January 2019 - ghc-tickets

[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] #15696: Derived Ord instance for enumerations with more than 8 elements seems to be incorrect
by GHC 18 Jan '19

18 Jan '19

#15696: Derived Ord instance for enumerations with more than 8 elements seems to be incorrect -------------------------------------+------------------------------------- Reporter: mrkkrp | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.6.1 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: -------------------------------------+------------------------------------- Sometimes. Here is the thread that explains the bug: https://github.com/haskell/containers/issues/568 I originally reported this as a bug on `containers` issue tracker, but we seem to have concluded that this is probably a bug in the GHC optimizer itself. I think the shortest repro so far is this: {{{#!hs import qualified Data.Set as S main = print $ let {-# noinline f #-} f () = T2 in S.fromList [f (), f ()] data T = T1 | T2 | T3 | T4 | T5 | T6 | T7 | T8 | T9 deriving (Show, Read, Eq, Ord, Bounded, Enum) }}} which prints {{{#!hs fromList [T2,T2] }}} The person who derived this from my original repro says: > And as I said earlier, comment out the T9 constructor => prints fromList [T2] as it should. Another interesting quote: > Can confirm. Tested with ghc-8.6.1, containers-0.6.0.1 and leancheck-0.7.5 (so it does not seem to depend on the testing framework). Error occurs: > > * with ghc -O1 and -O2 (but not with -O0) > * and if data type has at least 9 elements > > So, likely a bug in ghc's optimizer. > > in some cases, input has duplicates, but not always. This is a bad one, makes GHC 8.6.1 totally unusable for me. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15696> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #16029: Inadequate absence analysis
by GHC 18 Jan '19

18 Jan '19

#16029: Inadequate absence analysis -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.3 Component: Compiler | Version: 8.6.2 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 {{{ data S = MkS Int Int g1 :: S -> Int -> Int g1 (MkS x y) 0 = 0 g1 (MkS x y) n = g1 (MkS y x) (n-1) }}} With GHC 8.6 we get {{{ $wg1 :: S -> GHC.Prim.Int# -> GHC.Prim.Int# [Str=<S,1*H><S,1*U>] $wg1 = \ (w_s2oH :: S) (ww_s2oL :: GHC.Prim.Int#) -> case w_s2oH of { MkS x_s2pz y_s2pA -> case ww_s2oL of ds_X2nb { __DEFAULT -> Foo.$wg1 (Foo.MkS y_s2pA x_s2pz) (GHC.Prim.-# ds_X2nb 1#); 0# -> 0# }} g1 :: S -> Int -> Int [Str=<S,1*H><S(S),1*U(1*U)>m] g1 = \ (w_s2oH :: S) (w1_s2oI :: Int) -> case w_s2oH of w2_X2pG { MkS ipv_s2p2 ipv1_s2p3 -> case w1_s2oI of { GHC.Types.I# ww1_s2oL -> case Foo.$wg1 w2_X2pG ww1_s2oL of ww2_s2oP { __DEFAULT -> GHC.Types.I# ww2_s2oP }}} }}} What terrible code! We evaluate the S argument in the wrapper, and box and unbox it every time around the loop, even though it is never ultimately used. Here's what happens if the arguments are banged: {{{ data T = MkT !Int !Int g2 :: T -> Int -> Int g2 (MkT x y) 0 = 0 g2 (MkT x y) n = g2 (MkT y x) (n-1) }}} We get {{{ $wg2 GHC.Prim.Int# -> GHC.Prim.Int# -> GHC.Prim.Int# -> GHC.Prim.Int# [Str=<L,1*U><L,1*U><S,1*U>] Foo.$wg2 = \ (ww_s2ow :: GHC.Prim.Int#) (ww1_s2ox :: GHC.Prim.Int#) (ww2_s2oB :: GHC.Prim.Int#) -> case ww2_s2oB of ds_X2n0 { __DEFAULT -> Foo.$wg2 ww1_s2ox ww_s2ow (GHC.Prim.-# ds_X2n0 1#); 0# -> 0# } g2 :: T -> Int -> Int [Str=<S(SS),1*U(1*U,1*U)><S(S),1*U(1*U)>m ] g2 = \ (w_s2os :: T) (w1_s2ot :: Int) -> case w_s2os of { MkT ww1_s2ow ww2_s2ox -> case w1_s2ot of { GHC.Types.I# ww4_s2oB -> case Foo.$wg2 ww1_s2ow ww2_s2ox ww4_s2oB of ww5_s2oF { __DEFAULT -> GHC.Types.I# ww5_s2oF }}} }}} Still terrible. We pass the two components around the loop before discarding them at the end. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16029> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #16197: Strictness is not preserved under -O1
by GHC 18 Jan '19

18 Jan '19

#16197: Strictness is not preserved under -O1 -------------------------------------+------------------------------------- Reporter: alang9 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.4.1 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: -------------------------------------+------------------------------------- With -O0, the attached code prints: {{{ > /usr/local/ghc/ghc-8.4.1.0/bin/ghc -O0 src/foo.hs; and ./src/foo [1 of 1] Compiling Main ( src/foo.hs, src/foo.o ) [Optimisation flags changed] Linking src/foo ... ("exec",0) ("depth",0) ("exec",1) ("depth",0) }}} But with -O1, it prints: {{{ > /usr/local/ghc/ghc-8.4.1.0/bin/ghc -O1 src/foo.hs; and ./src/foo [1 of 1] Compiling Main ( src/foo.hs, src/foo.o ) Linking src/foo ... ("depth",0) ("depth",0) }}} Reproduced with 8.4.1 and 8.6.2. Doesn't seem to happen under 8.2.0 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16197> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #4941: SpecConstr generates functions that do not use their arguments
by GHC 18 Jan '19

18 Jan '19

#4941: SpecConstr generates functions that do not use their arguments -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: task | Status: new Priority: normal | Milestone: ⊥ Component: Compiler | Version: 7.0.1 Resolution: | Keywords: SpecConstr, | DemandAnalysis 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: | -------------------------------------+------------------------------------- Changes (by simonpj): * keywords: SpecConstr => SpecConstr, DemandAnalysis -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4941#comment:23> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #1171: GHC doesn't respect the imprecise exceptions semantics
by GHC 18 Jan '19

18 Jan '19

#1171: GHC doesn't respect the imprecise exceptions semantics -------------------------------------+------------------------------------- Reporter: neil | Owner: (none) Type: bug | Status: new Priority: low | Milestone: ⊥ Component: Compiler | Version: 6.6 Resolution: | Keywords: | DemandAnalysis Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: cg059 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * keywords: => DemandAnalysis -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/1171#comment:21> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #1885: Improve CPR analysis
by GHC 18 Jan '19

18 Jan '19

#1885: Improve CPR analysis -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: feature request | Status: new Priority: lowest | Milestone: Component: Compiler | Version: 6.8.1 Resolution: | Keywords: CPRAnalysis Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * keywords: => CPRAnalysis -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/1885#comment:21> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #3138: Returning a known constructor: GHC generates terrible code for cmonad
by GHC 18 Jan '19

18 Jan '19

#3138: Returning a known constructor: GHC generates terrible code for cmonad -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: lowest | Milestone: Component: Compiler | Version: 6.10.1 Resolution: | Keywords: | DemandAnalysis 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: | -------------------------------------+------------------------------------- Changes (by simonpj): * keywords: => DemandAnalysis -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/3138#comment:20> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #5075: CPR optimisation for sum types if only one constructor is used
by GHC 18 Jan '19

18 Jan '19

#5075: CPR optimisation for sum types if only one constructor is used -------------------------------------+------------------------------------- Reporter: batterseapower | Owner: simonpj Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.0.3 Resolution: | Keywords: SpecConstr, | CPRAnalysis 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: | -------------------------------------+------------------------------------- Changes (by simonpj): * keywords: SpecConstr => SpecConstr, CPRAnalysis -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5075#comment:28> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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