May 2015 - ghc-tickets

[GHC] #10390: Constraint order must match with RankNTypes
by GHC 20 Oct '15

20 Oct '15

#10390: Constraint order must match with RankNTypes -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 7.8.4 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- In the following code, GHC will not compile unless the constraints on the higher-rank function have the same *order* as the method declaration in `ApPair`. {{{#!hs {-# LANGUAGE RankNTypes #-} class ApPair r where apPair :: (forall a . (ApPair a, Num a) => Maybe a) -> Maybe r instance (ApPair a, ApPair b) => ApPair (a,b) where apPair = apPair' apPair' :: (ApPair b, ApPair c) => (forall a . (ApPair a, Num a) => Maybe a) -> Maybe (b,c) apPair' f = let (Just a) = apPair f (Just b) = apPair f in Just $ (a, b) }}} That is, the following does *not* compile: {{{#!hs apPair' :: (ApPair b, ApPair c) => (forall a . (Num a, ApPair a) => Maybe a) -> Maybe (b,c) apPair' f = let (Just a) = apPair f (Just b) = apPair f in Just $ (a, b) }}} GHC probably shouldn't care about lexical matching when checking constraints. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10390> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8648: Initialization of C statics broken in threaded runtime
by GHC 20 Oct '15

20 Oct '15

#8648: Initialization of C statics broken in threaded runtime ------------------------------------+------------------------------------- Reporter: edsko | Owner: simonmar Type: bug | 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: | ------------------------------------+------------------------------------- Consider a tiny package `static-value`, consisting of one Haskell file {{{ foreign import ccall unsafe "returnStaticValue" c_returnStaticValue :: IO CInt printStaticValue :: IO () printStaticValue = print =<< c_returnStaticValue }}} and one corresponding C file {{{ static int theStaticValue = 0; int returnStaticValue() { // Modify the static so the C compiler doesn't optimize it away return theStaticValue++; } }}} (test case is attached). If we call `printStaticValue` using the GHC API: {{{ runGhc (Just libdir) $ do flags0 <- getSessionDynFlags void $ setSessionDynFlags flags0 { hscTarget = HscInterpreted , ghcLink = LinkInMemory , ghcMode = CompManager } setContext $ [ IIDecl $ simpleImportDecl $ mkModuleName "StaticValue" ] _ <- runStmt "StaticValue.printStaticValue" RunToCompletion }}} then we see "0", as expected. However, if we compile this code using the threaded runtime, and we wrap the above code in a call to either `forkIO` or `forkOS`, then we see a different value printed (-907777, whatever that value is). Some notes: * I have been unable to reproduce this bug without using GHC as API; in particular, calling `printStaticValue` directly, wrapped in `forkIO` or `forkOS` or not, always works as expected. * If I change the initialization value of `staticValue` from 0 to anything else (say, 1234), we always get the right answer, never the uninitialized value. Presumably this is because non-zero values require some explicit code to be run (and it does get run), while a zero value gets initialized differently (and apparently, that's where the bug is). * I have reproduced this bug in both ghc 7.4 and 7.7.20131227. This ticket is the result of tracking down a problem with calling `createProcess` from within the GHC API, which would cause the parent process to stall. As it turns out, `runProcess.c` (from the `process` library) declares a `static long max_fd = 0`, and in `runInteractiveProcess` checks for this value to be 0, and if it is, does a syscall to figure out what the maximum FD is. But since this static does not get initialized properly (the bug reported in this ticket), it gets left at its (random? but always the same) value (281474975802879), so that the child process proceeds to close rather too many file descriptors (if close_fds was set to True) and the parent stalls. Indeed, changing the initialization to `static long max_fd = -1` (and adjusting the later check for zero accordingly) fixes this (so this is a viable workaround in `process` if we cannot track down the bug in GHC). -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8648> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9360: GHCi: Behave nicely on `-e`, like `ghc` and other programs
by GHC 19 Oct '15

19 Oct '15

#9360: GHCi: Behave nicely on `-e`, like `ghc` and other programs -------------------------------------+------------------------------------- Reporter: mcandre | 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: -------------------------------------+------------------------------------- Like many programming language environments, GHC offers a handy `-e` option for evaluating an expression, then returning to the shell. {{{ $ ghc -e '2 + 2' 4 }}} One would expect the interpreter, GHCi, to offer a similar flag, but it surprisingly rejects it. {{{ ghci -e '2 + 2' ghc: on the commandline: cannot use `--interactive' with `-e' Usage: For basic information, try the `--help' option. }}} I think this behavior is quite unintuitive--when I pass `-e <exp>` to ghci, or pass `--interactive -e <exp>` to ghc, I expect the expression to be evaluated as the leading expression in an interactive interpreter session. Could we please tweak ghc like this to make it slightly more intuitive when these flags are used together? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9360> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10216: Allow arr ∧ (first ∨ (***)) as minimal definition of Arrow instance
by GHC 13 Oct '15

13 Oct '15

#10216: Allow arr ∧ (first ∨ (***)) as minimal definition of Arrow instance -------------------------------------+------------------------------------- Reporter: strake888 | Owner: ekmett Type: bug | Status: new Priority: normal | Milestone: Component: Core | Version: 7.10.1 Libraries | Operating System: Unknown/Multiple Keywords: | Type of failure: None/Unknown Architecture: | Blocked By: Unknown/Multiple | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10216> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9319: nofib-analyze doesn’t provide per-benchmark compile time/alloc numbers
by GHC 08 Oct '15

08 Oct '15

#9319: nofib-analyze doesn’t provide per-benchmark compile time/alloc numbers -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: NoFib benchmark | Version: 7.8.2 suite | Differential Revisions: Keywords: | Architecture: Operating System: Unknown/Multiple | Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------- The compile time and allocation numbers calculated by nofib are only available per module. While this is useful to find the cause of problems, it would be nice if these numbers were also accumulated for each benchmark, for easy of general analysis and uniformity, especially with automated monitoring of benchmark numbers. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9319> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10153: GHC mode for converting files to explicit layout
by GHC 08 Oct '15

08 Oct '15

#10153: GHC mode for converting files to explicit layout -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: feature | Status: new request | Milestone: Priority: low | Version: 7.11 Component: Compiler | Operating System: Unknown/Multiple (Parser) | Type of failure: None/Unknown Keywords: | Blocked By: Architecture: | Related Tickets: Unknown/Multiple | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- One area where GHC is a little less friendly to newcomers is its Byzantine whitespace layout rules. Once you've gotten used to using GHC you can usually get what you want by abiding certain style, but before then it might be useful to see how GHC is inserting semicolons to your document. I don't think this would be too hard to do. The idea is to get the token stream and look for virtual curlies with no location. Once you have a list of them, you can go over the original buffer again and insert a brace or semicolon whenever you see a token like this. Might be a good beginner task. This function would also be useful for debugging layout, since knowing if semicolons are being inserted is useful. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10153> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9058: System.IO.openTempFile does not scale
by GHC 07 Oct '15

07 Oct '15

#9058: System.IO.openTempFile does not scale ------------------------------------+------------------------------------- Reporter: slyfox | Owner: Type: bug | 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: | ------------------------------------+------------------------------------- In search of a bug in darcs http://bugs.darcs.net/issue2364 i've notice very bad property of openTempFile: it's pattern is very predictable and has O(n^2) of already created temp files. Predictability allows very fun bugs survive in buggy programs, like: {{{ thread1: (fn, fh) <- openTempFile "." "hello" renameFile fn "something" -- some time after when (some_rare_buggy_condition) $ -- oops, reused temp name, but too late, other thread killed it writeFileFile fn thread2: (fn, fh) <- openTempFile "." "hello" workWithFn fn -- nobody should touch it, right? }}} It's _very_ hard to debug data corruption when all temp files are named "foo${pid}" and sometimes "foo${pid+1}". And more serious bug: the more threads you have trying to create similar temps performance drops significantly: Attached program shows the following numbers: {{{ $ time ./bench-temps same 2000 real 0m2.795s user 0m1.516s sys 0m1.190s $ time ./bench-temps diff 2000 real 0m0.161s user 0m0.043s sys 0m0.115s }}} It's O(N^2) growing open() storm. https://github.com/ghc/ghc/blob/master/libraries/base/System/IO.hs#L465 {{{ FileExists -> findTempName (x + 1) }}} This is the source of the problem. I'd suggest always using random name for it. For portability reasons I suggest adding at least insecure random '''rand()''' value from C library. That way we will succeed in opening temp file at the first attempt. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9058> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8364: equip GHC with an accurate internal model of floating point
by GHC 07 Oct '15

07 Oct '15

#8364: equip GHC with an accurate internal model of floating point ----------------------------------------------+---------------------------- Reporter: carter | Owner: Type: feature request | Status: new Priority: high | Milestone: 7.10.1 Component: Compiler | Version: 7.6.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Project (more than a week) | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: ----------------------------------------------+---------------------------- currently there really isn't a systematic / portable story for doing compile time reduction / evaluation for floating point numbers. there are some folks in the larger haskell community who are working on high precision soft float implementations / modelling. Once that work becomes available, might be worth exploring having something like that for doing controlled high precision evaluation of intermediate expressions in ghc proper. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8364> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8335: Create more specialized entries to GC
by GHC 06 Oct '15

06 Oct '15

#8335: Create more specialized entries to GC ------------------------------------+------------------------------------- Reporter: jstolarek | Owner: Type: task | 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 this Cmm code (taken from #8326): {{{ {offset cFd: // stack check if ((Sp + -16) < SpLim) goto cFr; else goto cFs; cFr: // not enough place on the stack, call GC R2 = R2; R1 = A.f_closure; call (stg_gc_fun)(R2, R1) args: 8, res: 0, upd: 8; cFs: // scrutinize (x ># 0#) _sEU::I64 = R2; _sEV::I64 = %MO_S_Gt_W64(R2, 0); if (_sEV::I64 != 1) goto cFg; else goto cFo; cFg: // False branch Hp = Hp + 16; if (Hp > HpLim) goto cFy; else goto cFx; cFy: // not enough heap, call GC HpAlloc = 16; I64[Sp - 16] = cFf; R1 = _sEV::I64; I64[Sp - 8] = _sEU::I64; Sp = Sp - 16; call stg_gc_unbx_r1(R1) returns to cFf, args: 8, res: 8, upd: 8; cFf: // re-do the False branch _sEU::I64 = I64[Sp + 8]; Sp = Sp + 16; _sEV::I64 = R1; goto cFg; cFx: // RHS of False branch I64[Hp - 8] = GHC.Types.I#_con_info; I64[Hp] = -_sEU::I64; R1 = Hp - 7; call (P64[Sp])(R1) args: 8, res: 0, upd: 8; cFo: // True branch Hp = Hp + 16; if (Hp > HpLim) goto cFv; else goto cFu; cFv: // not enough heap, call GC HpAlloc = 16; I64[Sp - 16] = cFn; R1 = _sEV::I64; I64[Sp - 8] = _sEU::I64; Sp = Sp - 16; call stg_gc_unbx_r1(R1) returns to cFn, args: 8, res: 8, upd: 8; cFn: // re-do the True branch _sEU::I64 = I64[Sp + 8]; Sp = Sp + 16; _sEV::I64 = R1; goto cFo; cFu: // RHS of True branch I64[Hp - 8] = GHC.Types.I#_con_info; I64[Hp] = _sEU::I64; R1 = Hp - 7; call (P64[Sp])(R1) args: 8, res: 0, upd: 8; } }}} We perform two different calls to garbage collection here. Calling GC after making initial stack check obviously can't use any stack, so we pass parameter to GC in regsiters: {{{ R2 = R2; R1 = A.f_closure; call (stg_gc_fun)(R2, R1) args: 8, res: 0, upd: 8; }}} But in the calls that are made later on inside the function we do put some parameters on the stack, thus increasing stack usage: {{{ cFv: // not enough heap, call GC HpAlloc = 16; I64[Sp - 16] = cFn; R1 = _sEV::I64; I64[Sp - 8] = _sEU::I64; Sp = Sp - 16; call stg_gc_unbx_r1(R1) returns to cFn, args: 8, res: 8, upd: 8; }}} If we had more specialized entries to GC that would allow us to pass parameters in registers (or in some fixed memory location associated with TCO?) then this function would not require any stack at all. Note that there is a separate issue here - `_sEV` variable is in fact dead and should be eliminated (see #8327). -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8335> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8920: Alternative GADT syntax
by GHC 05 Oct '15

05 Oct '15

#8920: Alternative GADT syntax ----------------------------------------------+---------------------------- Reporter: guest | Owner: Type: feature request | Status: new Priority: normal | Milestone: 7.10.1 Component: Compiler (Parser) | Version: Keywords: gadts | 7.8.1-rc2 Architecture: Unknown/Multiple | Operating System: Difficulty: Moderate (less than a day) | Unknown/Multiple Blocked By: | Type of failure: Other Related Tickets: | Test Case: | Blocking: ----------------------------------------------+---------------------------- An alternative syntax for GADTs Instead of {{{ data Term x where K :: Term (a -> b -> a) S :: Term ((a -> b -> c) -> (a -> b) -> a -> c) Const :: a -> Term a (:@) :: Term (a -> b) -> (Term a) -> Term b }}} You can write {{{ data Term x = K :: Term (a -> b -> a) | S :: Term ((a -> b -> c) -> (a -> b) -> a -> c) | Const a :: Term a | (:@) (Term (a -> b)) (Term a) :: Term b }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8920> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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