December 2017 - ghc-tickets

[GHC] #13153: Several Traversable instances have an extra fmap
by GHC 24 Feb '18

24 Feb '18

#13153: Several Traversable instances have an extra fmap -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: dfeuer Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Core | Version: 8.1 Libraries | Keywords: | 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: -------------------------------------+------------------------------------- For example, we define {{{#!hs instance Traversable ZipList where traverse f (ZipList xs) = ZipList <$> traverse f xs }}} If the list is very short, the extra `fmap` could be bad. We can fix this by inlining the inner `traverse`. However, I suspect a better approach would be to add a method to `Traversable`: {{{#!hs mapTraverse :: Applicative f => (t b -> r) -> (a -> f b) -> t a -> f r mapTraverse p f xs = p <$> traverse f xs }}} but I need to work through whether this is enough power to solve enough problems. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13153> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11826: unsafe causes bug, news @ 11
by GHC 24 Feb '18

24 Feb '18

#11826: unsafe causes bug, news @ 11 -------------------------------------+------------------------------------- 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: -------------------------------------+------------------------------------- I know there has been plenty of levity here lately but here goes :) I didn't expect ‘isUndefined’ to be safe but this may be interesting to the devs (I am obedient and report it thusly and hereby-ly per GHC's request) {{{#!haskell % ghci -ignore-dot-ghci GHCi, version 8.1.20160117: http://www.haskell.org/ghc/ :? for help Prelude> import Control.Exception Prelude Control.Exception> import System.IO.Unsafe Prelude Control.Exception System.IO.Unsafe> import Unsafe.Coerce Prelude Control.Exception System.IO.Unsafe Unsafe.Coerce> isUndefined x = unsafePerformIO $ catch ((unsafeCoerce x :: IO ()) >> return False) (const $ return True :: SomeException -> IO Bool) Prelude Control.Exception System.IO.Unsafe Unsafe.Coerce> isUndefined (succ True) True Prelude Control.Exception System.IO.Unsafe Unsafe.Coerce> isUndefined (succ False) <interactive>: internal error: stg_ap_v_ret (GHC version 8.1.20160117 for x86_64_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Aborted (core dumped) % }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11826> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12524: RFC: Allow prefixing result evaluated at GHCi prompt
by GHC 24 Feb '18

24 Feb '18

#12524: RFC: Allow prefixing result evaluated at GHCi prompt -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature | Status: new request | Priority: lowest | Milestone: Component: GHCi | Version: 8.0.1 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: -------------------------------------+------------------------------------- Similar to `prompt`, `prompt-cont` (previously `prompt2`): allowing users to supply a prefix to evaluation results with `result` has two use cases I can think of {{{ :set prompt " ghci> " :set prompt-cont " ghci| " :set result " " }}} the desired outcome would be `↓` as indented code for IRC, also reddit code format: {{{ ghci> 1 + 2 + 3 6 ghci> reverse "hello" "olleh" }}} ---- The other case came from [https://twitter.com/rightfold/status/767301255140179968 this tweet] {{{ :set prompt "" :set result "-- " }}} {{{ 1 + 2 + 3 -- 6 reverse "hello" -- "olleh" }}} This can be copied/pasted back to a separate GHCi session where the result is ignored. ---- I haven't seen this feature in any other repl, I'm not sure if this would see much use. This wouldn't work with IO actions without some magic. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12524> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14328: ld.gold -r brokenness breaks SplitSections=YES builds
by GHC 21 Feb '18

21 Feb '18

#14328: ld.gold -r brokenness breaks SplitSections=YES builds -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: 14291 | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- See https://sourceware.org/bugzilla/show_bug.cgi?id=22266. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14328> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14564: CAF isn't floated
by GHC 21 Feb '18

21 Feb '18

#14564: CAF isn't floated -------------------------------------+------------------------------------- Reporter: neil.mayhew | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Keywords: | 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: -------------------------------------+------------------------------------- In the following code, `pattern` isn't floated, and the execution time is 20x slower than it should be: {{{#!hs import Text.Regex.TDFA (Regex, makeRegex, match) main :: IO () main = do entries <- map parseFilename . lines <$> getContents let check (Right (_, t)) = last t == 'Z' check _ = False print $ all check entries parseFilename :: String -> Either String (String, String) parseFilename fn = case (pattern `match` fn :: [[String]]) of [[_, full, _, time]] -> Right $ (full, time) _ -> Left fn where pattern :: Regex pattern = makeRegex "^\\./duplicity-(full|inc|new)(-signatures)?\\.\ \([0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]T[0-9][0-9][0-9][0-9][0-9][0-9]Z)\\." }}} Curiously, it is floated when profiling is enabled, so the profiled program ends up being 7x faster than the unprofiled one. I can float the code manually (by taking out `where` and unindenting) and the program then runs at an acceptable speed. I get the same behaviour with 8.0.2, 8.2.1 and 8.2.2 but 7.10.3 is OK. I don't think the OS and architecture makes a difference, but for the record I'm on various flavours of 64-bit Linux. Test input data is [https://gist.github.com/neilmayhew/247a30738c0e294902e7c2830ca2c6f5 here] -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14564> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14536: Ghc panics while building stage2 with -dstg-lint
by GHC 21 Feb '18

21 Feb '18

#14536: Ghc panics while building stage2 with -dstg-lint -------------------------------------+------------------------------------- Reporter: duog | 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: -------------------------------------+------------------------------------- Building ghc at 54fda257d4a7bfddaa0c1fa0be698d1a849c4124 with the following mk/build.mk: {{{ BuildFlavour = validate ifneq "$(BuildFlavour)" "" include mk/flavours/$(BuildFlavour).mk endif GhcStage2HcOpts += -dcore-lint -dstg-lint -dcmm-lint }}} building with: {{{ make compiler/stage2/build/Exception.o }}} panics with: {{{ ghc-stage1: panic! (the 'impossible' happened) (GHC version 8.3.20171127 for x86_64-unknown-linux): ASSERT failed! dataConInstArgTys Unit# [k0_10, a_11] ['TupleRep '[], 'IntRep, State# RealWorld, Int#] Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1208:22 in ghc:Outputable assertPprPanic, called at compiler/basicTypes/DataCon.hs:1256:76 in ghc:DataCon Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable pprPanic, called at compiler/utils/Outputable.hs:1206:5 in ghc:Outputable assertPprPanic, called at compiler/basicTypes/DataCon.hs:1256:76 in ghc:DataCon Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} Once this is resolved, is there any reason that -dstg-ling and -dcmm-lint are not set on validate (at least with SLOW)? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14536> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14541: Stg lint failure while building ghc-stage2: compiler/stage2/build/Hoopl/Block.o
by GHC 21 Feb '18

21 Feb '18

#14541: Stg lint failure while building ghc-stage2: compiler/stage2/build/Hoopl/Block.o -------------------------------------+------------------------------------- Reporter: duog | 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: -------------------------------------+------------------------------------- With the tree at: https://github.com/duog/ghc/tree/trac-14536 Which fixes ticket:14536 With build.mk: {{{ BuildFlavour = validate ifneq "$(BuildFlavour)" "" include mk/flavours/$(BuildFlavour).mk endif GhcStage2HcOpts += -dcore-lint -dstg-lint -dcmm-lint }}} building with: {{{ make compiler/stage2/build/Hoopl/Blocks.o }}} I get the following Stg lint error (extracts, full dump attached): {{{ <no location info>: warning: [in body of lambda with binders ds_s4dH :: a_a3as -> b_a3at, ds1_s4dI :: MaybeO ex_a3ao a_a3as] In some algebraic case alternative, number of arguments doesn't match constructor: JustO (arity 2) [a1_s4dK] }}} ... {{{ $fFunctorMaybeO_$cfmap :: forall ex a b. (a -> b) -> MaybeO ex a -> MaybeO ex b [GblId, Arity=2, Caf=NoCafRefs, Str=<L,1*C1(U)><S,1*U>, Unf=OtherCon []] = [] \r [ds_s4dH ds1_s4dI] case ds1_s4dI of { JustO a1_s4dK [Occ=Once] -> let { sat_s4dL [Occ=Once] :: b_a3at [LclId] = [ds_s4dH a1_s4dK] \u [] ds_s4dH a1_s4dK; } in JustO [sat_s4dL]; NothingO -> $WNothingO; }; }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14541> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14001: Inlining does not work between modules
by GHC 20 Feb '18

20 Feb '18

#14001: Inlining does not work between modules -------------------------------------+------------------------------------- Reporter: danilo2 | Owner: (none) Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 8.0.2 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: -------------------------------------+------------------------------------- Hi! I discovered today something that looks like a terrifying bug. I was discussing it on IRC and so far there is no explanation why it happens. Let's grab a test code from here: https://github.com/luna/dependent- state/tree/irc-testing (branch irc-testing). If you download it and execute `stack --stack-yaml stack-develop.yaml bench dependent-state:layered-state-benchmark` benchmarks from `test/bench/Main.hs` will be executed. Everything is compiled with `-O2` etc (see stack-develop.yaml for details). And now (everything regarding file `Control/Monad/State/Layered.hs`): 1. If you change lines `94-111` to use pointfree instead of normal arg, you get 40x slowdown 2. If you then uncomment lines `80-81`, everything is fast again. The problem is that lines `80-81` are: {{{ (.) :: (b -> c) -> (a -> b) -> a -> c (.) f g = \x -> f (g x) ; {-# INLINE (.) #-} }}} It seems that this function is inlined ONLY if is defined in this module. If we use the one from base OR if we move this function to separate module, we can observe the slowdown. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14001> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11285: Static linking is really slow sometimes
by GHC 20 Feb '18

20 Feb '18

#11285: Static linking is really slow sometimes -------------------------------------+------------------------------------- Reporter: ezyang | 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: -------------------------------------+------------------------------------- I'm testing on Cabal `Setup.hs`, which links against Cabal. On a beefy machine with many cores and lots of RAM, I see a x2 regression in linking time from GHC 7.10.2 to GHC 8.0 (a recent HEAD) using GNU ld (not gold): {{{ [ezyang@hs01 ezyang]$ rm A; time ghc --make A.hs -fforce-recomp [1 of 1] Compiling Main ( A.hs, A.o ) Linking A ... real 0m1.273s user 0m0.990s sys 0m0.210s [ezyang@hs01 ezyang]$ rm A; time ghc-8.0/usr/bin/ghc --make A.hs -fforce- recomp [1 of 1] Compiling Main ( A.hs, A.o ) Linking A ... real 0m3.270s user 0m2.727s sys 0m0.523s }}} On a puny eight year-old laptop, I see a x2 regression from 7.6 to 7.10 (with not much change with 8.0) {{{ ezyang@sabre:~$ rm A; time ghc --make -O0 A.hs -fforce-recomp rm: cannot remove ‘A’: No such file or directory [1 of 1] Compiling Main ( A.hs, A.o ) Linking A ... real 0m3.058s user 0m1.860s sys 0m1.164s ezyang@sabre:~$ rm A; time ghc-7.10 --make -O0 A.hs -fforce-recomp [1 of 1] Compiling Main ( A.hs, A.o ) Linking A ... real 0m7.139s user 0m4.616s sys 0m2.488s }}} There must be something which is causing the linker to run slowly in one case, and quickly in the other. It would be really good to figure out what this is. Slow linking is NOT NICE. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11285> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14434: GHC 8.2.1 picks wrong instances with -O in a presence of an OVERLAPPABLE instance
by GHC 20 Feb '18

20 Feb '18

#14434: GHC 8.2.1 picks wrong instances with -O in a presence of an OVERLAPPABLE instance -------------------------------------+------------------------------------- Reporter: dredozubov | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: Incorrect result (amd64) | at runtime Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- We experienced code breakage while moving to 8.2.1 - in some circumstances it picks wrong instances if an instance in the lines of `instance {-# OVERLAPPABLE #-} C a ` is defined. @s9gf4ult was really nice to create a repo demonstrating an issue, you can look at the repo here: [https://github.com/s9gf4ult/isogen-minimal-example] Running `chech.sh` will trigger three builds: {{{ GHC 8.0.2, -O, succeeds GHC 8.2.1, -O0, succeeds GHC 8.2.1, -O, fails }}} If you'll check out to either of fix- branches: {{{ fix-move-to_string fix-no-catchall-instance fix-replace-to_string fix-add-irrelevant-instance }}} then the last test should succeed also. Those branches contain changes over master which fixes the bug in different ways. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14434> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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