October 2015 - ghc-tickets

[GHC] #8662: GHC does not inline cheap inner loop when used in two places
by GHC 04 Aug '16

04 Aug '16

#8662: GHC does not inline cheap inner loop when used in two places ------------------------------+-------------------------------------------- Reporter: nh2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime performance bug Unknown/Multiple | Test Case: Difficulty: Unknown | Blocking: Blocked By: | Related Tickets: | ------------------------------+-------------------------------------------- When I made a Criterion benchmark of Neil Michell's "Tight Inner Loop" blog post (http://neilmitchell.blogspot.co.uk/2014/01/optimising-haskell- for-tight-inner-loop.html), I noticed that GHC 7.6.3 will not inline the performance-critical function when it's called from two different places (inside the same module). {{{ break0_2pleaseinline :: (Char -> Bool) -> ByteString0 -> (ByteString, ByteString0) break0_2pleaseinline f (BS0 bs) = (BS.unsafeTake i bs, BS0 $ BS.unsafeDrop i bs) where i = Internal.inlinePerformIO $ BS.unsafeUseAsCString bs $ \ptr -> do let start = castPtr ptr :: Ptr Word8 let end = go start return $! Ptr end `minusPtr` start go s@(Ptr a) | c == '\0' || f c = a | otherwise = go $ inc s where c = chr s versionInl1 :: ByteString0 -> (ByteString, ByteString0) versionInl1 str = break0_2pleaseinline test str where test x = x <= '$' && (x == ' ' || x == '\r' || x == '\n' || x == '$') versionInl2 :: ByteString0 -> (ByteString, ByteString0) versionInl2 str = break0_2pleaseinline test str where test x = x <= '$' && (x == ' ' || x == '\r' || x == '\n' || x == '$') }}} Full code here: https://github.com/nh2/inner-loop- benchmarks/blob/6715d1e9946d6b5e6d9bb53203982ed3d2ed32ff/Bench.hs#L166. {{{break0_2pleaseinline}}} does not get inlined, which makes {{{versionInl1}}} and {{{versionInl2}}} over 4 times slower than when inlined. The inlining doens't happen, not even with {{{-O2}}} and {{{-O3}}}, only an {{{INLINE}}} pragma will move GHC to do it. If I was a compiler, I would so inline that function! I am surprised that GHC doesn't decide to do so. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8662> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8814: 7.8 optimizes attoparsec improperly
by GHC 04 Aug '16

04 Aug '16

#8814: 7.8 optimizes attoparsec improperly ------------------------------+-------------------------------------------- Reporter: joelteon | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.1-rc1 Keywords: | Operating System: MacOS X Architecture: x86_64 | Type of failure: Runtime performance bug (amd64) | Test Case: Difficulty: Unknown | Blocking: Blocked By: | Related Tickets: | ------------------------------+-------------------------------------------- {{{ #!haskell {-# LANGUAGE OverloadedStrings #-} import Control.Applicative import qualified Data.Attoparsec.Text as A import Data.Text (Text) import qualified Data.Text as T testParser :: Text -> Either String Int testParser f = fmap length . A.parseOnly (many (A.char 'b' <|> A.anyChar)) $ f main :: IO () main = print . testParser $ T.replicate 50000 "a" }}} On GHC 7.6.3 with `-O2`: {{{ real 0m0.062s user 0m0.022s sys 0m0.007s }}} On GHC 7.8 tip with `-O2`: {{{ real 0m12.700s user 0m12.504s sys 0m0.165s }}} On GHC 7.6.3 with `-O0`: {{{ real 0m0.077s user 0m0.025s sys 0m0.007s }}} On GHC 7.8 tip with `-O0`: {{{ real 0m0.055s user 0m0.026s sys 0m0.007s }}} This seems to be related to the use of `<|>`; if I change the program so that the second branch (`A.anyChar`) is never taken, 7.8 behavior is roughly the same as 7.6 under any optimization level. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8814> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9798: Frustrating behaviour of the INLINE pragma
by GHC 03 Aug '16

03 Aug '16

#9798: Frustrating behaviour of the INLINE pragma -------------------------------------+------------------------------------- Reporter: mojojojo | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: MacOS X Architecture: x86_64 (amd64) | Type of failure: Runtime Difficulty: Unknown | performance bug Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- So I have a function "a", which uses another function "b" from another module. * Step 1. I benchmark its performance and get 250ns. * Step 2. I go and put the "INLINE" pragma on the function "b", run the benchmark again and get 500ns. That is twice as long. * Step 3. I go and add an explicit invocation of the [http://hackage.haskell.org/package/base-4.7.0.1/docs/GHC- Exts.html#v:inline inline] function over function "b" in function "a" and finally get the desired optimization: 208ns. You can reproduce the issue by executing {{{cabal bench decoding --benchmark-options=numeric}}} after cloning the trees of the following commits: * [https://github.com/nikita-volkov/postgresql- binary/commit/31e08b4f6887445c28118d25f9da808e7ca9fc86 Step 1] * [https://github.com/nikita-volkov/postgresql- binary/commit/8f29b8daebc3161dc0fa35fbd70d3f3305370a4e Step 2] * [https://github.com/nikita-volkov/postgresql- binary/commit/3d3e4378bcd8ea85cda868d6a4bf07a76a9cad2c Step 3] -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9798> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10710: More self-explanatory pragmas for inlining phase control
by GHC 03 Aug '16

03 Aug '16

#10710: More self-explanatory pragmas for inlining phase control -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 (Parser) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Eventually implement [[Proposal/SelfExplinatoryInlinePragmas]]. And while at it: Extend the docs where they say “You shouldn't ever need to do this, unless you're very cautious about code size.” -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10710> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10766: user manual: INLINE's interaction with optimization levels is not clear
by GHC 03 Aug '16

03 Aug '16

#10766: user manual: INLINE's interaction with optimization levels is not clear -------------------------------------+------------------------------------- Reporter: osa1 | Owner: Type: task | Status: new Priority: normal | Milestone: Component: | Version: 7.11 Documentation | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Other Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- user manual: INLINE's interaction with optimization levels is not clear From the user manual it's not possible to say how should INLINE and actual inlining behavior should interact with optimization levels. What the user manuals says: - Without INLINE GHC tries to inline depending on size of definitions and current optimization level(user manual says -O is needed, but can we assume that for any inlining to be done we need at least -O? Because I have an example in which inlining is happening even with -O0). - With INLINE GHC tries hard to inline. But it doesn't say which optimization level is needed. I think we should add word or two about this. Does anyone know the answer here? My guess is: Even though INLINE makes sure GHC saves RHS of the definition in .hi files to be able to inline later, optimization levels still have an effect on deciding whether to inline or not. Because some INLINEd RHSs may be too big in which case it may decide not to inline with -O1 but inline with -O2 etc. Furthermore, without -O, GHC doesn't mark definitions as INLINE. But if we have INLINEd definitions and compiling a code with -O0, it may still inline those definitions, because there are two different mechanisms to decide whether to mark as INLINE and whether to inline the RHS, and while without -O GHC doesn't mark anything as INLINE, it can still inline previously marked definitions. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10766> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8473: Generate table of cost-centre numbers with source locations
by GHC 01 Aug '16

01 Aug '16

#8473: Generate table of cost-centre numbers with source locations ------------------------------------+------------------------------------- Reporter: lars_e_krueger | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Profiling | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: 7105 | ------------------------------------+------------------------------------- .prof files contain a cost centre number, which I assume is cc_key from CostCentre.lhs. It would be immensely helpful if the compiler creates a simpe table. This way, one could look up the number in .prof, check the table and the exact source location. It would also help with #7105, I think. Suggestion for the format: {{{ no. file begin end 1 main.hs 127:7 128:14 }}} Meaning: Cost centre 1 is in file main.hs, begins at line 127, character 7 and ends at line 128, character 14. If somebody points me in the right direction, and the whole thing is not too complicated, I can try my hand at implementing it myself. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8473> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10957: getExecutablePath adds " (deleted)" suffix if executable was deleted under linux
by GHC 22 Jul '16

22 Jul '16

#10957: getExecutablePath adds " (deleted)" suffix if executable was deleted under linux -------------------------------------+------------------------------------- Reporter: aslpavel | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Core | Version: 7.10.2 Libraries | Keywords: | Operating System: Linux Architecture: | Type of failure: Incorrect result Unknown/Multiple | at runtime Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- If we remove current executable or update it with a new one, getExecutablePath returns path with added " (deleted)" suffix. I think it is incorrect behavior , for example if you updated binary and want to exec a new one, the obvious way to go is to call getExecutablePath and use it. Moreover it is inconsistent between platforms. Problem stems from the fact the getExecutablePath is implemented as readlink on /proc/self/exe and it contains gibberish once file has been deleted. Example: {{{#!hs module Main (main) where import System.Environment (getExecutablePath) import System.Directory (removeFile) main :: IO () main = do before <- getExecutablePath putStrLn $ "Before: " ++ before removeFile before after <- getExecutablePath putStrLn $ "After: " ++ after }}} Output: {{{ $ ./getExecutablePath Before: /mnt/data/Maggot/getExecutablePath After: /mnt/data/Maggot/getExecutablePath (deleted) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10957> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8732: Global big object heap allocator lock causes contention
by GHC 16 Jul '16

16 Jul '16

#8732: Global big object heap allocator lock causes contention ------------------------------------+------------------------------------- Reporter: tibbe | Owner: simonmar Type: bug | Status: new Priority: normal | Milestone: Component: Runtime System | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- The lock allocate() takes when allocating big objects hurts scalability of I/O bound application. Network.Socket.ByteString.recv is typically called with a buffer size of 4096, which causes a ByteString of that size to be allocate. The size of this ByteString causes it to be allocated from the big object space, which causes contention of the global lock that guards that space. See http://www.yesodweb.com/blog/2014/02/new-warp for a real world example. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8732> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10647: Notice about lack of SIMD support.
by GHC 12 Jul '16

12 Jul '16

#10647: Notice about lack of SIMD support. -------------------------------------+------------------------------------- Reporter: mniip | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: Compile-time (amd64) | crash Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- In some cases, when SIMD primitives are used without the `-fllvm` flag, instead of giving the friendly `SIMD vector instructions require the LLVM back-end.`, GHC crashes with varying messages. The simplest example is {{{#!hs {-# LANGUAGE MagicHash #-} module Foo where import GHC.Prim data V = V Int8X16# }}} In 7.8.4 this crashes with {{{ ghc: panic! (the 'impossible' happened) (GHC version 7.8.4 for x86_64-unknown-linux): Size.intSize W128 }}} According to osa1, in HEAD this still crashes, but with {{{ ghc-stage2: panic! (the 'impossible' happened) (GHC version 7.11.20150717 for x86_64-unknown-linux): Format.intFormat W128 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10647> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8308: Resurrect ticky code for counting constructor arity
by GHC 12 Jul '16

12 Jul '16

#8308: Resurrect ticky code for counting constructor arity ----------------------------------------------+---------------------------- Reporter: jstolarek | Owner: Type: task | Status: new Priority: normal | Milestone: Component: Profiling | Version: 7.7 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Moderate (less than a day) | Type of failure: Other Blocked By: | Test Case: Related Tickets: | Blocking: ----------------------------------------------+---------------------------- There is a dead piece of ticky profiling code that computes histograms of data constructor arities (including tuples) and size of vector returns. The function responsible is `bumpHistogram` in [[GhcFile(compiler/codeGen/StgCmmTicky.hs)]], which masks the commented out function `bumpHistogramE`. We can improve ticky profiling by resurrecting this dead code. There are following things that need to be done: 1. Replace current no-op `bumpHistogram` with `bumpHistogramE`. Note that current implementation of `bumpHistogramE` computes constructor arity at runtime. This is unnecessary because we know arity at compile time so we can avoid runtime check by doing sth like this: {{{ bumpHistogram lbl n = do let offset = n `min` 8 emit (addToMem cLong (cmmIndexExpr cLongWidth (CmmLit (CmmLabel (mkRtsDataLabel lbl))) (CmmReg (CmmInt (fromIntegral offset) cLongWidth))) 1) }}} Note that `mkRtsDataLabel` function does not exist anymore but we should be able to replace that call with `mkCmmDataLabel rtsPackageId lbl` (speculation). 2. We need to declare arrays that will store histogram values. Declarations of variables used for storing ticky statistics are in [[GhcFile(includes/stg/Ticky.h)]]. We also need to initialize the declared array to contain only zeros. 3. Having declared the arrays we need to fix printing of computed arities. This is done in [[GhcFile(rts/Ticky.c)]]. Note that there is a lot of code that is commented out (`/* krc: comment out some of this stuff temporarily */`) or disabled with `#if FALSE` pragma. We need to resurect *some* of it. There is a `PR_HST` macro for printing one histogram entry. This seems bad. We should probably rework the part of code responsible for printing out results of historgams. 4. Current code counts arities from 0 to 8. Everything above 8 is put into the same bin as 8. This magical 8 is spread all over the code. We should declare a constant that is visible both in Haskell sources (see `bumpHistogram` in 1.) and RTS files and have all code behave properly depending on that constant - we should have loops instead of statically printing 9 elements of histogram array. 5. Some ticky functions that count arity histograms are not used. For example `tickyReturnNewCon` should probably be called by `cgConApp` like this: {{{ ; emit =<< fcode_init ; tickyReturnNewCon ; emitReturn [idInfoToAmode idinfo] } }}} The above is a an outline, which should be fairly accurate, but unexpected things may show up along the way. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8308> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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