. However, if I delete `Test.o` all object files are restored. Another related issue reported by Neil Mitchell (which may be worth a separate ticket) is that even if we corrupt `Test.o` and rerun `ghc -split-objs Test.hs`, the compiler doesn't restore the object file (nor missing split objects) to the consistent state. ---- [=#note Note:] The problem I have is described here: https://github.com/snowleopard/shaking-up-ghc/issues/30. It's not necessary to follow the link to understand this ticket, however I'd appreciate any insight you might have about the reasons behind the problem and/or a possible solution. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11315> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9832: Get rid of PERL dependency of `ghc-split`
by GHC 08 Mar '19

08 Mar '19

#9832: Get rid of PERL dependency of `ghc-split` -------------------------------------+------------------------------------- Reporter: hvr | Owner: Type: task | Status: new Priority: normal | Milestone: 7.12.1 Component: Compiler | Version: Keywords: | Operating System: Windows Architecture: Unknown/Multiple | Type of failure: Difficulty: Unknown | None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- (writeme) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9832> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8964: split_marker_entry assert breaks -split-objs and -ddump-opt-cmm
by GHC 08 Mar '19

08 Mar '19

#8964: split_marker_entry assert breaks -split-objs and -ddump-opt-cmm -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: bug | Status: new Priority: lowest | Milestone: ⊥ Component: Compiler | Version: 7.9 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Compile-time Difficulty: Easy (less than 1 | crash hour) | Test Case: Blocked By: | Blocking: Related Tickets: | -------------------------------------+------------------------------------- ...since the pretty-printer attempts to force the blocks list of the split_marker_entry proc. I'm not really sure what the right way to fix this is: just provide an empty block list, or hack the pretty-printer to treat split markers specially. But should be easy to fix. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8964> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8685: -split-objs doesn't work for executables
by GHC 08 Mar '19

08 Mar '19

#8685: -split-objs doesn't work for executables ------------------------------------+------------------------------------- Reporter: refold | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Driver | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- While investigating [this bug report](https://github.com/haskell/cabal/issues/1611), I've noticed that the `-split-objs` option only works for libraries, but not for executables. That is, unlike in the case of libraries, the driver feeds only `A.o B.o ...` files to the linker instead of `A_split_0.o A_split_1.o ... B_split_0.o B_split_1.o ...`. It'd be nice if `-split-objs` could be changed to also work on executables. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8685> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8629: Option 'split-objs' being ignored when trying to reduce object code size in iOS cross-compilation
by GHC 08 Mar '19

08 Mar '19

#8629: Option 'split-objs' being ignored when trying to reduce object code size in iOS cross-compilation ------------------------------------+--------------------------------- Reporter: f1rstmistake | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Keywords: ios | Operating System: MacOS X Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+--------------------------------- I have been trying to statically link Pandoc packages to iOS application following the guide in https://ghc.haskell.org/trac/ghc/wiki/Building/CrossCompiling/iOS and it's great so far! I have finally successfully use Pandoc library in iOS app, but the binary size is very big! 1. More than 50MB when using Pandoc library. 2. Around 5MB for simple haskell code. I try to add “split-objs: True” in .cabal/config file, but when running cabal I got this message: {{{ Preprocessing library pandoc-1.12.2.1... unused terminals: 1 on the commandline: Warning: ignoring -fsplit-objs }}} And if I ignore that, at the end of build, I get another error message that stop the build. {{{ cabal: dist-i386/build/Text/Pandoc_o_split: does not exist }}} I try with another package and got similar error message. It seems that split-objs not only being ignored by cross-compilation ghc, but will also fail the build. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8629> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8095: TypeFamilies painfully slow
by GHC 08 Mar '19

08 Mar '19

#8095: TypeFamilies painfully slow -------------------------------------+------------------------------------- Reporter: MikeIzbicki | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.6.3 checker) | Operating System: Unknown/Multiple Keywords: | Type of failure: Compile-time Architecture: Unknown/Multiple | performance bug Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: 5321 | -------------------------------------+------------------------------------- I'm using the TypeFamilies extension to generate types that are quite large. GHC can handle these large types fine when they are created manually, but when type families get involved, GHC's performance dies. Unlike in ticket #5321, using tail recursion does not eliminate the problem, and the order of arguments greatly affects compile time. I've attached a file Types.hs that demonstrates the problems. This file generates another Haskell file which has the problems. It takes 3 flags. The first is the size of the type to generate, the second is which type family function to use, and the third is whether to call the type family or just use a manually generated type. Here are my performance results: Using non-tail recursion, I get these results. I have to increase the stack size based on the size of the type I want to generate. {{{ $ ./Types 200 a a > test.hs && time ghc test.hs > /dev/null -fcontext- stack=250 real 0m2.973s $ ./Types 300 a a > test.hs && time ghc test.hs > /dev/null -fcontext- stack=350 real 0m6.018s $ ./Types 400 a a > test.hs && time ghc test.hs > /dev/null -fcontext- stack=450 real 0m9.995s $ ./Types 500 a a > test.hs && time ghc test.hs > /dev/null -fcontext- stack=550 real 0m15.645s }}} Tail recursion generates much slower compile times for some reason, and I still need to adjust the stack size: {{{ $ ./Types 200 b a > test.hs && time ghc test.hs > /dev/null -fcontext- stack=250 real 0m16.120s }}} Changing the order of arguments to the recursive type family greatly changes the run times: {{{ $ ./Types 200 c a > test.hs && time ghc test.hs > /dev/null -fcontext- stack=250 real 0m6.095s }}} Without the type family, I get MUCH better performance: {{{ $ ./Types 10000 a d > test.hs && time ghc test.hs > /dev/null real 0m2.271s }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8095> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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