April 2014 - ghc-tickets

[GHC] #8400: Migrate the RTS to use libuv (or libev, or libevent)
by GHC 29 Sep '18

29 Sep '18

#8400: Migrate the RTS to use libuv (or libev, or libevent) ----------------------------------------------+---------------------------- Reporter: schyler | Owner: Type: feature request | simonmar Priority: normal | Status: new Component: Runtime System | Milestone: Keywords: | Version: Architecture: Unknown/Multiple | Operating System: Difficulty: Project (more than a week) | Unknown/Multiple Blocked By: | Type of failure: Related Tickets: 635, 7353 | None/Unknown | Test Case: | Blocking: ----------------------------------------------+---------------------------- This is mainly a reference discussion ticket. libuv (https://github.com/joyent/libuv) is a lightweight library which allows asynchronous IO across OpenBSD, Linux, Darwin, Windows etc by utilizing the fastest implementation on each system (epoll, kqueue, IOCP, event ports). These specialized IO polling methods are '''much''' faster on their respective platforms than just using select() like the RTS currently does. Additionally, it also provides cross platform threads, mutex, condition vars, terminal input/output and term settings w/ cross platform ANSI escape code handling, thread pools, cross platform HRC's etc. It's currently significantly faster than libevent and slightly faster than libev (and no doubt faster than rolling our own stuff). Because it's maintained and utilized heavily by Node.js it's in extremely active and maintained development. Rewriting a portion of the RTS to utilize libuv would have the following benefits; * We could ditch basically all of our platform specific code. Everything under rts/win32 and rts/posix, except the SEH stuff, could be deleted. * libuv is tuned for speed on each platform. This would be an optimization to all our async IO stuff. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8400> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8809: Prettier error messages?
by GHC 26 Sep '18

26 Sep '18

#8809: Prettier error messages? ------------------------------------+------------------------------------- Reporter: joelteon | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.9 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- `clang` has very nice-looking error messages. {{{ #!html <pre>pretty.c:6:7: warning: incompatible pointer to integer conversion passing 'char [14]' to parameter of type 'int' [-Wint-conversion] foo("Hello, world!"); ^~~~~~~~~~~~~~~ pretty.c:1:14: note: passing argument to parameter 'i' here void foo(int i) { ^ 1 warning generated.</pre> }}} `ghc`'s error messages are not so good. {{{ #!html <pre>ugly.hs:7:18: Couldn't match expected type ‘()’ with actual type ‘[Char]’ In the first argument of ‘f’, namely ‘"Hello, world!"’ In the second argument of ‘($)’, namely ‘f "Hello, world!"’ In the expression: print $ f "Hello, world!"</pre> }}} In my opinion, there are three independent improvements that could be made to GHC error messages and warnings: color, context and whitespace. Currently they're blobs of text. Consider all three applied to error messages: {{{ #!html <pre>ugly.hs:7:18: error: Argument to 'f' is type '[Char]' but expected 'Int' main = print $ f "Hello, world!" ^~~~~~~~~~~~~~~ ugly.hs:3:1: note: type of 'f' is given here: f :: () -> IO () ^~</pre> }}} or {{{ #!html <pre>ugly.hs: note: type of 'f' is inferred: f :: forall m. Monad m => () -> m () ^~</pre> }}} In my opinion, context and whitespace are more important that color. Even without color, compare this error message to the one shown above: {{{ ugly.hs:7:18: error: Argument to 'f' is type '[Char]' but expected 'Int' main = print $ f "Hello, world!" ^~~~~~~~~~~~~~~ ugly.hs:3:1: note: type of 'f' is given here: f :: () -> IO () ^~ }}} In my opinion this is much easier to visually process than GHC's current messages. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8809> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8578: Improvements to SpinLock implementation
by GHC 24 Sep '18

24 Sep '18

#8578: Improvements to SpinLock implementation ------------------------------------+------------------------------------- Reporter: parcs | Owner: parcs Type: task | Status: new Priority: normal | Milestone: Component: Runtime System | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- The SpinLock implementation has a number of deficiencies. Here is a pair of patches that improves its implementation. Let me know what you think. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8578> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8281: The impossible happened: primRepToFFIType
by GHC 21 Sep '18

21 Sep '18

#8281: The impossible happened: primRepToFFIType -----------------------------------+--------------------------------------- Reporter: tibbe | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time crash Unknown/Multiple | Test Case: Difficulty: Unknown | Blocking: Blocked By: | Related Tickets: | -----------------------------------+--------------------------------------- I ran into this error while trying to use GHCi on the hashable package: {{{ $ cabal repl Preprocessing library hashable-1.2.0.10... GHCi, version 7.6.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package array-0.4.0.1 ... linking ... done. Loading package deepseq-1.3.0.1 ... linking ... done. Loading package bytestring-0.10.0.2 ... linking ... done. Loading package text-0.11.3.1 ... linking ... done. Loading object (static) dist/build/cbits/fnv.o ... done Loading object (static) dist/build/cbits/getRandomBytes.o ... done final link ... done [1 of 4] Compiling Data.Hashable.RandomSource ( Data/Hashable/RandomSource.hs, interpreted ) [flags changed] [2 of 4] Compiling Data.Hashable.Class ( Data/Hashable/Class.hs, interpreted ) [flags changed] ghc: panic! (the 'impossible' happened) (GHC version 7.6.2 for x86_64-apple-darwin): primRepToFFIType Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8281> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8414: ghc-pkg prevents dynamic library only packages
by GHC 04 Sep '18

04 Sep '18

#8414: ghc-pkg prevents dynamic library only packages ------------------------------------+------------------------------------- Reporter: AndreasVoellmy | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: ghc-pkg | Version: Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Other Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- GHC supports compiling Haskell libraries as dynamic libraries and building executables against dynamic libraries. GHCi seems to support loading modules from dynamic libraries, too. So it would seem that it should be possible to have packages that only include dynamic libraries, omitting the .a and .o files typically generated. cabal-install allows building dynamic libraries only, for example with the command "cabal install --disable-library-vanilla --enable-shared --disable-library-profiling". However, ghc-pkg register refuses to install the package without the .a files present. This seems to be a bug with ghc-pkg. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8414> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8441: Allow family instances in an hs-boot file
by GHC 02 Sep '18

02 Sep '18

#8441: Allow family instances in an hs-boot file ------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- Compiling an .hs-boot file with a data instance leads to this error: {{{ Illegal family instance in hs-boot file }}} Yet, data instances make good sense in an hs-boot file, in case another module wants to access a constructor declared in the instance. Furthermore, it may sometimes be desirable to allow type family instances in a hs-boot file, as well, in case some including modules need the instance for type simplification. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8441> 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] #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] #8944: Warn instead of stopping on misplaced Haddock comments
by GHC 16 Aug '18

16 Aug '18

#8944: Warn instead of stopping on misplaced Haddock comments -------------------------------------+------------------------------------- Reporter: Fuuzetsu | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Parser) | Version: 7.9 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | -------------------------------------+------------------------------------- Given a simple module like {{{ module H where data F = F () -- ^ Comment for first type argument () }}} and trying to run Haddock on it, we'll get back {{{ /tmp/H.hs:4:12: parse error on input ‘(’ }}} The error message is rather sub-par in this scenario. It'd be great if we could instead print a warning saying that a Haddock comment is unexpected in that position and then continue on parsing. Best case scenario would be a more informative message such as “Documenting each constructor argument is not supported” but that might be quite a bit harder. Filing on GHC Trac as it's the parser that needs changing I believe. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8944> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8128: Standalone deriving fails for GADTs due to inaccessible code
by GHC 24 Jul '18

24 Jul '18

#8128: Standalone deriving fails for GADTs due to inaccessible code -------------------------------------+------------------------------------- Reporter: adamgundry | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.7 checker) | Operating System: Unknown/Multiple Keywords: | Type of failure: GHC rejects Architecture: Unknown/Multiple | valid program Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | -------------------------------------+------------------------------------- Consider the following: {{{ {-# LANGUAGE StandaloneDeriving, GADTs, FlexibleInstances #-} module StandaloneDerivingGADT where data T a where MkT1 :: T Int MkT2 :: (Bool -> Bool) -> T Bool deriving instance Show (T Int) }}} This gives the error: {{{ StandaloneDerivingGADT.hs:9:1: Couldn't match type ‛Int’ with ‛Bool’ Inaccessible code in a pattern with constructor MkT2 :: (Bool -> Bool) -> T Bool, in an equation for ‛showsPrec’ In the pattern: MkT2 b1 In an equation for ‛showsPrec’: showsPrec a (MkT2 b1) = showParen ((a >= 11)) ((.) (showString "MkT2 ") (showsPrec 11 b1)) When typechecking the code for ‛showsPrec’ in a standalone derived instance for ‛Show (T Int)’: To see the code I am typechecking, use -ddump-deriv In the instance declaration for ‛Show (T Int)’ }}} The derived instance declaration matches on all the constructors, even if they cannot possibly match. It should omit obviously inaccessible constructors so that this example is accepted. For reference, the derived code is: {{{ instance GHC.Show.Show (StandaloneDerivingGADT.T GHC.Types.Int) where GHC.Show.showsPrec _ StandaloneDerivingGADT.MkT1 = GHC.Show.showString "MkT1" GHC.Show.showsPrec a_aij (StandaloneDerivingGADT.MkT2 b1_aik) = GHC.Show.showParen ((a_aij GHC.Classes.>= 11)) ((GHC.Base..) (GHC.Show.showString "MkT2 ") (GHC.Show.showsPrec 11 b1_aik)) GHC.Show.showList = GHC.Show.showList__ (GHC.Show.showsPrec 0) }}} The same problem applies to other derivable classes (e.g. `Eq`). -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8128> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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