May 2015 - ghc-tickets

[GHC] #9285: IO manager startup procedure somewhat odd
by GHC 15 Jan '18

15 Jan '18

#9285: IO manager startup procedure somewhat odd ------------------------------------+------------------------------------- Reporter: edsko | Owner: Type: bug | Status: new Priority: low | 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 RTS startup procedure for the threaded runtime with a single capability looks something like this: '''1. Capability gets initialized''' {{{ hs_main real_main hs_init_ghc initScheduler initCapabilities }}} '''2. IO Manager is started''' {{{ hs_main real_main hs_init_ghc ioManagerStart }}} '''3. ensureIOManagerIsRunning''' The IO manager of course is mostly written in Haskell, so `ioManagerStart` does {{{ cap = rts_lock(); rts_evalIO(&cap,&base_GHCziConcziIO_ensureIOManagerIsRunning_closure,NULL); rts_unlock(cap); }}} '''4. Bound task is created and we start running Haskell code''' Since this is the first call to `rts_lock`, it creates a bound task to correspond to the current OS thread. '''5. Haskell code does a safe FFI call: create worker task 1''' The first safe FFI call that the IO manager makes (ignoring `labelThread`) is to `c_setIOManagerControlFd`. At this point, the bound task gets suspended and we create a worker task. '''6. Worker task 1 starts executing `GHC.Event.Manager.loop`''' The worker tasks gets suspended due to an explicit call to `yield` in `GHC.Event.Manager.step`, at which point the bound task continues execution. It continues and finishes (`ensureIOManagerRunning` completes). The worker task takes over. '''7. Worker task 1 does a safe FFI call to `I.poll`, create new worker task 2''' (This safe call is from `GHC.Event.Manager.step`) '''8. Worker task 2 executes `GHC.Event.TimerManager.loop`, and does safe FFI call to `I.poll`; create worker task 3''' This worker task will remain spare for now. '''9. Start executing the actual main function''' The IO manager calls `rts_unlock`, which verifies if there are any spare workers available; it finds that there are and doesn't start a new one. Then when we start `main` itself of course we go through this `rts_lock, rts_evalIO, rts_unlock` sequence ''again''. This is somewhat confusing, and makes tracing the startup of the code more difficult to interpret. It seems that this could be simpler: only a single `rts_lock` should be necessary, in which we first call `ensureIOManagerIsRunning` and then `main`, followed by a single `rts_unlock`. A similar simplification should be possible in `forkProcess`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9285> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10245: panic in new integer switch logic with "out-of-range" literals
by GHC 07 Jan '18

07 Jan '18

#10245: panic in new integer switch logic with "out-of-range" literals -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 7.11 (CodeGen) | Operating System: Unknown/Multiple Keywords: | Type of failure: Compile-time Architecture: x86_64 | crash (amd64) | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Compiling this module {{{ module D (f) where f :: Int -> String f n = case n of 0x8000000000000000 -> "yes" _ -> "no" }}} crashes with the error {{{ [1 of 1] Compiling D ( /tmp/D.hs, /tmp/D.o ) ghc-stage1: panic! (the 'impossible' happened) (GHC version 7.11.20150403 for x86_64-unknown-linux): Map.findMin: empty map has no minimal element }}} The constant does not have to be exactly `0x8000000000000000`, everything I tested from there up to `0xffffffffffffffff` yields the same crash. Also occurs with `Word` and negative literals. The bug seems to be tied to the target's word size somehow, though: a 64-bit compiler does not panic on `Int32` and `0x80000000`, but a 32-bit compiler does. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10245> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8487: Debugger confuses variables
by GHC 02 Jan '18

02 Jan '18

#8487: Debugger confuses variables ------------------------------------+------------------------------------- Reporter: edsko | Owner: Type: bug | Status: new Priority: normal | 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: | ------------------------------------+------------------------------------- Consider the following example: {{{ import Control.Exception f = do ma <- try $ evaluate a case ma of Right str -> return a Left err -> return $ show (err :: SomeException) where a :: String a = error "hi" }}} Load into ghci, set a breakpoint on the 5th line, then run f: {{{ :break 5 f }}} You will get {{{ Stopped at Ex.hs:(5,3)-(7,53) _result :: IO String = _ a :: Either SomeException String = Left _ }}} This doesn't make sense. `a` has type `String`; it is `ma` that has type `Either SomeException String`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8487> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8642: Allow GHCi to print non-pretty forms of things.
by GHC 29 Dec '17

29 Dec '17

#8642: Allow GHCi to print non-pretty forms of things. ------------------------------------+------------------------------------- Reporter: Fuuzetsu | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: GHCi | 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: | ------------------------------------+------------------------------------- Currently when we ask GHCi for :info or :type of something, it will do things like splitting over multiple lines and all that eye candy that people like to see. This however makes it more difficult for people to write text editor plugins &c that rely on GHCi output. Currently one has to write parsers to stitch the GHCi output back together which is fairly error prone and not really future-proof. Is there any chance for a more machine-friendly output? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8642> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9291: Don't reconstruct sum types if the type subtly changes
by GHC 28 Dec '17

28 Dec '17

#9291: Don't reconstruct sum types if the type subtly changes ----------------------------------------------+---------------------------- Reporter: schyler | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Moderate (less than a day) | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: ----------------------------------------------+---------------------------- Consider this test case: {{{ module Main (main) where fun :: Either Int String -> Either String String fun x = case x of Left int -> Left (show int) Right str -> Right str {-# NOINLINE fun #-} main :: IO () main = do l <- getLine print $ fun (Right l) }}} The core we get for fun looks like: {{{ Main.fun [InlPrag=NOINLINE] :: Data.Either.Either GHC.Types.Int GHC.Base.String -> Data.Either.Either GHC.Base.String GHC.Base.String [GblId, Arity=1, Caf=NoCafRefs, Str=DmdType <S,1*U>] Main.fun = \ (x_aur :: Data.Either.Either GHC.Types.Int GHC.Base.String) -> case x_aur of _ [Occ=Dead] { Data.Either.Left int_aus -> Data.Either.Left @ GHC.Base.String @ GHC.Base.String (GHC.Show.$fShowInt_$cshow int_aus); Data.Either.Right str_aCG -> Data.Either.Right @ GHC.Base.String @ GHC.Base.String str_aCG } }}} There would be less allocations and probably perform better if we just coerced `x` into the new type. Because the coercion is common code, this also means that if hypothetically some other sum type which had 15 constructors, and only 3 had subtle type changes, 12 of the case branches could be CSE'd into the single coerce greatly reducing code generated and also hinting the inliner better. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9291> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10012: Cheap-to-compute values aren't pushed into case branches inducing unnecessary register pressure
by GHC 22 Dec '17

22 Dec '17

#10012: Cheap-to-compute values aren't pushed into case branches inducing unnecessary register pressure -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- When characterizing bytestring's `Builder` interface[1] I noticed that some benchmarks involving repeated appends (e.g. `Host endian/1MB of Word8 in chunks of 16`) perform inexplicably much worse than others. A glance at the assembly revealed that a large number of `Word8`s are being evaluated and saved to registers, only to be later stored. E.g., ```#!asm # First we evaluate a number of Word8s, saving them in registers movzbl %bl,%ebx leaq 1(%r14),%rcx movzbl %cl,%ecx leaq 2(%r14),%rdx movzbl %dl,%edx leaq 3(%r14),%rsi movzbl %sil,%esi leaq 4(%r14),%r9 movzbl %r9b,%r9d leaq 5(%r14),%r10 movzbl %r10b,%r10d ... # Eventually we run out of registers and start spilling to the stack movq %rax,64(%rsp) leaq 7(%r14),%rax movzbl %al,%eax movq %rbx,72(%rsp) ... # Only after evaluating all of the needed words do we actually consume # them movq %rax,-152(%r12) movq 72(%rsp),%rax movq %rax,-144(%r12) movq 80(%rsp),%rax movq %rax,-136(%r12) movq 88(%rsp),%rax movq %rax,-128(%r12) ... movq %rsi,-56(%r12) movq %r9,-48(%r12) movq %r10,-40(%r12) movq %r11,-32(%r12) movq %r14,-24(%r12) ``` This is due to the fact that the `Word`s are bound outside of a case analysis and GHC is reluctant to push them inside of the branches. The float-in pass will only float a binding into a case if the value is "small" and at least one branch doesn't use the binding. Unfortunately the case expression in question has only two branches. This is demonstrated in the attached testcase. [1] https://github.com/kolmodin/binary/pull/65 [2] https://www.haskell.org/pipermail/ghc-devs/2015-January/007997.html -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10012> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9244: Compiler could warn about type variable shadowing, and hint about ScopedTypeVariables
by GHC 18 Dec '17

18 Dec '17

#9244: Compiler could warn about type variable shadowing, and hint about ScopedTypeVariables -------------------------+------------------------------------------------- Reporter: | Owner: stusmith | Status: new Type: | Milestone: feature request | Version: 7.6.3 Priority: | Operating System: Unknown/Multiple normal | Type of failure: Incorrect warning at Component: | compile-time Compiler | Test Case: Keywords: | Blocking: Architecture: | Unknown/Multiple | Difficulty: | Unknown | Blocked By: | Related Tickets: | -------------------------+------------------------------------------------- GHC already warns about variable shadowing: {{{ timesTwoPlusOne x = timesTwo x + 1 where timesTwo x = x * 2 Warning: This binding for `x' shadows the existing binding bound at <location> }}} However the similar warning doesn't happen for type variables. {{{ tryMaybe :: IO a -> IO (Maybe a) tryMaybe action = do result <- (try action) :: IO (Either SomeException a) return $ case result of Left _ -> Nothing Right v -> Just v Couldn't match type `a' with `a1' `a' is a rigid type variable bound by the type signature for tryMaybe :: IO a -> IO (Maybe a) at types.hs:<line>:13 `a1' is a rigid type variable bound by an expression type signature: IO (Either SomeException a1) at types.hs:<line>:15 Expected type: IO a1 Actual type: IO a ... }}} Here, I thought that the 'a' in the function's type declaration was the same 'a' in the expression type declaration. However in Haskell 98, they are completely different variables. Suggestion: if a type variable is renamed by the compiler due to a clash with another type variable, issue a warning that the second shadows the first, and give a hint about using -XScopedTypeVariables and forall. Alternative suggestion: if an error is displayed, where the error contains a renamed type variable, issue a hint that the second shadows the first, and give a hint about using -XScopedTypeVariables and forall. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9244> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8272: testing if SpLim=$rbp and Sp=$rsp changed performance at all
by GHC 16 Dec '17

16 Dec '17

#8272: testing if SpLim=$rbp and Sp=$rsp changed performance at all ----------------------------------------------+---------------------------- Reporter: carter | Owner: carter Type: task | Status: new Priority: normal | Milestone: 7.10.1 Component: Compiler | Version: 7.7 Keywords: callingConvention | Operating System: Architecture: x86_64 (amd64) | Unknown/Multiple Difficulty: Project (more than a week) | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: ----------------------------------------------+---------------------------- testing if SpLim=$rbp and Sp=$rsp changed performance at all would need a stack check but then push could be used to spill to the stack Idea via Nathan Howell. At the very least, the x86 PUSH instruction has a more succinct encoding than MOV. worth hacking out to see if this can measurable shift ghc perf on nofib or not. this would be part of a larger effort to explore ways to improve GHC's calling convention for modern hardware -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8272> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9364: GHCi (or haskeline?) confused by non-single-width characters
by GHC 14 Dec '17

14 Dec '17

#9364: GHCi (or haskeline?) confused by non-single-width characters -------------------------------------+------------------------------------- Reporter: cheater | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 7.8.3 Keywords: linux, terminal, | Operating System: colour, color, prompt | Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Other Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Hi, there's an issue in the GHCi prompt. To reproduce: 1. Do everything in Linux (I don't have other operating systems); I'm using a version of the gnome 2 gnome-terminal. 2. Use the following to colourise the prompt in GHCi: echo -e :set prompt '"\033[32;1m%s\033[34;1m>\033[0m "' >> ~/.ghc/ghci.conf 3. Import quite a few modules (say, 5 to 10 should be enough) 4. Give the prompt enough text that it wraps around. It's best if you start the text with -- so that it's a comment and doesn't produce any output. For example, say I entered "-- bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb", I now see the following: *Types Control.Monad.Writer Control.Monad Data.Functor Control.Applicative Data.Monoid> -- bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb (do note that the first set of b's is located on the same line as the prompt, and the second is located under the prompt) 5. Press Enter. You should now see: *Types Control.Monad.Writer Control.Monad Data.Functor Control.Applicative Data.Monoid> -- bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb *Types Control.Monad.Writer Control.Monad Data.Functor Control.Applicative Data.Monoid> 6. Press Ctrl-P or the up arrow key to select the previous history item. You will now see the following: *Types Control.Monad.Writer Control.Monad Data.Functor Control.Applicative Data.Monoid> -- bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb *Types Control.Monad.Writer Control.Monad Data.Functor Control.Applicative Data.Monoid> -- bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb 7. Press Ctrl-A or the Home cursor key to go to the beginning of the text entry, and then type X. You will now see the last prompt to be: *Types Control.Monad.Writer Control.Monad Data.Functor Control.Applicative Data.Monoid> -- bbbbbbbbbbbbX-- bbbbbbbbbbbbbbbbb bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb 8. If you press Enter, you will find out that prompt was actually (without line breaks): X-- bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb I tried resizing the terminal window to 45 chars to make the lines above shorter; I imported Data.Monoid and Data.Functor in a new GHCi. The prompt contained garbage, most likely badly wrapped escape codes. See attachment. As both likely are a single bug in disguise, I include both reports in this ticket. Creating a test case for whether things actually work in a terminal might be difficult, but creating test for whether things get wrapped correctly (and not testing how things work out in a terminal) should be simple if annoying. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9364> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9669: Long compile time/high memory usage for modules with many deriving clauses
by GHC 12 Dec '17

12 Dec '17

#9669: Long compile time/high memory usage for modules with many deriving clauses -------------------------------------+------------------------------------- Reporter: snoyberg | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: Linux Architecture: x86_64 (amd64) | Type of failure: Compile- Difficulty: Unknown | time performance bug Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- I've seen many different people complain about this in various different contexts. One of the most common I personally see is when using Persistent with a large number of entities, which results in multi-minute build times for that single module. Usually, these reports have come from private repositories, which makes for a difficult repro. Also, the usage of Template Haskell in those modules tends to confuse the issue. So I'd like to report this issue from a completely separate project, with the guess that this represents an issue that is affecting many other users. Steps to reproduce: * `cabal unpack fpco-api-1.2.0.1` * `cd fpco-api-1.2.0.1` * `cabal install` * `ghc --make -isrc/library src/library/FP/API/Types.hs` * `touch src/library/FP/API/Types.hs` Then, to see the time and memory usage of compiling just the one module, run: * `time ghc -O2 --make -isrc/library src/library/FP/API/Types.hs +RTS -s` On my system (16GB RAM, quadcore i7), the results are: {{{ $ time ghc -O2 --make -isrc/library src/library/FP/API/Types.hs +RTS -s [3 of 3] Compiling FP.API.Types ( src/library/FP/API/Types.hs, src/library/FP/API/Types.o ) 51,846,533,568 bytes allocated in the heap 6,799,246,288 bytes copied during GC 389,297,584 bytes maximum residency (22 sample(s)) 11,330,472 bytes maximum slop 1041 MB total memory in use (0 MB lost due to fragmentation) Tot time (elapsed) Avg pause Max pause Gen 0 2041 colls, 0 par 9.19s 9.19s 0.0045s 0.0998s Gen 1 22 colls, 0 par 4.33s 4.33s 0.1969s 0.5436s TASKS: 4 (1 bound, 3 peak workers (3 total), using -N1) SPARKS: 0 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled) INIT time 0.00s ( 0.00s elapsed) MUT time 34.21s ( 37.74s elapsed) GC time 13.52s ( 13.53s elapsed) EXIT time 0.07s ( 0.07s elapsed) Total time 47.81s ( 51.34s elapsed) Alloc rate 1,515,662,937 bytes per MUT second Productivity 71.7% of total user, 66.8% of total elapsed gc_alloc_block_sync: 0 whitehole_spin: 0 gen[0].sync: 0 gen[1].sync: 0 real 0m51.375s user 0m50.864s sys 0m0.456s }}} By contrast, with optimizations turned off: {{{ $ time ghc -O0 --make -isrc/library src/library/FP/API/Types.hs +RTS -s [3 of 3] Compiling FP.API.Types ( src/library/FP/API/Types.hs, src/library/FP/API/Types.o ) 12,767,593,936 bytes allocated in the heap 1,078,202,664 bytes copied during GC 179,551,768 bytes maximum residency (13 sample(s)) 6,684,544 bytes maximum slop 439 MB total memory in use (0 MB lost due to fragmentation) Tot time (elapsed) Avg pause Max pause Gen 0 623 colls, 0 par 1.24s 1.24s 0.0020s 0.0651s Gen 1 13 colls, 0 par 0.76s 0.76s 0.0586s 0.2397s TASKS: 4 (1 bound, 3 peak workers (3 total), using -N1) SPARKS: 0 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled) INIT time 0.00s ( 0.00s elapsed) MUT time 8.28s ( 9.23s elapsed) GC time 2.00s ( 2.00s elapsed) EXIT time 0.05s ( 0.05s elapsed) Total time 10.34s ( 11.28s elapsed) Alloc rate 1,542,443,897 bytes per MUT second Productivity 80.6% of total user, 73.9% of total elapsed gc_alloc_block_sync: 0 whitehole_spin: 0 gen[0].sync: 0 gen[1].sync: 0 real 0m11.304s user 0m11.108s sys 0m0.172s }}} and -O1: {{{ $ time ghc -O1 --make -isrc/library src/library/FP/API/Types.hs +RTS -s [3 of 3] Compiling FP.API.Types ( src/library/FP/API/Types.hs, src/library/FP/API/Types.o ) 45,550,443,664 bytes allocated in the heap 5,721,700,512 bytes copied during GC 358,036,456 bytes maximum residency (21 sample(s)) 9,167,176 bytes maximum slop 906 MB total memory in use (0 MB lost due to fragmentation) Tot time (elapsed) Avg pause Max pause Gen 0 1642 colls, 0 par 7.70s 7.70s 0.0047s 0.1031s Gen 1 21 colls, 0 par 3.68s 3.69s 0.1756s 0.4968s TASKS: 4 (1 bound, 3 peak workers (3 total), using -N1) SPARKS: 0 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled) INIT time 0.00s ( 0.00s elapsed) MUT time 30.21s ( 33.09s elapsed) GC time 11.38s ( 11.39s elapsed) EXIT time 0.07s ( 0.07s elapsed) Total time 41.68s ( 44.55s elapsed) Alloc rate 1,507,571,106 bytes per MUT second Productivity 72.7% of total user, 68.0% of total elapsed gc_alloc_block_sync: 0 whitehole_spin: 0 gen[0].sync: 0 gen[1].sync: 0 real 0m44.577s user 0m44.164s sys 0m0.368s }}} Note that this module is essentially just 900 lines of data declarations, with a large number of derived classes, and a few manually written helper functions and instances. This high memory usage hasn't just resulted in user complaints: automated Jenkins and Travis jobs will often times fail without optimizations disabled, which can be problematic for proper testing and production code. In the case of fpco-api, I've worked around this by adding `-O0` to the cabal file, but it would be much nicer to not have to do that. For some other examples of complaints along these lines: * https://groups.google.com/d/msg/yesodweb/MX1bnOFu8Hc/VLQXhnvpIkMJ * https://groups.google.com/d/msg/yesodweb/XPWixNjuOnM/FN26bmkudgwJ I believe there are a few other threads discussing this, if it would be helpful. I did my testing on GHC 7.8.3 64-bit, Ubuntu 12.04. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9669> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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