July 2018 - ghc-tickets

[GHC] #15363: Do some cleaning up of the testsuite driver
by GHC 01 Sep '18

01 Sep '18

#15363: Do some cleaning up of the testsuite driver -------------------------------------+------------------------------------- Reporter: lantti | Owner: (none) Type: task | Status: new Priority: low | Milestone: 8.6.1 Component: Test Suite | Version: 8.4.3 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: -------------------------------------+------------------------------------- When trying to understand the GHC test suite I noticed some small improvements that I could do to it. This would probably work nicely as my first task on GHC: Rewrite the timeout.hs in python, it doesn't seem to do anything that would strictly need the Haskell libraries to be used and it is the only part of the test suite driver that is not written in python. Or; See if the timeout scripts could be eliminated altogether as the python subprocess module that we are using can now (since python 3.3) handle timeouts by itself and using the timeout scripts effectively doubles the number of processes we need to create for each test case. Notice that the timeout scripts do more than just generating the timeout. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15363> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14827: Recognize when inlining would create a join point
by GHC 31 Aug '18

31 Aug '18

#14827: Recognize when inlining would create a join point -------------------------------------+------------------------------------- Reporter: ersetzen | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Keywords: JoinPoints | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- [https://www.reddit.com/r/haskell/comments/7yh8ar/i_wrote_a_program_that_run… This discussion] revolved around a program that runs 10x faster under ghci. One way to solve this is to remove a superfluous inline pragma which allows the following transformation to happen: {{{ letrec { f a = case e of { p1 -> f a'; p2 -> (# l, r #); }} in case f e2 of { (# l, r #) -> e3; } }}} into {{{ joinrec { f a = case e of { p1 -> jump f a'; p2 -> e3; }} in jump f e2 }}} More generally a recursive let binding that is called exactly once from the outside. If all recursive calls are tail calls and the outside one isn't then we could safely replace the call with the binding and end up with join points. In this case it means a 10x speedup so it might be worth doing generally. {{{ letrec { fi = ei; } in ... (fj e) ... => ... (joinrec { fi = ei; } in fj e) ... }}} [https://gist.github.com/AndreasPK/8e6f0cbf253f0930f4cda81e685ac136 Self contained example] -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14827> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15412: "Instance head is not headed by a class" when `Constraint` replaced with `type C = Constraint`
by GHC 30 Aug '18

30 Aug '18

#15412: "Instance head is not headed by a class" when `Constraint` replaced with `type C = Constraint` -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 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: -------------------------------------+------------------------------------- {{{#!hs {-# Language DataKinds, TypeInType, TypeFamilies #-} import Data.Kind newtype I a = I a type C = Constraint type family UnitC :: Constraint where UnitC = () instance UnitC => Functor I where fmap = undefined }}} this works fine, but if I try to use the constraint synonym `C` (`UnitC :: C where`) fails with "Instance head is not headed by a class" which has '''no''' Google hits outside of the [https://hackage.haskell.org/package/ghc-8.4.1/docs/src/TcValidity.html#chec… compiler] {{{ $ ghci -ignore-dot-ghci hs/249.hs GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( hs/249.hs, interpreted ) hs/249.hs:13:10: error: • Instance head is not headed by a class • In the instance declaration for ‘Functor I’ | 13 | instance UnitC => Functor I where | ^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. Prelude> }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15412> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8279: bad alignment in code gen yields substantial perf issue
by GHC 29 Aug '18

29 Aug '18

#8279: bad alignment in code gen yields substantial perf issue ------------------------------+-------------------------------------------- Reporter: carter | Owner: Type: bug | Status: new Priority: highest | 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: | ------------------------------+-------------------------------------------- independently, a number of folks have noticed that in various ways, GHC currently has quite a few different memory alignment related performance problems that can have >= 10% perf impact! Nicolas Frisby notes {{{ On my laptop, a program showed a consistent slowdown with -fdicts-strict I didn't find any obvious causes in the Core differences, so I turned to Intel's Performance Counter Monitor for measurements. After trying a few counters, I eventuall saw that there are about an order of magnitude more misaligned memory loads with -fdicts-strict than without, so I think that may be a significant part of the slowdown. I'm not sure if these are code or data reads. Can anyone suggest how to validate this hypothesis about misaligned reads? A subsequent commit has changed the behavior I was seeing, so I'm not interested in alternatives means to determine if -fdicts-strict is somehow at fault — I'm just asking specifically about data/code memory alignment in GHC and how to diagnose/experiment with it. }}} Reid Barton has independently noted {{{ so I did a nofib run with llvm libraries, ghc quickbuild so there's this really simple benchmark tak, https://github.com/ghc/nofib/blob/master/imaginary/tak/Main.hs it doesn't use any libraries at all in the main loop because the Ints all get unboxed but it's still 8% slower with quick-llvm (vs -fasm) weird right? [14:36:30] <carter> could you post the asm it generates for that function? [14:36:49] <rwbarton> well it's identical between the two versions <rwbarton> but they get linked at different offsets because some llvm sections are different sizes <rwbarton> if I add a 128-byte symbol to the .text section to move it to the same address... then the llvm libs version is just as fast <rwbarton> well, apparently 404000 is good and 403f70 is bad <rwbarton> I guess I can test other alignments easily enough <rwbarton> I imagine it wants to start on a cache line <rwbarton> but I don't know if it's just a coincidence that it worked with the ncg libraries <rwbarton> that it got a good location <rwbarton> for this program every 32-byte aligned address is 10+% faster than any merely 16-byte aligned address <rwbarton> and by alignment I mean alignment of the start of the Haskell code section <carter> haswell, sandybridge, ivy bridge, other? <rwbarton> dunno <rwbarton> I have similar results on Intel(R) Core(TM)2 Duo CPU T7300 @ 2.00GHz <rwbarton> and on Quad-Core AMD Opteron(tm) Processor 2374 HE <carter> ok <rwbarton> trying a patch now that aligns all *_entry symbols to 32 bytes }}} the key point in there is that on the tak benchmark, better alignment for the code made a 10% perf differnce on TAk on Core2 and opteron cpus! benjamin scarlet and Luite are speculating that this may be further induced by Tables next to code (TNC) accidentally creating bad alignment so theres cache line pollution / conflicts between the L1 Instruction- cache and data-caches. So one experiment would be to have the TNC transform pad after the table so the function entry point starts on the next cacheline? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8279> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11350: Allow visible type application in patterns
by GHC 28 Aug '18

28 Aug '18

#11350: Allow visible type application in patterns -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.1 Keywords: | Operating System: Unknown/Multiple TypeApplications PatternSynonyms | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Constructors (and pattern synonyms) when treated as expressions may be applied to types: {{{#!hs {-# LANGUAGE TypeApplications #-} Nothing :: Maybe a Nothing @() :: Maybe () pattern Pair :: a -> a -> (a, a) pattern Pair x y = (x, y) Pair :: a -> a -> (a, a) Pair @Int :: Int -> Int -> (Int, Int) }}} But using them in patterns won't parse: {{{#!hs -- parse error in pattern: @Int maybeToList :: Maybe Int -> [Int] maybeToList (Nothing @Int) = [] maybeToList (Just @Int x) = [x] -- parse error in pattern: @Int add :: (Int, Int) -> Int add (Pair @Int x y) = x + y }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11350> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15305: Erroneous "non-exhaustive pattern match" using nested GADT with strictness annotation
by GHC 27 Aug '18

27 Aug '18

#15305: Erroneous "non-exhaustive pattern match" using nested GADT with strictness annotation -------------------------------------+------------------------------------- Reporter: jkoppel | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | error/warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- In the following code, `fun` contains an exhaustive pattern match, but, when compiling with -Wall, ghc erroneously reports a non-exhaustive pattern match. {{{#!hs data (:+:) f g a = Inl !(f a) | Inr !(g a) data A data B data Foo l where Foo :: Foo A data Bar l where Bar :: Bar B type Sig = Foo :+: Bar fun :: Sig B -> Int fun (Inr Bar) = 1 }}} This report came from https://stackoverflow.com/questions/16225281/gadts- failed-exhaustiveness-checking . Without strictness annotations, this is indeed a failed exhaustive check, due to bottom. I spoke to Richard Eisenberg at PLDI a few days ago, and he informed me that, if this warning did not disappear with strictness annotations, it was a bug. I added strictness annotations, and it did not disappear. I've tried all combinations of using strictness annotations and/or running with `{-# LANGUAGE Strict #-}`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15305> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14021: 8.2.1 deb8 bindist fails to install on Windows 10 WSL
by GHC 26 Aug '18

26 Aug '18

#14021: 8.2.1 deb8 bindist fails to install on Windows 10 WSL -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.2.2 Component: Build System | 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: -------------------------------------+------------------------------------- In principle the GHC 8.2.1 bindist for Debian 8 should work fine on WSL. However, in practice it fails during `make install` with this error, {{{ }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14021> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8733: I/O manager causes unnecessary syscalls in send/recv loops
by GHC 26 Aug '18

26 Aug '18

#8733: I/O manager causes unnecessary syscalls in send/recv loops ------------------------------------+------------------------------------- Reporter: tibbe | Owner: simonmar Type: bug | Status: new Priority: normal | Milestone: Component: Runtime System | 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: | ------------------------------------+------------------------------------- Network applications often call send followed by recv, to send a message and then read an answer. This causes syscall traces like this one: {{{ recvfrom(13, ) -- Haskell thread A sendto(13, ) -- Haskell thread A recvfrom(13, ) = -1 EAGAIN -- Haskell thread A epoll_ctl(3, ) -- Haskell thread A (a job for the IO manager) recvfrom(14, ) -- Haskell thread B sendto(14, ) -- Haskell thread B recvfrom(14, ) = -1 EAGAIN -- Haskell thread B epoll_ctl(3, ) -- Haskell thread B (a job for the IO manager) }}} The recvfrom call always fails, as the response from the partner we're communicating with won't be available right after we send the request. We ought to consider descheduling the thread as soon as sending is "done". The hard part is to figure out when that is. See http://www.yesodweb.com/blog/2014/02/new-warp for a real world example. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8733> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14483: getExecutablePath from System.Environment in base should follow symlinks properly.
by GHC 25 Aug '18

25 Aug '18

#14483: getExecutablePath from System.Environment in base should follow symlinks properly. -------------------------------------+------------------------------------- Reporter: angerman | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.3 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: -------------------------------------+------------------------------------- During the implementation of relocation capabilities in GHC for linux and macOS it came to light, that windows (which has had this support for quite some time), uses a custom implementation of `getExecutablePath`. Ideally this logic should be moved into `base`. Phyx- noted in D4121 that this should be possible for 8.6, as at that point Win32 should be new enough to be usable in base for this. The result would be that the custom code in GHC (SysTools) and the older copy in GhcPkg of the same code can be unified with the linux and macOS CPP conditional branch. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14483> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14460: Can't decompose ghc.exe path
by GHC 25 Aug '18

25 Aug '18

#14460: Can't decompose ghc.exe path -------------------------------------+------------------------------------- Reporter: astert | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Windows Architecture: | Type of failure: GHC doesn't work Unknown/Multiple | at all Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I downloaded the haskell platform, but when I try to use ghci I get this message: ghc.exe: panic!(the 'impossible' happened) (GHC version 8.2.1 for x86_64-unknown-mingw32): can't decompose ghc.exe path: "H:\\Programs\\Haskell\\8.2.1\\bin\\ghc.exe" -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14460> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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