October 2015 - ghc-tickets

[GHC] #10945: Typed Template Haskell splices broken in HEAD (7.11)
by GHC 11 Jan '19

11 Jan '19

#10945: Typed Template Haskell splices broken in HEAD (7.11) -------------------------------------+------------------------------------- Reporter: jstolarek | Owner: Type: bug | Status: new Priority: high | Milestone: 8.0.1 Component: Template | Version: 7.11 Haskell | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | -------------------------------------+------------------------------------- In the latest HEAD (e2b579e8d77357e8b36f57d15daead101586ac8e) when I say: {{{#!hs $$(return [ SigD (mkName "m") (ForallT [PlainTV (mkName "a")] [] (AppT (AppT ArrowT (VarT (mkName "a"))) (VarT (mkName "a")))) , FunD (mkName "m") [Clause [VarP (mkName "x")] (NormalB (VarE (mkName "x"))) []] ]) }}} I get: {{{ ghc-stage2: panic! (the 'impossible' happened) (GHC version 7.11.20151007 for x86_64-unknown-linux): runRnSplice $$(return [SigD (mkName "m") (ForallT [PlainTV (mkName "a")] [] (AppT (AppT ArrowT (VarT (mkName "a"))) (VarT (mkName "a")))), FunD (mkName "m") [Clause [VarP (mkName "x")] (NormalB (VarE (mkName "x"))) []]]) }}} This code compiles correctly in GHC 7.10.1. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10945> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10949: Document typed Template Haskell
by GHC 11 Jan '19

11 Jan '19

#10949: Document typed Template Haskell -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: high | Milestone: 8.0.1 Component: Template | Version: 7.10.2 Haskell | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Documentation Unknown/Multiple | bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | -------------------------------------+------------------------------------- As far as I can tell, typed Template Haskell is completely missing from the manual. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10949> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8326: Place heap checks common in case alternatives before the case
by GHC 07 Jan '19

07 Jan '19

#8326: Place heap checks common in case alternatives before the case ------------------------------------+------------------------------------- Reporter: jstolarek | Owner: Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: 8317 Related Tickets: #1498 | ------------------------------------+------------------------------------- We would like to have functions that check whether an `Int#` is a valid tag to represent `Bool` (see Note [Optimizing isTrue#] in ghc-prim): {{{ isTrue# :: Int# -> Bool isTrue# 1# = True isTrue# _ = False isFalse# :: Int# -> Bool isFalse# 0# = True isFalse# _ = False }}} We could use them with comparison primops like this: {{{ f :: Int# -> Int f x | isTrue# (x ># 0#) = I# x | otherwise = -(I# x) }}} `isTrue#` is optimized away at the Core level: {{{ A.f = \ (x_aqM :: GHC.Prim.Int#) -> case GHC.Prim.># x_aqM 0 of _ { __DEFAULT -> GHC.Types.I# (GHC.Prim.negateInt# x_aqM); 1 -> GHC.Types.I# x_aqM } }}} but the code genrator produces very bad Cmm code, because it pushes heap checks into case alternatives: {{{ {offset cFd: // stack check if ((Sp + -16) < SpLim) goto cFr; else goto cFs; cFr: // not enough place on the stack, call GC R2 = R2; R1 = A.f_closure; call (stg_gc_fun)(R2, R1) args: 8, res: 0, upd: 8; cFs: // scrutinize (x ># 0#) _sEU::I64 = R2; _sEV::I64 = %MO_S_Gt_W64(R2, 0); if (_sEV::I64 != 1) goto cFg; else goto cFo; cFg: // False branch Hp = Hp + 16; if (Hp > HpLim) goto cFy; else goto cFx; cFy: // not enough heap, call GC HpAlloc = 16; I64[Sp - 16] = cFf; R1 = _sEV::I64; I64[Sp - 8] = _sEU::I64; Sp = Sp - 16; call stg_gc_unbx_r1(R1) returns to cFf, args: 8, res: 8, upd: 8; cFf: // re-do the False branch _sEU::I64 = I64[Sp + 8]; Sp = Sp + 16; _sEV::I64 = R1; goto cFg; cFx: // RHS of False branch I64[Hp - 8] = GHC.Types.I#_con_info; I64[Hp] = -_sEU::I64; R1 = Hp - 7; call (P64[Sp])(R1) args: 8, res: 0, upd: 8; cFo: // True branch Hp = Hp + 16; if (Hp > HpLim) goto cFv; else goto cFu; cFv: // not enough heap, call GC HpAlloc = 16; I64[Sp - 16] = cFn; R1 = _sEV::I64; I64[Sp - 8] = _sEU::I64; Sp = Sp - 16; call stg_gc_unbx_r1(R1) returns to cFn, args: 8, res: 8, upd: 8; cFn: // re-do the True branch _sEU::I64 = I64[Sp + 8]; Sp = Sp + 16; _sEV::I64 = R1; goto cFo; cFu: // RHS of True branch I64[Hp - 8] = GHC.Types.I#_con_info; I64[Hp] = _sEU::I64; R1 = Hp - 7; call (P64[Sp])(R1) args: 8, res: 0, upd: 8; } }}} This results in average 2.5% increase in binary size. By contrast, if we use `tagToEnum#` instead of `isTrue#` heap check will be placed before `case` expression and the code will be significantly shorter (this is done by a special case-on-bool optimization in the code generator - see #8317). What we would like to do here is: 1. compile case alternatives without placing heap checks inside them 2. each compiled alternative should return amount of heap it needs to allocate 3. code generator inspects amounts of heap needed by each alternative and either adds heap checks in alternatives or puts a single check before the case expression. Getting this right might be a bit tricky. 1. if all branches allocate some heap then we can just put a common heap check before the case. Note that we must allocate the higgest amount required by any of the alternatives and then alternatives that use less heap must retract the heap pointer accordingly. 2. if we have two alternatives, one of which allocates heap and the other does not, we should place the heap check only in the alternative that allocates the stack. This will solve #1498. 3. it is not clear to me what to do if we have combination of the above (more than one branch that allocates heap and at least one branch that does not). If we place heap check before the `case` expression we lose optimization of recursive functions and face the problem described in #1498. If we push heap checks into branches that allocate heap then we get code duplication, i.e. the problem that we're addressing in this ticket. I guess the only way to make correct decission here is to try different aproaches and measure their performance. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8326> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8611: nofib’s cacheprof’s allocations nondeterminisitic
by GHC 01 Jan '19

01 Jan '19

#8611: nofib’s cacheprof’s allocations nondeterminisitic -----------------------------------------+--------------------------------- Reporter: nomeata | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: NoFib benchmark suite | Version: 7.6.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: -----------------------------------------+--------------------------------- This seems to be neither expected nor desired, and hence worth investigating: {{{ ./cacheprof +RTS -t <<ghc: 127588752 bytes, 243 GCs, 2261288/6551032 avg/max bytes residency (6 samples), 15M in use, 0.00 INIT (0.00 elapsed), 0.12 MUT (0.12 elapsed), 0.05 GC (0.05 elapsed) :ghc>> ./cacheprof +RTS -t <<ghc: 127392144 bytes, 242 GCs, 2261304/6551576 avg/max bytes residency (6 samples), 15M in use, 0.00 INIT (0.00 elapsed), 0.11 MUT (0.11 elapsed), 0.04 GC (0.05 elapsed) :ghc>> }}} The allocations numbers change, when they shoudn’t. Reproduced with 7.6.3 and current HEAD. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8611> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8316: GHCi debugger segfaults when trying force a certain variable
by GHC 28 Dec '18

28 Dec '18

#8316: GHCi debugger segfaults when trying force a certain variable ------------------------------------+------------------------------------- Reporter: guest | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: GHCi crash Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- The file Test.hs has following definitions: {{{ whnf :: a -> IO () whnf a = a `seq` (return ()) foo :: [Int] foo = [1..] }}} Calling ghci as: {{{ ghci Test.hs -ignore-dot-ghci }}} and bebugging foo like this: {{{ *Main> :b foo Breakpoint 0 activated at Test.hs:5:7-11 *Main> foo Stopped at Test.hs:5:7-11 _result :: [Int] = _ [Test.hs:5:7-11] *Main> :p foo foo = (_t1::[Int]) [Test.hs:5:7-11] *Main> whnf _t1 }}} results in this segault: {{{ <interactive>: internal error: TSO object entered! (GHC version 7.6.3 for x86_64_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug [1] 5445 abort (core dumped) ghci Test.hs -ignore-dot-ghci }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8316> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9627: Type error with functional dependencies
by GHC 24 Dec '18

24 Dec '18

#9627: Type error with functional dependencies -------------------------------------+------------------------------------- Reporter: augustss | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- The attached file should compile. The types T1, T2, and T3 are all equivalent types (T1 and T2 normalize to T3), but with foo::T1 and bar::T2 there is a type error. Rewriting the example using type families instead of FD makes it work. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9627> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8949: switch -msse2 to be on by default
by GHC 21 Dec '18

21 Dec '18

#8949: switch -msse2 to be on by default ----------------------------------------------+---------------------------- Reporter: errge | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (CodeGen) | Version: 7.9 Keywords: | Operating System: Architecture: x86 | Unknown/Multiple Difficulty: Moderate (less than a day) | Type of failure: Other Blocked By: | Test Case: Related Tickets: | Blocking: ----------------------------------------------+---------------------------- I propose msse2 to be on by default. I guess the default was chosen way back, when Pentium III support was still relevant. Nowadays we don't really win on the CPU support, because e.g. https://github.com/tibbe/hashable/blob/master/hashable.cabal is built by default with sse2 on the injected C code level. And hashable has a lot of reverse depends, therefore on an end user system (RedHat or Debian) the user is most probably unlucky with a Pentium III CPU anyways. Flipping this default would also fix the excess precision problem for most users of GHC on the i686 platform. GHC should provide a -mno-sse2 flag for the cases when this needs to be disabled. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8949> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8655: Evaluate know-to-terminate-soon thunks
by GHC 19 Dec '18

19 Dec '18

#8655: Evaluate know-to-terminate-soon thunks ------------------------------------+------------------------------------- Reporter: nomeata | Owner: nomeata Type: task | Status: new Priority: normal | Milestone: Component: Compiler | 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: | ------------------------------------+------------------------------------- I guess I’ll better put my interior monologue in a ticket than on ghc- dev... In order to implement nested CPR (#1600), I had to enrich the CPR type to keep track of whether something, when entered, will converge for sure. My code does not solve the halting problem, but does only simple stuff, so if something is known to converge, is is very likely to be cheap. Simon suggested to measure the effect of evaluating a let-bound thunk with the known-to-terminate property, as if the body was using it strictly. This tickets tracks this suggestion, and reports progress. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8655> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8598: IO hack in demand analyzer gets in the way of CPR
by GHC 19 Dec '18

19 Dec '18

#8598: IO hack in demand analyzer gets in the way of CPR ------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: task | Status: new Priority: normal | Milestone: Component: Compiler | 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: | ------------------------------------+------------------------------------- After a lot of staring at code and comparing unexpected nofib results I found the following: The IO hack in the demand analyzer (see `dmdAnalAlt` in `StrAnal.lhs` and #1592 for a good discussion) prevents CPR in any function that uses a C call. This is a small example, reduced from the `scaleFloat` method for doubles: {{{ module Float(fun) where import GHC.Float (Double(..)) import GHC.Integer (decodeDoubleInteger, encodeDoubleInteger) fun :: Double -> Double fun x | isFix = x | otherwise = case x of (D# x#) -> case decodeDoubleInteger x# of (# i, j #) -> D# (encodeDoubleInteger i j) where isFix = isDoubleFinite x == 0 foreign import ccall unsafe "isDoubleFinite" isDoubleFinite :: Double -> Int }}} Here, `fun` does current not get the CPR property, and the work gets type `GHC.Prim.Double# -> GHC.Types.Double`. Why? Because in core, there will be a {{{ case {__pkg_ccall main isDoubleFinite GHC.Prim.Double# -> GHC.Prim.State# GHC.Prim.RealWorld -> (# GHC.Prim.State# GHC.Prim.RealWorld, GHC.Prim.Int# #)}_dQD ds_dQA GHC.Prim.realWorld# of _ [Occ=Dead, Dmd=<L,A>] { (# ds_dQC [Dmd=<L,A>, OS=OneShot], ds_dQB [Dmd=<S,1*U>] #) -> ... }}} where the case body has `DmdType m {dQz-><L,1*U(U)> dQA-><L,U>}`, but `dmdAnalAlt` detects that this is evaluating a possibly exiting function and throws aways the information using `alt_ty `lubDmdType` topDmdType`. Would it be ok to `lub` only the demand on the free variables, but keep the CPR information? In nofib (if I managed to compare the right results) this does nothing for almost all benchmarks, `-9.2%` of allocations for `mandel` and `+4.9%` for reverse-complement (but these numbers are not obtained very cleanly, and all is happening on top of the better-ho-cardinality branch. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8598> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10905: Incorrect number of parameters in "role" errors
by GHC 19 Dec '18

19 Dec '18

#10905: Incorrect number of parameters in "role" errors -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: roles | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- The following code produces the error: {{{ import Data.Coerce import Data.Functor.Trans.Tagged toTT :: Tagged t [a] -> TaggedT t [] a toTT = coerce }}} {{{ Couldn't match representation of type ‘[a]’ with that of ‘Data.Functor.Identity.Identity [a]’ arising from trying to show that the representations of ‘Tagged t [a]’ and ‘TaggedT t [] a’ are the same Relevant role signatures: type role [] representational type role Data.Functor.Identity.Identity representational type role TaggedT nominal nominal phantom representational nominal The data constructor ‘Data.Functor.Identity.Identity’ of newtype ‘Data.Functor.Identity.Identity’ is not in scope Relevant bindings include toTT :: Tagged t [a] -> TaggedT t [] a (bound at Main.hs:9:1) In the expression: coerce In an equation for ‘toTT’: toTT = coerce }}} First, kudos for correctly identifying the problem: I failed to import the `Identity` constructor. However, it seems that the report for the roles of the `TaggedT` type has too many parameters. It is defined in tagged-transformer [https://hackage.haskell.org/package/tagged-transformer-0.8/docs/Data- Functor-Trans-Tagged.html] as `newtype TaggedT s m b = TagT { untagT :: m b }`, but is listed with 5 roles. I did try to move the definition of `TaggedT/Tagged` into my module so I didn't have to import the library, but when I did the error didn't show up: TaggedT was listed with the expected roles `phantom representational nominal`. As a user, it wasn't clear to me why there are 5 roles listed for a type with 3 parameters, and I didn't know which roles I should be looking at to help me debug the `coerce` error. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10905> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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