November 2015 - ghc-tickets

[GHC] #10271: Typed Template Haskell splice difficulty when resolving overloading
by GHC 11 Jan '19

11 Jan '19

#10271: Typed Template Haskell splice difficulty when resolving overloading -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- J Garrett Morris describes the following surprising behaviour for typed Template Haskell {{{ {-# LANGUAGE TemplateHaskell, FlexibleInstances #-} module PrintfLib where import Language.Haskell.TH class Printf t where printf :: String -> Q (TExp String) -> Q (TExp t) instance Printf [Char] where tf s t | '%' `notElem` s = [|| $$t ++ s ||] | otherwise = fail ("Unexpected format %" ++ [c]) where (_, _:c:_) = break ('%' ==) s instance Printf t => Printf (Char -> t) where printf s t | c /= 'c' = fail ("Unexpected format %" ++ [c] ++ " for character") | otherwise = [|| \c -> $$(printf s'' [|| $$t ++ s' ++ [c] ||]) ||] where (s', '%':c:s'') = break ('%' ==) s ------------------------- {-# LANGUAGE TemplateHaskell #-} module Printf where import PrintfLib f :: Char -> String f = $$(printf "foo %c" [||""||]) h :: Char -> String h y = $$(printf "foo %c" [||""||]) y }}} Oddly, `f` typechecks but `h` does not, even though `h` is just an eta- expanded version of `f`: {{{ Printf.hs:9:10: No instance for (Printf t0) arising from a use of ‘printf’ The type variable ‘t0’ is ambiguous Note: there are several potential instances: instance Printf t => Printf (Char -> t) -- Defined in ‘PrintfLib’ instance Printf [Char] -- Defined in ‘PrintfLib’ In the expression: printf "foo %c" [|| "" ||] }}} What is going on? Here's the deal * To run the splice, GHC must solve any constraints that arise form the expression `(printf "foo %c" ...)`. * Since `printf` is overloaded, and overloaded on its result type, the type needed by the context of the splice is what determines which instance of `Printf` is needed. * In `f` the context needs `Char -> String`, and so the call to `printf` must have type `TExpr (Char -> String)`, so we get the constraint `Printf (Char -> String)` which we can solve. * But in `h` the rule for application tries to ''infer'' a type for the splice. So the context for the call just says `TExp t0` for some unification variable `t0`; and that leads to the insoluble constraint. You may say that GHC should be cleverer, and push that type signature information into the application. And perhaps it should. But you can never win. For example: {{{ hard x = [ $$(printf "gluk" [|| "" ||]), undefined :: Char -> String ] }}} Here the RHS of `hard` is a 2-element list. Since all elements of a list have the same type, the splice must have the same type as the second element of the list, namely `Char->String`. But seeing that would mean that we'd have to typecheck right-to-left. In general GHC tries very very hard NOT to depend on traveral order. There is no way in general to ensure that we have all the information now that constraint solving may ultimately produce. I'm not sure what to do about this. * It seldom matters, because resolving the overloading on the splice seldom depends on the result type. * When it does matter, you can fix it by giving a type signature to the splice itself. But it seems unsatisfactory. Ideas welcome. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10271> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10946: Typed hole inside typed Template Haskell bracket causes panic
by GHC 11 Jan '19

11 Jan '19

#10946: Typed hole inside typed Template Haskell bracket causes panic -------------------------------------+------------------------------------- Reporter: jstolarek | Owner: Type: bug | Status: new Priority: high | Milestone: 8.0.1 Component: Template | Version: 7.10.1 Haskell | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time Unknown/Multiple | crash Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | -------------------------------------+------------------------------------- When I say: {{{#!hs m :: a -> a m x = $$([||_||]) }}} I get: {{{ T10946.hs:47:13:ghc: panic! (the 'impossible' happened) (GHC version 7.10.1 for x86_64-unknown-linux): No skolem info: a_apJ[sk] }}} This happens both with GHC 7.10.1 and latest HEAD (e2b579e8d77357e8b36f57d15daead101586ac8e). -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10946> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[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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