October 2018 - ghc-tickets

[GHC] #15562: `-XStrict -XNoStrict` is not neutral
by GHC 14 Dec '18

14 Dec '18

#15562: `-XStrict -XNoStrict` is not neutral -------------------------------------+------------------------------------- Reporter: hvr | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 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 assumed we already had a ticket about this, but it appears we have not. I'm not saying this is a necessarily a bug in the implementation; but we should make sure this is properly documented in the user's guide, and ideally justify why it's done this way rather than the alternatives: As described in StrictPragma, > `Strict` implies `StrictData` Currently however the inverse case does not hold, i.e. `NoStrict` does *not* imply `NoStrictData` This has the surprising property (assuming these were left-most flags on the CLI) that - `-XStrict -XNoStrict` == `-XStrictData` - `-XStrictData -XNoStrict` == `-XStrictData` However, if `-XNoStrict` was to naively imply `-XNoStrictData`, we'd have the properties - `-XStrict -XNoStrict` == *id* - `-XStrictData -XNoStrict` == *id* This might be a bit less confusing; another variant would be - `-XStrict -XNoStrict` == *id* - `-XStrictData -XNoStrict` == `-XStrictData` Btw, I'm not sure what the following means: - `-XStrict -XNoStrictData` == ? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15562> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 6

[GHC] #15333: Weird cachegrind results for binary-trees
by GHC 14 Dec '18

14 Dec '18

#15333: Weird cachegrind results for binary-trees -------------------------------------+------------------------------------- Reporter: sgraf | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Research needed Component: NoFib | Version: 8.5 benchmark suite | 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 currently investigating an alleged regression in my branch of the late lambda lift and hit a confusing data point. Note that I'm very much relying on cachegrinds counted instructions/memory accesses for my findings. Check out the most recent version of `nofib` and run the following script: {{{#!sh #! /usr/bin/env zsh sed -i 's/import Debug.Trace//g' Main.hs # Make the following line idempotent echo "import Debug.Trace" | cat - Main.hs > Main.tmp && mv Main.tmp Main.hs # add the import for trace sed -i 's/trace "" \$ bit/bit/g' Main.hs # strip `trace $ ` prefixes in the call to `bit` ghc -O2 -XBangPatterns Main.hs -o bt1 sed -i 's/bit/trace "" $ bit/g' Main.hs # prepend `trace $ ` to the call to `bit` ghc -O2 -XBangPatterns Main.hs -o bt2 valgrind --tool=cachegrind ./bt1 12 2>&1 > /dev/null # without trace valgrind --tool=cachegrind ./bt2 12 2>&1 > /dev/null # with trace }}} This will compile two versions of `binary-trees`, one with an extra `trace "" $` call before the only call to the `bit` function. One would expect the version with the `trace` call (`bt1`) to allocate more than the version without (`bt2`). Indeed, the output of `+RTS -s` suggests that: {{{ $ ./bt1 12 +RTS -s ... 43,107,560 bytes allocated in the heap ... $ ./bt2 12 +RTS -s ... 43,116,888 bytes allocated in the heap ... }}} That's fine. A few benchmark runs by hand also suggested the tracing version is a little slower (probably due to IO). Compare that to the output of the above cachegrind calls: {{{ $ valgrind --tool=cachegrind ./bt1 12 > /dev/null ... I refs: 118,697,583 ... D refs: 43,475,212 ... $ valgrind --tool=cachegrind ./bt2 12 > /dev/null ... I refs: 116,340,710 ... D refs: 42,523,369 ... }}} It's the other way round here! How's that possible? Even if this isn't strictly a bug in GHC or NoFib, it's relevant nonetheless, as our benchmark infrastructure currently relies on instruction counts. I couldn't reproduce this by writing my own no-op `trace _ a = a; {-# NOINLINE trace #-}`, btw. I checked this on GHC 8.2.2 and a semi-recent HEAD commit (bb539cfe335eeec7989332b859b1f3162c5e105a). -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15333> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 8

[GHC] #14473: Implement Underscores in Numeric Literals Proposal (NumericUnderscores extension)
by GHC 14 Dec '18

14 Dec '18

#14473: Implement Underscores in Numeric Literals Proposal (NumericUnderscores extension) -------------------------------------+------------------------------------- Reporter: takenobu | Owner: (none) Type: task | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: #13126 #9224 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Implement Underscores in Numeric Literals Proposal. GHC supports various numeric literals such as decimal, octal, hexadecimal, binary, and floating point numbers. However, large numeric literals are hard to read. This proposal improves the readability, quality, expressiveness of numeric literals. This proposal allows underscores to numeric literals when the `NumericUnderscores` language extension is enabled. Underscores (`_`) in numeric literals are simply ignored. The specification of the feature is available here:\\ https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0009 -numeric-underscores.rst For a discussion:\\ https://github.com/ghc-proposals/ghc-proposals/pull/76 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14473> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 12

[GHC] #15718: Build panic on ghc-8.6.1 under FreeBSD when using -fllvm
by GHC 13 Dec '18

13 Dec '18

#15718: Build panic on ghc-8.6.1 under FreeBSD when using -fllvm -------------------------------------+------------------------------------- Reporter: gwright | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.1 Keywords: | Operating System: FreeBSD Architecture: x86_64 | Type of failure: Compile-time (amd64) | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This looks like a straightforward oversight. I am building my pure haskell LU solver under FreeBSD. I use the -fllvm compiler option because I need extra numerical performance. When I build, I get (this is the output from GitLab CI, which is in turn running 'stack build') {{{ Running with gitlab-runner 11.2.0 (35e8515d) on stacker.18clay.com a2111f63 Using Shell executor... Running on stacker.18clay.com... Fetching changes... Removing .stack-work/ HEAD is now at f8fe3b4 Use llvm60 on FreeBSD. From https://gitlab.18clay.com/software/luSolve f8fe3b4..c993a26 streamlined -> origin/streamlined Checking out c993a262 as streamlined... Skipping Git submodules setup $ stack build --pedantic luSolve-0.5: configure (lib) Configuring luSolve-0.5... luSolve-0.5: build (lib) Preprocessing library for luSolve-0.5.. Building library for luSolve-0.5.. [1 of 1] Compiling Numeric.LinearAlgebra.LUSolve ( src/Numeric/LinearAlgebra/LUSolve.hs, .stack- work/dist/x86_64-freebsd/Cabal-2.4.0.1/build/Numeric/LinearAlgebra/LUSolve.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.6.1 for x86_64-portbld-freebsd): Failed to lookup the datalayout for x86_64-unknown-freebsd; available targets: ["i386-unknown-windows","i686-unknown-windows","x86_64 -unknown-windows","arm-unknown-linux-gnueabihf","armv6-unknown-linux- gnueabihf","armv6l-unknown-linux-gnueabihf","armv7-unknown-linux-gnueabihf ","armv7a-unknown-linux-gnueabi","armv7l-unknown-linux-gnueabihf","aarch64 -unknown-linux-gnu","aarch64-unknown-linux","i386-unknown-linux-gnu","i386 -unknown-linux","x86_64-unknown-linux-gnu","x86_64-unknown-linux","armv7 -unknown-linux-androideabi","aarch64-unknown-linux-android","powerpc64le- unknown-linux","amd64-portbld-freebsd","arm-unknown-nto-qnx-eabi","i386 -apple-darwin","x86_64-apple-darwin","armv7-apple-ios","aarch64-apple- ios","i386-apple-ios","x86_64-apple-ios","aarch64-unknown-freebsd","armv6 -unknown-freebsd-gnueabihf","armv7-unknown-freebsd-gnueabihf"] CallStack (from HasCallStack): error, called at compiler/llvmGen/LlvmCodeGen.hs:96:24 in ghc:LlvmCodeGen Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug -- While building custom Setup.hs for package luSolve-0.5 using: /home/gitlab-runner/.stack/setup-exe-cache/x86_64-freebsd/Cabal- simple_mPHDZzAJ_2.4.0.1_ghc-8.6.1 --builddir=.stack- work/dist/x86_64-freebsd/Cabal-2.4.0.1 build lib:luSolve --ghc-options " -ddump-hi -ddump-to-file" Process exited with code: ExitFailure 1 ERROR: Job failed: exit status 1 }}} The bug seems simple. The compiler is looking for the datalayout for x86_64-unknown-freebsd; but what is available is amd64-portbld-freebsd. Is this just a simple naming error? The stack resolver was the cutting- edge nightly-2018-10-6; if it is a problem in their packaging rather than ghc itself I will file a report with them (though ghc did panic here, and as the breath ebbed from its ASTs and light faded from the code generator, it pleaded that a bug report be filed). I've attached the .cabal file in case it helps. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15718> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 9

[GHC] #14345: Warning when linking with C++ code
by GHC 13 Dec '18

13 Dec '18

#14345: Warning when linking with C++ code -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | error/warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When linking with a C++ file, GHC 8.2.1.20170928 produces a warning because it runs gcc with 'std=gnu99', even when I pass 'std=c++11' to cc- options (when using stack, I see that the command is something like 'gcc std=gnu99 ... std=c++11', so it isn't *ignoring* my input, just appending it.) I believe this warning is new as of 8.2. The attached example demonstrates the problem: {{{ > cabal configure > cabal build cc1plus: warning: command line option ‘-std=gnu99’ is valid for C/ObjC but not for C++ cc1plus: warning: command line option ‘-std=gnu99’ is valid for C/ObjC but not for C++ }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14345> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 3

[GHC] #15080: List Operators Sorted by Precedence in GHCi
by GHC 13 Dec '18

13 Dec '18

#15080: List Operators Sorted by Precedence in GHCi -------------------------------------+------------------------------------- Reporter: sjs | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: 8.6.1 Component: GHCi | Version: 8.2.2 Keywords: operator | Operating System: Unknown/Multiple precedence | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- In a large project, it can be difficult to keep track of the precedence of operators. It would be a nice quality-of-life improvement if there were a GHCi command to list the current operators in scope by precedence. It could list them in a table like this: {{{ :operators | prec | nonassoc | leftassoc | rightassoc | |------+----------+-----------+------------| | 9 | !! | | . | | 8 | | | ^ ^^ ** | | 7 | * / | | | | 6 | + - | | | | 5 | | | : ++ | | 4 | | == | | }}} Perhaps it could list all infix operators, as well as functions that have a fixity declaration? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15080> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 6

[GHC] #15774: SIGKILL only reports backtrace for one capability
by GHC 13 Dec '18

13 Dec '18

#15774: SIGKILL only reports backtrace for one capability -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Runtime | Version: 8.6.1 System | 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: -------------------------------------+------------------------------------- SIGKILL is intended to mirror the same signal's functionality provided by the JVM. That is, provide a snapshot of a processes' state in the form of backtraces on stderr. However, GHC's implementation currently only prints a backtrace for a single, arbitrary thread. This should be fixed. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15774> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 4

[GHC] #15485: GHC uses 300% CPU when calling into blocking C call
by GHC 13 Dec '18

13 Dec '18

#15485: GHC uses 300% CPU when calling into blocking C call -------------------------------------+------------------------------------- Reporter: oconnore | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Runtime | Version: 8.4.3 System | Keywords: | Operating System: Unknown/Multiple Architecture: x86_64 | Type of failure: None/Unknown (amd64) | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Hello, I'm trying to write a program that modifies /etc/passwd safely, and so I wrote a function that looks like this: {{{#!hs lockPwd f = bracket main recover (\_ -> f) where mode = unionFileModes ownerReadMode ownerWriteMode main = do fd <- openFd "/etc/.pwd.lock" WriteOnly (Just mode) defaultFileFlags putStrLn "waiting to set lock" waitToSetLock fd (WriteLock, AbsoluteSeek, 0, 0) putStrLn "got lock" return fd recover = flip setLock (Unlock, AbsoluteSeek, 0, 0) }}} When I run it, my fans go wild, and CPU usage hits 300%. /u/gelisam did some more in depth investigation here: https://www.reddit.com/r/haskell/comments/94wbfc/systemunixiowaittosetlock_… > Googling confirms that the parallel gc is using spin locks. So I think what is happening is that the waitToSetLock call makes the current thread unavailable for garbage collection (I don't know if that means the thread is "descheduled" as in the linked issue), which then causes the other threads to spin-lock while waiting for that thread at the next parallel GC. The problem still occurs with the latest release, GHC 8.4.3, and I could not find an existing issue describing the problem, so please file a ticket. Maybe an argument could be made that this waitToSetLock call should be converted to be [[https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ffi- chap.html#interruptible-foreign-calls|interruptible]], but it also seems like I should be able to make my haskell program block patiently if I want it to. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15485> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 3

[GHC] #15267: Extend TVar/MVar to N capacity / Add primitive channal type
by GHC 13 Dec '18

13 Dec '18

#15267: Extend TVar/MVar to N capacity / Add primitive channal type -------------------------------------+------------------------------------- Reporter: winter | Owner: (none) Type: feature | Status: new request | Priority: high | Milestone: 8.6.1 Component: Runtime | Version: 8.4.3 System | 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: -------------------------------------+------------------------------------- The current concurrent primitives `TVar` and `MVar` have a fixed single capacity for holding the value to be synchronized. This pose great limitations on implementing many higher level concurrent structures efficiently, e.g. bound channel, semaphore, resource pool, etc. For example current <semaphore's implementation http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Concurren…> is not ideal due to the extensive usage of multiple `MVar` s. If we have a good `BoundedChan a` type, semaphores can be easily implemented with `BoundedChan ()`. Currently we have various bound channel implementations, but these implementations are often not very efficient or too complex. For example the `BoundedChan` from <BoundedChan http://hackage.haskell.org/package/BoundedChan-1.0.3.0/docs/Control- Concurrent-BoundedChan.html> is based on an array of `MVar` s, thus consumes much more memory than a primitve channel which only have to record parked `StgTSO` s. I'm thinking adding something like: ``` typedef struct { StgHeader header; struct StgMVarTSOQueue_ *head; struct StgMVarTSOQueue_ *tail; StgInt capacity; // the channel's capacity StgInt read_index; // the reading end's index StgInt write_index; // the writing end's index StgClosure *payload[]; // payload array in continuous memory } StgMChan; ``` I still can't work out all the details, but I'm confident something similar to this will work. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15267> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 3

[GHC] #15768: Using oneshot kqueue() on macOS
by GHC 13 Dec '18

13 Dec '18

#15768: Using oneshot kqueue() on macOS -------------------------------------+------------------------------------- Reporter: kazu-yamamoto | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Core | Version: 8.6.1 Libraries | Keywords: KQueue, | Operating System: MacOS X oneshot, parallel build | Architecture: | Type of failure: Other Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Oneshot kqueue() is disabled on macOS due to https://ghc.haskell.org/trac/ghc/ticket/7651 Recall that the paper "Mio: A High-Performance Multicore IO Manager for GHC" says: "Mio also uses kqueue on OS X, since Darwin, the foundation for Apple’s OS X, is a variant of BSD. However, we encountered problems running parallel builds of the GHC compiler using Mio as described above. We have been unable to uncover the underlying source of the problem to our satisfaction. However, several Internet discussions suggest that the implementation of kqueue and pipe on OS X are unstable. We were able to resolve the observed prob- lems on OS X by avoiding the use of one-shot mode on OS X, instead unregistering events after they are delivered, and by send- ing wakeup writes to a pipe monitored by the dispatcher threads’ kqueue instances on every event registration. With these changes, the behavior on OS X, including parallel builds of GHC, has been stable." Now the serious bug on KQueue has been fixed: - https://ghc.haskell.org/trac/ghc/ticket/13903 - https://phabricator.haskell.org/D3692 So, I guess it's time to use oneshot on macOS. A patch is attached. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15768> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 6