July 2017 - ghc-tickets

[GHC] #9622: GHCi command to solve a constraint
by GHC 06 Sep '18

06 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] #13499: "Panic: no skolem info" with StaticPointers and typed hole
by GHC 06 Sep '18

06 Sep '18

#13499: "Panic: no skolem info" with StaticPointers and typed hole -------------------------------------+------------------------------------- Reporter: Otini | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Linux StaticPointers, hole, skolem, | panic | Architecture: x86_64 | Type of failure: Compile-time (amd64) | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When compiling this minimal example: {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE StaticPointers #-} import Data.Typeable (Typeable) import GHC.StaticPtr (StaticPtr) f :: Typeable a => StaticPtr (a -> a) f = static (\a -> _) main :: IO () main = return () }}} I get this output: {{{ Bug.hs:8:19: error:ghc: panic! (the 'impossible' happened) (GHC version 8.0.1 for x86_64-unknown-linux): No skolem info: a_aJo[sk] Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} Unlike similar reported bugs, this happens on both 8.0.1 and 8.0.2. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13499> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10249: Suboptimal error message with deferred type errors
by GHC 05 Sep '18

05 Sep '18

#10249: Suboptimal error message with deferred type errors -------------------------------------+------------------------------------- Reporter: | Owner: Iceland_jack | Status: new Type: feature | Milestone: request | Version: 7.10.1 Priority: normal | Operating System: Unknown/Multiple Component: GHCi | Type of failure: None/Unknown Keywords: | Blocked By: Architecture: | Related Tickets: Unknown/Multiple | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- {{{#!hs $ ghci -fdefer-type-errors -ignore-dot-ghci GHCi, version 7.10.0.20150316: http://www.haskell.org/ghc/ :? for help Prelude> _ <interactive>:2:1: Warning: Found hole ‘_’ with type: IO t0 Where: ‘t0’ is an ambiguous type variable In the first argument of ‘GHC.GHCi.ghciStepIO :: IO a_aly -> IO a_aly’, namely ‘_’ In a stmt of an interactive GHCi command: it <- GHC.GHCi.ghciStepIO :: IO a_aly -> IO a_aly _ *** Exception: <interactive>:2:1: Found hole ‘_’ with type: IO t0 Where: ‘t0’ is an ambiguous type variable In the first argument of ‘GHC.GHCi.ghciStepIO :: IO a_aly -> IO a_aly’, namely ‘_’ In a stmt of an interactive GHCi command: it <- GHC.GHCi.ghciStepIO :: IO a_aly -> IO a_aly _ (deferred type error) Prelude> }}} It should ideally not expose `ghciStepIO` to the user. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10249> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10756: Allow users to indicate inaccessible patterns
by GHC 04 Sep '18

04 Sep '18

#10756: Allow users to indicate inaccessible patterns -------------------------------------+------------------------------------- Reporter: edsko | Owner: Type: feature | Status: new request | 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 Revisions: | -------------------------------------+------------------------------------- Consider {{{#!hs {-# LANGUAGE KindSignatures #-} {-# LANGUAGE GADTs #-} data Elem :: * -> * -> * where EZ :: Elem (f , fs) f ES :: Elem fs f -> Elem (f', fs) f elemAbsurd :: Elem () f -> a elemAbsurd _ = error "inaccessible" }}} This catch-all in `elemAbsurd` is very unfortunate, because if I change my datatype and not all cases are inaccessible anymore the type checker will not be warning me. Although of course there is recent work by SPJ et al on inferring more inaccessible cases, I think it would be tremendously useful for users to be able to indicate that patterns are in fact inaccesible. I'd be very happy if I could write {{{#!hs elemAbsurd :: Elem () f -> a elemAbsurd EZ = inaccessible elemAbsurd (ES _) = inaccessible }}} Note that ghc ''can already detect that these patterns are in fact inaccessible'': it won't let me write the above: {{{ T.hs:12:12: Couldn't match type ‘()’ with ‘(f, fs)’ Inaccessible code in a pattern with constructor EZ :: forall f fs. Elem (f, fs) f, in an equation for ‘elemAbsurd’ }}} So I think it should be very very simple to change the type checker to allow to write cases like this if and only if the RHS is a specially designated symbol "inaccessible", which could be defined simply as {{{#!hs inaccessible :: a inaccessible = error "inaccessible" }}} in `GHC.Prim` or something. This would seem like very little work, and a correspondingly tiny patch, but with quite a large payoff. Future work in automatically detecting inaccessible cases would not make existing code using this style wrong, it just means that we have to write fewer inaccessible cases by hand; but even with more sophisticated inference algorithms for inaccessible cases there will still be inaccessible cases remaining. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10756> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13243: make test in non-validate configuration fails with a variety of ghci errors
by GHC 04 Sep '18

04 Sep '18

#13243: make test in non-validate configuration fails with a variety of ghci errors -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: GHCi | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Other Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Starting with a fresh checkout of `ghc`, testing in a non-validate configuration fails in a rather peculiar way. e.g., {{{ $ ./boot $ ./configure $ make $ make test }}} Results in a number of test failures of the `ghci` way which suggest that GHCi is having trouble identifying types. e.g., {{{ =====> break026(ghci) 50 of 101 [0, 46, 0] cd "./ghci.debugger/scripts/break026.run" && HC="/mnt/work/ghc/ghc- testing/inplace/test spaces/ghc-stage2" HC_OPTS="-dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn-missed-specialisations -fshow- warning-groups -fdiagnostics-color=never -fno-diagnostics-show-caret -dno- debug-output " "/mnt/work/ghc/ghc-testing/inplace/test spaces/ghc- stage2" -dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn- missed-specialisations -fshow-warning-groups -fdiagnostics-color=never -fno-diagnostics-show-caret -dno-debug-output --interactive -v0 -ignore- dot-ghci -fno-ghci-history +RTS -I0.1 -RTS -ignore-dot-ghci< break026.script Actual stderr output differs from expected: diff -uw "/dev/null" "./ghci.debugger/scripts/break026.run/break026.run.stderr.normalised" --- /dev/null 2017-01-24 15:20:36.872346204 -0500 +++ ./ghci.debugger/scripts/break026.run/break026.run.stderr.normalised 2017-02-08 15:52:09.290099692 -0500 @@ -0,0 +1,14 @@ + +<interactive>:16:1: + No instance for (Show t1) arising from a use of ‘print’ + Cannot resolve unknown runtime type ‘t1’ + Use :print or :force to determine these types + Relevant bindings include it :: t1 (bound at <interactive>:16:1) + These potential instances exist: + instance Show Ordering -- Defined in ‘GHC.Show’ + instance Show Integer -- Defined in ‘GHC.Show’ + instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’ + ...plus 22 others + ...plus 11 instances involving out-of-scope types + (use -fprint-potential-instances to see them all) + In a stmt of an interactive GHCi command: print it *** unexpected failure for break026(ghci) =====> break027(ghci) 51 of 101 [0, 47, 0] cd "./ghci.debugger/scripts/break027.run" && HC="/mnt/work/ghc/ghc- testing/inplace/test spaces/ghc-stage2" HC_OPTS="-dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn-missed-specialisations -fshow- warning-groups -fdiagnostics-color=never -fno-diagnostics-show-caret -dno- debug-output " "/mnt/work/ghc/ghc-testing/inplace/test spaces/ghc- stage2" -dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn- missed-specialisations -fshow-warning-groups -fdiagnostics-color=never -fno-diagnostics-show-caret -dno-debug-output --interactive -v0 -ignore- dot-ghci -fno-ghci-history +RTS -I0.1 -RTS -ignore-dot-ghci< break027.script Actual stdout output differs from expected: diff -uw "./ghci.debugger/scripts/break027.run/break027.stdout.normalised" "./ghci.debugger/scripts/break027.run/break027.run.stdout.normalised" --- ./ghci.debugger/scripts/break027.run/break027.stdout.normalised 2017-02-08 15:52:10.750125249 -0500 +++ ./ghci.debugger/scripts/break027.run/break027.run.stdout.normalised 2017-02-08 15:52:10.750125249 -0500 @@ -1,8 +1,8 @@ Breakpoint 0 activated at QSort.hs:4:12-13 Breakpoint 1 activated at QSort.hs:5:16-51 Stopped in QSort.qsort, QSort.hs:5:16-51 -_result :: [Integer] = _ -a :: Integer = 3 -left :: [Integer] = _ -right :: [Integer] = _ -a :: Integer -- Defined in ‘interactive:Ghci1’ +_result :: [a] = _ +a :: a = _ +left :: [a] = _ +right :: [a] = _ +a :: a -- Defined in ‘interactive:Ghci1’ *** unexpected failure for break027(ghci) =====> print001(ghci) 11 of 101 [0, 7, 0] cd "./ghci.debugger/scripts/print001.run" && HC="/mnt/work/ghc/ghc- testing/inplace/test spaces/ghc-stage2" HC_OPTS="-dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn-missed-specialisations -fshow- warning-groups -fdiagnostics-color=never -fno-diagnostics-show-caret -dno- debug-output " "/mnt/work/ghc/ghc-testing/inplace/test spaces/ghc- stage2" -dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn- missed-specialisations -fshow-warning-groups -fdiagnostics-color=never -fno-diagnostics-show-caret -dno-debug-output --interactive -v0 -ignore- dot-ghci -fno-ghci-history +RTS -I0.1 -RTS -ignore-dot-ghci< print001.script Actual stdout output differs from expected: diff -uw "./ghci.debugger/scripts/print001.run/print001.stdout.normalised" "./ghci.debugger/scripts/print001.run/print001.run.stdout.normalised" --- ./ghci.debugger/scripts/print001.run/print001.stdout.normalised 2017-02-08 15:51:08.413033889 -0500 +++ ./ghci.debugger/scripts/print001.run/print001.run.stdout.normalised 2017-02-08 15:51:08.413033889 -0500 @@ -1,10 +1,9 @@ li = (_t1::[Maybe Integer]) Just 0 -li = Just 0 : (_t2::[Maybe Integer]) +li = (_t2::[Maybe Integer]) 6 -li = [Just 0,(_t3::Maybe Integer),(_t4::Maybe Integer), - (_t5::Maybe Integer),(_t6::Maybe Integer),(_t7::Maybe Integer)] -li = [Just 0,_,_,_,_,_] +li = (_t3::[Maybe Integer]) +li = _ [Just 0,Just 1,Just 2,Just 3,Just 4,Just 5] -li = [Just 0,Just 1,Just 2,Just 3,Just 4,Just 5] -li = [Just 0,Just 1,Just 2,Just 3,Just 4,Just 5] +li = (_t4::[Maybe Integer]) +li = _ *** unexpected failure for print001(ghci) }}} It would be nice to know what is going on here. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13243> 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] #10374: Can't build GHC with a dynamic only GHC installation
by GHC 04 Sep '18

04 Sep '18

#10374: Can't build GHC with a dynamic only GHC installation -------------------------------------+------------------------------------- Reporter: jessicah | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Build | Version: 7.10.1 System | Operating System: Unknown/Multiple Keywords: | Type of failure: Building GHC Architecture: | failed Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- When an installation of GHC doesn't include the vanilla "way", GHC cannot be built from this existing installation of GHC. E.g. using the following mk/build.mk: {{{ V = 0 GhcLibWays = dyn SRC_HC_OPTS = -O -H64m GhcStage1HcOpts = -O -fasm GhcStage2HcOpts = -O2 -fasm GhcHcOpts = -Rghc-timing GhcLibHcOpts = -O2 DYNAMIC_BY_DEFAULT = YES NoFibWays = STRIP_CMD = : }}} Steps to reproduce: 1. build & install GHC using above configuration 2. rebuild GHC using above configuration, --with-ghc pointing to the installation created in step 1 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10374> 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] #13930: Cabal configure
by GHC 02 Sep '18

02 Sep '18

#13930: Cabal configure -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 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: -------------------------------------+------------------------------------- `cabal configure` apparently has a space leak when compiled with GHC HEAD but not 8.0. It's not clear whether this is really a GHC regression yet, but here it is certainly GHC version-specific. See https://github.com/haskell/cabal/issues/4589 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13930> 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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