April 2017 - ghc-tickets

[GHC] #12469: Memory fence on writeIORef missing on ARM
by GHC 06 Oct '18

06 Oct '18

#12469: Memory fence on writeIORef missing on ARM ------------------------------------+------------------------------------- Reporter: rrnewton | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Keywords: memory model | Operating System: Unknown/Multiple Architecture: arm | Type of failure: None/Unknown Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: ------------------------------------+------------------------------------- The memory model question has been debated now and again. his thread from ten years back (https://mail.haskell.org/pipermail/haskell-prime/2006-April/001237.html) lays out the basic situation with thunk update, writeIORef, and memory fences. But we recently began experimenting with GHC on ARM platforms, and it seems to lack a memory fence that the participants in the cited thread expect it to have. Here's an attempt to construct a program which writes fields of a data structure, and then writes the pointer to that structure to an IORef, without the proper fence inbetween: ```Haskell import Data.IORef import Control.Concurrent data Foo = Foo Int deriving Show {-# NOINLINE mkfoo #-} mkfoo x = Foo x {-# NOINLINE dowrite #-} dowrite r n = writeIORef r $! mkfoo n main = do r <- newIORef (Foo 3) forkIO (dowrite r 4) x <- readIORef r print x ``` Here's the relevant bits of the CMM that results when compiled on an ARM 64 machine: ```C mkfoo_rn1_entry() // [] { [] } {offset c40i: P64[MainCapability+872] = P64[MainCapability+872] + 16; if (P64[MainCapability+872] > I64[MainCapability+880]) goto c40m; else goto c40l; c40m: I64[MainCapability+928] = 16; P64[MainCapability+24] = mkfoo_rn1_closure; call (I64[MainCapability+16])(R1) args: 16, res: 0, upd: 8; c40l: I64[P64[MainCapability+872] - 8] = Foo_con_info; P64[P64[MainCapability+872]] = P64[I64[MainCapability+856]]; P64[MainCapability+24] = P64[MainCapability+872] - 7; I64[MainCapability+856] = I64[MainCapability+856] + 8; call (I64[P64[I64[MainCapability+856]]])(R1) args: 8, res: 0, upd: 8; } } ``` ```C dowrite_entry() // [] { [] } {offset c44j: call a_r3Dy_entry() args: 24, res: 0, upd: 8; } } a_r3Dy_entry() // [R1] { [] } {offset c41D: if (I64[MainCapability+856] - 16 < I64[MainCapability+864]) goto c41H; else goto c41I; c41H: P64[MainCapability+24] = a_r3Dy_closure; call (I64[MainCapability+16])(R1) args: 24, res: 0, upd: 8; c41I: I64[I64[MainCapability+856] - 8] = block_c41B_info; P64[I64[MainCapability+856] - 16] = P64[I64[MainCapability+856] + 8]; I64[MainCapability+856] = I64[MainCapability+856] - 16; call mkfoo_rn1_entry() args: 16, res: 8, upd: 8; } } block_c41B_entry() // [R1] { [] } {offset c41B: _s3Ep::P64 = P64[I64[MainCapability+856] + 8]; I64[I64[MainCapability+856] + 8] = block_c41G_info; _s3Es::P64 = P64[MainCapability+24]; P64[MainCapability+24] = _s3Ep::P64; P64[I64[MainCapability+856] + 16] = _s3Es::P64; I64[MainCapability+856] = I64[MainCapability+856] + 8; if (P64[MainCapability+24] & 7 != 0) goto u41S; else goto c41K; u41S: call block_c41G_entry(R1) args: 0, res: 0, upd: 0; c41K: call (I64[I64[P64[MainCapability+24]]])(R1) args: 8, res: 8, upd: 8; } } block_c41G_entry() // [R1] { [] } {offset c41G: _s3Ev::P64 = P64[P64[MainCapability+24] + 7]; P64[_s3Ev::P64 + 8] = P64[I64[MainCapability+856] + 8]; call "ccall" arg hints: [PtrHint, PtrHint] result hints: [] dirty_MUT_VAR(MainCapability+24, _s3Ev::P64); P64[MainCapability+24] = ()_closure+1; I64[MainCapability+856] = I64[MainCapability+856] + 16; call (I64[P64[I64[MainCapability+856]]])(R1) args: 8, res: 0, upd: 8; } } ``` The fence should happen before the write of the pointer into the IORef. I can't find the fence, and can't find a codepath in the compiler that would insert it (i.e. with MO_WriteBarrier). `dirty_MUT_VAR` is actually too late to perform the fence, but it doesn't either: ```C void dirty_MUT_VAR(StgRegTable *reg, StgClosure *p) { Capability *cap = regTableToCapability(reg); if (p->header.info == &stg_MUT_VAR_CLEAN_info) { p->header.info = &stg_MUT_VAR_DIRTY_info; recordClosureMutated(cap,p); } } ``` (Neither does `recordClosureMutated`.) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12469> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13477: Turn cIntegerLibraryType into a dynflag
by GHC 05 Oct '18

05 Oct '18

#13477: Turn cIntegerLibraryType into a dynflag -------------------------------------+------------------------------------- Reporter: nomeata | Owner: (none) Type: task | Status: new Priority: low | Milestone: Component: GHC API | Version: 8.1 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: -------------------------------------+------------------------------------- Currently, at configure time we tell GHC whether integers are built using `integer-gmp` or `integer-simple`, and this gets baked in via the global constant `cIntegerLibraryType`. This is all fine for normal use of GHC (the program), but users of ghc (the library) might want to have more leeway here. A good solution seems to be turn this into a dynflag setting, so that users of the GHC API can adjust that as fit. From a cursory glance at the code, this does not seem to be a big problem. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13477> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8708: Kind annotation in tuple not parsed
by GHC 04 Oct '18

04 Oct '18

#8708: Kind annotation in tuple not parsed -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Parser) | 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: {{{ {-# LANGUAGE KindSignatures #-} foo :: (Int, Int :: *) foo = undefined }}} HEAD reports {{{ /Users/rae/temp/Bug.hs:5:18: parse error on input ‛::’ }}} Changing the line to {{{ foo :: (Int, (Int :: *)) }}} fixes the problem. Note the extra parentheses. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8708> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11622: Annotating types in type familiy equations without parentheses
by GHC 04 Oct '18

04 Oct '18

#11622: Annotating types in type familiy equations without parentheses -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 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: -------------------------------------+------------------------------------- When annotating types in right-hand side of type family equations parentheses are required: {{{#!hs type family F a where F _ = (Int :: Type) }}} instead of {{{#!hs type family F a where F _ = Int :: Type }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11622> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11251: isInstance does not work on Typeable with base-4.8 anymore
by GHC 04 Oct '18

04 Oct '18

#11251: isInstance does not work on Typeable with base-4.8 anymore -------------------------------------+------------------------------------- Reporter: songzh | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: | Version: 7.10.1 libraries/base | Keywords: Typeable, | Operating System: Unknown/Multiple isInstance | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!hs > :set -XTemplateHaskell > :m +Data.Tyepable > :m +Language.Haskell.TH --- With GHC 7.8 or earlier versions of GHC (I only tried with GHC 7.6, but my friends tried on 7.8 and earlier versoin) > $(isInstance ''Typeable [ConT ''Char] >>= stringE.show) "True" --- With GHC 7.10.1 > $(isInstance ''Typeable [ConT ''Char] >>= stringE.show) "False" }}} I have noticed that standalone derivings of Typeable instances in Data.Typeable are disappared magically in base-4.8 of GHC 7.10,also one cannot query the instances of it by using `:i Typeable` in GHCi, however, obviously, I can use `typeOf` function in base-4.8 to get the TypeRep of Char. The problem is that `isInstance` in template-haskell library doesn't know Char and many other types are instances of Typeable anymore. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11251> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13365: Kind-inference for poly-kinded GADTs
by GHC 04 Oct '18

04 Oct '18

#13365: Kind-inference for poly-kinded GADTs -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | 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: -------------------------------------+------------------------------------- With `-XPolyKinds`, GADTs require kind signatures where they should be inferred. Moreover, the error when these kind sigs are omitted is baffling. {{{ {-# LANGUAGE PolyKinds, GADTs #-} import GHC.TypeLits data C (x :: k) a where C1 :: (KnownNat x) => a -> Foo x a C2 :: a -> Foo Int a }}} {{{ error: • Expected kind ‘k’, but ‘x1’ has kind ‘Nat’ • In the first argument of ‘Foo’, namely ‘x’ In the type ‘Foo x a’ In the definition of data constructor ‘C1’ }}} From this error, I would never expect that putting a kind signature on `a` would help here. But a signature shouldn't be required at all: it's clear from the GADT constructors that `a :: *`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13365> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12556: `stimes` adds extra power to Semigroup
by GHC 03 Oct '18

03 Oct '18

#12556: `stimes` adds extra power to Semigroup -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: task | Status: new Priority: normal | Milestone: Component: | Version: 8.0.1 libraries/base | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect result Unknown/Multiple | at runtime Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{ ghci> stimes 0 (undefined :: [_]) [] }}} ghci> stimes 0 undefined () }}} makes it seem like `stimes 0` has some knowledge about `mempty` from `Monoid` but it has the following type {{{#!hs stimes 0 :: Semigroup a => a -> a }}} Desired behaviour? Given that type (assuming it only has the power of `<>`) I would have assumed this behaviour {{{ ghci> data L a = N | C a (L a) deriving Show ghci> instance Semigroup (L a) where (<>) = undefined ghci| ghci> stimes 0 (undefined :: L _) *** Exception: stimes: positive multiplier expected }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12556> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9136: Constant folding in Core could be better
by GHC 02 Oct '18

02 Oct '18

#9136: Constant folding in Core could be better ------------------------------+-------------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime performance bug Unknown/Multiple | Test Case: Difficulty: Unknown | Blocking: Blocked By: | Related Tickets: | ------------------------------+-------------------------------------------- Constant folding in Core is still a bit feeble. For example, when staring at the code for `nofib/shootout/k-nucleotide` I saw this expression: {{{ (x +# 8) -# 1 }}} which would be relatively easy to optimise. The difficulty is knowing where to stop. What about `(8 +# x) -# 1`? Anyway, this ticket is to record the issue. Currently constant folding is done by the built-in RULES in `compiler/prelude/PrelRules`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9136> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10791: Add a --disable-large-memory-space configure option
by GHC 02 Oct '18

02 Oct '18

#10791: Add a --disable-large-memory-space configure option -------------------------------------+------------------------------------- Reporter: erikd | Owner: erikd Type: feature | Status: new request | Priority: normal | Milestone: 7.12.1 Component: Build | Version: 7.11 System | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- GHC recently started using large (1TB) address space on 64 bit systems. This feature is enabled by default but does not work on arm64-linux or amd64-solaris. Even just for testing, it would be nice to be able to disable this. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10791> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10793: Incorrect blocked on MVar detection
by GHC 02 Oct '18

02 Oct '18

#10793: Incorrect blocked on MVar detection -------------------------------------+------------------------------------- Reporter: | Owner: simonmar NeilMitchell | Type: bug | Status: new Priority: normal | Milestone: Component: Runtime | Version: 7.10.1 System | Keywords: | Operating System: Windows Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Given the code: {{{#!hs import Control.Concurrent main = do v <- newEmptyMVar forkFinally (putStrLn $ let x = x in x) (putMVar v) print =<< takeMVar v }}} The spawned thread should raise a {{{NonTermination}}} exception (assuming idle GC), then put it in the MVar {{{v}}}, which should cause the main thread to abort printing {{{Left <<loop>>}}}. However, in reality, the main thread aborts with {{{Exception: thread blocked indefinitely in an MVar operation}}}. My debugging shows that the spawned thread raises {{{NonTermination}}} and at the same time the main thread raises {{{BlockedIndefinitelyOnMVar}}}. Of course, the main thread is not really blocked on the exception, and after the finally bit runs, the MVar is filled. Adding tracing shows that the {{{putMVar}}} is called. Hypothesis is that the GC notices it can raise the non-termination exception, and then opportunistically excludes that thread from further GC scanning, and detects the MVar is unreferenced, raising the subsequent exception. Of course, given you can catch the exception, that isn't safe. Roman Leshchinskiy worries that in the wrong circumstances the MVar might really be being GC'd and thus could corrupt memory. This issue was detected while debugging an error in the Shake build system for GHC itself. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10793> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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