September 2018 - ghc-tickets

[GHC] #15056: Wrappers inlined too late
by GHC 18 Jan '19

18 Jan '19

#15056: Wrappers inlined too late -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 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: -------------------------------------+------------------------------------- Consider these two modules {{{ module Foo where test :: Int -> Bool test 0 = True test n = test (n-1) foo :: Foldable t => Int -> t Int -> Int foo n xs | test n = foldr (+) n xs | otherwise = n+7 }}} and {{{ module Bar where import Foo blam x = f 3 [1..x] }}} If we simply inline `foo` into `blam`, we get `foldr/build` fusion. And that is exactly what happens if you compile `Foo` with `-fno-strictness`. But what actually happens is * When compiling `Foo`, strictness analysis does worker/wrapper for `foo`. * The wrapper currently gets an aciviation of "active only in phase 0"; see `Note [Wrapper activation]` in `WorkWrap`. * So `foo` is ultimately inlined (actually its worker is inlined too) but too late for fusion to take place. This is bad: optimising `Foo` makes the overall runtime increase. I have seen other examples of this. The general pattern is: * Without worker/wrapper, a function `f` may inline, which exposes opportunities for functions in its body to participate in rule rewrites * With worker/wrapper, no inlining happens until phase 0; and that is too late for the rules to fire. Obvious thought: allow wrappers from strictness analysis to inline earlier. I'll try that. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15056> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11126: Entered absent arg in a Repa program
by GHC 18 Jan '19

18 Jan '19

#11126: Entered absent arg in a Repa program -------------------------------------+------------------------------------- Reporter: tuplanolla | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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: -------------------------------------+------------------------------------- Consider the following program. {{{#!hs module Main where import Data.Array.Repa data Stuff = !(Array U DIM1 Double) `With` !Double deriving Show through :: Maybe Double -> Stuff -> Stuff m `through` (a `With` _) = let b = a +^ (negate `smap` sumS (extend (Z :. All :. (1 :: Int)) a)) c = maybe b (const (negate `smap` a)) m in computeUnboxedS c `With` sumAllS b main :: IO () main = print $ Just 1 `through` (fromListUnboxed (Z :. 1) [1] `With` 1) }}} It should produce the following result once run. {{{#!hs AUnboxed (Z :. 1) (fromList [-1.0]) `With` 0.0 }}} However, when built using `repa-3.4.0.1` and compiled with the options `-O3 -Wall -funfolding-keeness-factor1000 -funfolding-use-threshold1000`, it crashes as follows. {{{#!hs Main: Oops! Entered absent arg arr2 Array D DIM1 Double }}} Adding `-fno-strictness` to the compiler options or removing strictness annotations from the code makes the problem disappear, so this looks like a strictness analyzer problem. The libraries used were * `QuickCheck-2.8.1`, * `array-0.5.1.0`, * `base-4.8.1.0`, * `bytestring-0.10.6.0`, * `containers-0.5.6.2`, * `deepseq-1.4.1.1`, * `ghc-prim-0.4.0.0`, * `integer-gmp-1.0.0.0`, * `pretty-1.1.2.0`, * `primitive-0.6`, * `random-1.1`, * `repa-3.4.0.1`, * `template-haskell-2.10.0.0`, * `tf-random-0.5`, * `time-1.5.0.1`, * `transformers-0.4.2.0` and * `vector-0.10.12.3`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11126> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14816: Missed Called Arity opportunity?
by GHC 18 Jan '19

18 Jan '19

#14816: Missed Called Arity opportunity? -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When I compile {{{#!hs test :: (a -> a -> a) -> Int -> a -> HashMap Int a -> HashMap Int a test f k a m = insertModifying a (blink (f a)) k m blink :: (a -> b) -> a -> (# b #) -- Or blink g = \a -> (# g a #) ; it makes no difference. blink g a = (# g a #) }}} I get {{{ test = \ (@ a_a9o3) (f_a7iP :: a_a9o3 -> a_a9o3 -> a_a9o3) (k_a7iQ :: Int) (a1_a7iR :: a_a9o3) (m_a7iS :: HashMap Int a_a9o3) -> case k_a7iQ of { GHC.Types.I# ww1_sa1Z -> RULES.$w$sinsertModifying @ a_a9o3 a1_a7iR (let { g_s9E1 [Dmd=<L,C(U)>] :: a_a9o3 -> a_a9o3 [LclId] g_s9E1 = f_a7iP a1_a7iR } in \ (a2_a7iU :: a_a9o3) -> (# g_s9E1 a2_a7iU #)) ww1_sa1Z m_a7iS } }}} We build `g_s9E1 = f_a7iP a1_a7iR` for no apparent reason. Trouble persists into STG: {{{ RULES.test :: forall a. (a -> a -> a) -> GHC.Types.Int -> a -> Data.HashMap.Base.HashMap GHC.Types.Int a -> Data.HashMap.Base.HashMap GHC.Types.Int a [GblId, Arity=4, Str=<L,1*C1(C(U))><S(S),1*U(U)><L,U><S,1*U>, Unf=OtherCon []] = [] \r [f_saiX k_saiY a1_saiZ m_saj0] case k_saiY of { GHC.Types.I# ww1_saj2 [Occ=Once] -> let { g_saj3 [Occ=OnceL!, Dmd=<L,C(U)>] :: a_a9o3 -> a_a9o3 [LclId] = [f_saiX a1_saiZ] \u [] f_saiX a1_saiZ; } in let { sat_saj6 [Occ=Once] :: a_a9o3 -> (# a_a9o3 #) [LclId] = [g_saj3] \r [a2_saj4] let { sat_saj5 [Occ=Once] :: a_a9o3 [LclId] = [g_saj3 a2_saj4] \u [] g_saj3 a2_saj4; } in Unit# [sat_saj5]; } in RULES.$w$sinsertModifying a1_saiZ sat_saj6 ww1_saj2 m_saj0; }; }}} `insertModifying` uses its function argument at most once, so there is no possible benefit to this partial application. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14816> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15226: GHC doesn't know that seq# produces something in WHNF
by GHC 18 Jan '19

18 Jan '19

#15226: GHC doesn't know that seq# produces something in WHNF -------------------------------------+------------------------------------- Reporter: dfeuer | 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: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!hs data Str a = Str !a bar :: Maybe a -> IO (Str (Maybe a)) bar x = do x' <- evaluate x pure (Str x') }}} This compiles to {{{#!hs Test.bar1 = \ (@ a_a3Ld) (x_a3Ah :: Maybe a_a3Ld) (s_i3Nz :: GHC.Prim.State# GHC.Prim.RealWorld) -> case GHC.Prim.seq# @ (Maybe a_a3Ld) @ GHC.Prim.RealWorld x_a3Ah s_i3Nz of { (# ipv_i3NC, ipv1_i3ND #) -> (# ipv_i3NC, Test.$WStr @ (Maybe a_a3Ld) ipv1_i3ND #) } }}} We suspend the application of `$WStr` to `ipv1_i3ND`, when all we actually need to do is apply `Str` directly. We could work around this in `base` by defining {{{#!hs evaluate x = IO $ \s -> case seq# x s of (# s', !x' #) -> (# s', x' #) }}} but that seems more than a little bit silly. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15226> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15627: Absent unlifted bindings
by GHC 18 Jan '19

18 Jan '19

#15627: Absent unlifted bindings -------------------------------------+------------------------------------- Reporter: sgraf | Owner: (none) Type: task | Status: new Priority: normal | Milestone: ⊥ 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: #9279 #4328 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- As simonpj put it in ticket:9279#comment:20: > For a long time, the worker/wrapper splitter has given up on absent > arguments of certain unlifted types: see `Literal.absentLiteralOf` and > `Note [Absent errors]` in `WwLib`. This is very annoying because it means > that we get left with functions that take a bunch of arguments they do not > use, as in this ticket (#9279). > > For lifted types T we build an absent value as a thunk of form > {{{ > aBSENT_ERROR_ID @T "Used absent value" > }}} > This does two things > A. It gives us something, of the right type, to use in place of the value > we aren't passing any more. > B. It gives an extra sanity check: if that value is ever used (a compiler > bug) we'll get a runtime error message. > > For unlifted types we don't have thunks, so we can't do this. As you can > see in `absentLiteralOf`, for some types we just make up a silly value: > e.g. for `Char#` we use `'x#'`; for `Int#` we use `0#`. > > Note, however that > > * Substituting a particular value serves purpose (A) but not purpose > (B). A compiler bug would go undetected. This is sad: e.g. #11126 > is a real bug that was detected by (B). But I see no way out. > > * It doesn't work for `Array#`, `MutVar#`, `TVar#` etc because we have > no available literal values of those types. > > So Sebastian is suggesting that we add a new literal value -- call it > a '''rubbish value''' -- which can work for any (unlifted type), > extending `Literal` something like this > {{{ > data Literal = ... > | RubbishLit Type > }}} > We need to store the type so we can still do `literalType`. > > Now > * Maybe we could get rid of `MachNullAddr` in favour of this new literal. > > * I think -- but I am not sure -- that this literal should never occur > in code generation. For example, we should never pass a rubbish value > to a function. Before then dead-code elimination should have got rid > of it I'm not 100% certain, but if this was true, it'd be a great > sanity check. > > * Yes, `Literal` has `Eq` and `Ord` -- but I'm not sure why. Try removing > them and seeing what happens! (Generally I think it'd be better to > define `eqLit` and `cmpLit` and cal them, rather than use `==` and `>`; > so much easier to grep for! > > And in fact, we do have `eqType` and `cmpType`. > > * Do we need to spit out a `RubbishLit` in the `Binary` instance. > This seems more likely, because perhaps these rubbish values can occur > in unfoldings, which are serialised as their parse tree. But > the we can just serialise the `Type`. It won't happen much. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15627> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10918: Float once-used let binding into a recursive function
by GHC 18 Jan '19

18 Jan '19

#10918: Float once-used let binding into a recursive function -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | -------------------------------------+------------------------------------- Consider this code {{{#!hs let x = f x0 in let go 10 = x go i = go (i+1) in go (0::Int) }}} Currently, this is pretty much the core that comes out at the end. But what we want to see instead is {{{#!hs let go 10 = let x = f x 0 in x go i = go (i+1) in go (0::Int) }}} In general, we do not want to float a binding into a recursive function, because we would risk doing the allocation and/or evaluation of it multiple times. But in this case, we can see that it is going to be used at most once, so it is safe to do so. Even more: In the slightly less contrived examples that I was looking at, the call to `x` was happening in the a less likely code path, so this way we’d avoid doing the allocation in most cases, a clear win. It might be enough to simply make `CallArity` (or rather the cardinality analysis done by call arity) tell the rest of the compiler that it found that `x` is called at most once, and hopefully the simplifier knows what to make of that information. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10918> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11301: Using GHC's parser and rendering the results is unreasonably difficult
by GHC 18 Jan '19

18 Jan '19

#11301: Using GHC's parser and rendering the results is unreasonably difficult -------------------------------------+------------------------------------- Reporter: bitemyapp | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: GHC API | Version: 7.10.2 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: -------------------------------------+------------------------------------- After a couple hours, 30-some browser tabs, and attempting to use ghc- simple to paper over some annoyances with GHC's API (dynFlags/session- stuff, particularly), I gave up. I then opened up haskell-src-exts and found the function I wanted in a minute or two. {{{#!hs import qualified Language.Haskell.Exts.Parser as P parsePls' :: String -> IO () parsePls' s = print $ P.parseExp s }}} This is where I got stuck: {{{ Prelude> parsePls "1 + 1" Too late for parseStaticFlags: call it before runGhc or runGhcT }}} This is the code I was working with before I gave up on GHC: {{{#!hs module ParsePls where import DynFlags import FastString import qualified GHC import qualified GhcMonad as GM import HsSyn import Lexer import Outputable import Parser import RdrName import SrcLoc import StaticFlags import StringBuffer import qualified Language.Haskell.GHC.Simple as S import qualified Language.Haskell.GHC.Simple.Types as ST import System.Environment main :: IO () main = do [expr] <- getArgs parsePls expr parsePls :: String -> IO () parsePls s = do initStaticOpts let sbuf = stringToStringBuffer s srcloc = mkRealSrcLoc (mkFastString s) 1 1 (dynflags, _) <- S.getDynFlagsForConfig ST.defaultConfig -- dynflags <- undefined -- GHC.getSessionDynFlags -- unP :: P a -> PState -> ParseResult a -- mkPState :: DynFlags -> StringBuffer -> RealSrcLoc -> PState -- parseExpression :: P (LHsExpr RdrName) -- type LHsExpr id = Located (HsExpr id) -- Defined in ‘HsExpr’ let parseResult = unP parseExpression (mkPState dynflags sbuf srcloc) case parseResult of POk _ (L _ mdl) -> print $ showPpr dynflags mdl PFailed ss _ -> do putStrLn "Error occurred!" print ss }}} Is there a reason it has to be this difficult? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11301> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11606: name shadowing warnings don't trigger on standalone declarations in ghci
by GHC 17 Jan '19

17 Jan '19

#11606: name shadowing warnings don't trigger on standalone declarations in ghci -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: bug | Status: new Priority: high | Milestone: 8.0.1 Component: GHCi | Version: 7.10.3 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: -------------------------------------+------------------------------------- The name shadowing warnings in ghci catch shadowing from top-level binds and `let` statements, but not from standalone declarations (new in 8.0). {{{ rwbarton@morphism:/tmp$ ~/ghc-HEAD/bin/ghci -fwarn-name-shadowing GHCi, version 8.1.20160212: http://www.haskell.org/ghc/ :? for help Prelude> a <- return () Prelude> a <- return () <interactive>:2:1: warning: This binding for ‘a’ shadows the existing binding defined at <interactive>:1:1 Prelude> let b = return () Prelude> let b = return () <interactive>:4:5: warning: This binding for ‘b’ shadows the existing binding defined at <interactive>:3:5 Prelude> c = return () Prelude> c = return () Prelude> }}} I'm hoping this can be fixed for 8.0, since I would also very much like to turn on name shadowing warnings in ghci by default (in the interactive flags), to reduce confusion from examples like the below, which is now possible for the first time in 8.0: {{{ Prelude> fact 0 = 1 Prelude> fact n = n * fact (n-1) Prelude> fact 5 ^C^CInterrupted. Prelude> -- why did it run forever? }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11606> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12091: 'Variable not in scope" when using GHCi with `-fobject-code`
by GHC 17 Jan '19

17 Jan '19

#12091: 'Variable not in scope" when using GHCi with `-fobject-code` -------------------------------------+------------------------------------- Reporter: thomie | Owner: Type: bug | Status: new Priority: high | Milestone: Component: GHCi | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: #7253 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Since b98ff3ccb14e36145404f075349c8689762a2913 was landed (#7253), you don't need `let` to define stuff in GHCi: {{{ $ ghci Prelude> x = 3 Prelude> x 3 }}} But when using `-fobject-code`, this results in an error: {{{ $ ghci -fobject-code Prelude> x = 3 Prelude> x <interactive>:2:1: error: Variable not in scope: x }}} Very strange. CC @roshats, who implemented this feature, but the bug is probably in some part of the code he didn't touch. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12091> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15086: hT profiling option
by GHC 17 Jan '19

17 Jan '19

#15086: hT profiling option -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Documentation Unknown/Multiple | bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/profiling.h… #rts-options-for-heap-profiling The docs] indicate that you can pass `-hT` as an RTS option for memory profiling "by heap closure type". However, GHC 8.4.2 claims `invalid heap profile options: -hT`, and this option is not listed among the usage options printed with the error. The user guide for GHC 8.2.2 also mentions -hT, and I believe that GHC-8.2.2 also did not permit the option. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15086> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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