September 2014 - ghc-tickets

[GHC] #9622: GHCi command to solve a constraint
by GHC 07 Sep '18

07 Sep '18

#9622: GHCi command to solve a constraint -------------------------------------+------------------------------------- Reporter: monoidal | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.8.3 checker) | Operating System: Keywords: | Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Difficulty: Unknown | None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- It would be nice to have a `:solve` command for ghci that took a class constraint and returned either its derivation or an error message. A possible interface could look like so: {{{ ghci> :solve Eq (Maybe Type1, Bool) 1) instance Eq Type1 -- Defined in ‘MyModule’ 2) instance Eq a => Eq (Maybe a) -- Defined in ‘GHC.Classes’ 3) instance Eq (Maybe Type1) -- Put a ~ Int in 2) and use 1) 4) instance Eq Bool -- Defined in ‘GHC.Classes’ 5) instance (Eq a, Eq b) => Eq (a,b) -- Defined in ‘GHC.Classes’ 6) instance Eq (Maybe Type1, Bool) -- Put a ~ Maybe Type1, b ~ Bool in 5) and use 3), 4) ghci> :solve Eq (Int -> Int) No instance for Eq (Int -> Int) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9622> 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 03 Sep '18

03 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] #9441: Merge identical top-level expressions following simplification when it is safe to do so
by GHC 18 Aug '18

18 Aug '18

#9441: Merge identical top-level expressions following simplification when it is safe to do so -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- If I redefine {{{#!hs {-# INLINE reverse #-} reverse :: [a] -> [a] reverse xs = build $ \c n -> foldl (\a x -> x `c` a) n xs }}} and then write a couple test cases: {{{#!hs appRev xs ys = xs ++ reverse ys revAppRev xs ys = reverse xs ++ reverse ys }}} I end up getting some rather annoying code duplication (lots of stuff omitted from the following): {{{#!hs Rec { poly_go_r2v3 poly_go_r2v3 = \ @ a_a2nF ds_a2zc eta_Xl -> case ds_a2zc of _ { [] -> eta_Xl; : y_a2zh ys_a2zi -> poly_go_r2v3 ys_a2zi (: y_a2zh eta_Xl) } end Rec } reverse reverse = \ @ a_a2nF eta_B1 -> poly_go_r2v3 eta_B1 ([]) Rec { revAppRev2 revAppRev2 = \ @ a_a2y7 ds_a2zc eta_B1 -> case ds_a2zc of _ { [] -> eta_B1; : y_a2zh ys_a2zi -> revAppRev2 ys_a2zi (: y_a2zh eta_B1) } end Rec } Rec { revAppRev1 revAppRev1 = \ @ a_a2y7 ds_a2zc eta_B1 -> case ds_a2zc of _ { [] -> eta_B1; : y_a2zh ys_a2zi -> revAppRev1 ys_a2zi (: y_a2zh eta_B1) } end Rec } Rec { appRev1 appRev1 = \ @ a_a2xE ds_a2zc eta_B1 -> case ds_a2zc of _ { [] -> eta_B1; : y_a2zh ys_a2zi -> appRev1 ys_a2zi (: y_a2zh eta_B1) } end Rec } }}} The `reverse` function was inlined three times. In each case, there was no fusion, so `build` was inlined and the resulting copy of the `reverse` worker lifted to the top level. It would seem to me that once simplification is complete, it should be safe to merge all these copies into one. They are all `Rec {\ ... -> ...}` forms, so I don't think this has any potential to introduce undesirable sharing. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9441> 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 17 Aug '18

17 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] #9214: UNPACK support for sum types
by GHC 11 Aug '18

11 Aug '18

#9214: UNPACK support for sum types ------------------------------------+------------------------------------- Reporter: mojojojo | Owner: Type: feature request | 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: | ------------------------------------+------------------------------------- Currently the pragma only supports single-constructor types. So in the following example it will simply be ignored: {{{ data A = A1 Char | A2 {-# UNPACK #-} !B data B = B1 Int | B2 Bool }}} However the problem seems to be easily solvable by denormalizing the type {{{A}}} to something like the following during unpacking: {{{ data A = A1 Char | A2_1 Int | -- from B1 A2_2 Bool -- from B2 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9214> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9334: Implement "instance chains"
by GHC 09 Aug '18

09 Aug '18

#9334: Implement "instance chains" -------------------------------------+------------------------------------- Reporter: diatchki | Owner: diatchki Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.9 checker) | Differential Revisions: Keywords: | Architecture: Operating System: Unknown/Multiple | Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------- It would be useful to implement a version of "instance chains" [1] in GHC, which would eliminate the need for OVERLAPPING_INSTANCES in most (all practcial?) programs. The idea is that programmers can explicitly group and order instances into an "instance chain". For example: {{{ instance (Monad m) => StateM (StateT s m) s where ... else (MonadTrans t, StateM m s) => StateM (t m) s where ... }}} When GHC searches for instances, the instances in a chain are considered together and in order, starting with the first one: 1. If the goal matches the current instance's head, then this instance is selected and the rest are ignored, as if they were not there; 2. If the goal does not match the current instance's head, AND it does not unify with the current instance's head, then we skip the instance and proceed to the next member of the chain; 3. If the goal does not match the current instance's head, but it does unify with it, then we cannot use this chain to solve the goal. In summary: earlier instances in a chain "hide" later instances, and later instances can be reached only if we are sure that none of the previous instance will match. [1] http://web.cecs.pdx.edu/~mpj/pubs/instancechains.pdf -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9334> 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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