#16191: GHC Panic deriving unbox vector (StrictData/unbox-strict-fields interaction) -------------------------------------+------------------------------------- Reporter: dbeacham | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: Operating System: Linux | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by dbeacham): * Attachment "ghc-panic-deriving-unbox.cabal" added. cabal file -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16191 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#16191: GHC Panic deriving unbox vector (StrictData/unbox-strict-fields interaction) -------------------------------------+------------------------------------- Reporter: dbeacham | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: Operating System: Linux | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by dbeacham: Old description:

The attached program fails to compile with the following error:

{{{ ghc: panic! (the 'impossible' happened) (GHC version 8.6.3 for x86_64-unknown-linux): mkNewTyConRhs MV_V [Int#, MVector s_a7WC[sk:1] a_a7WA[sk:1], MVector s_a7WC[sk:1] a_a7WA[sk:1]] Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable pprPanic, called at compiler/iface/BuildTyCl.hs:65:27 in ghc:BuildTyCl }}}

Removing either of the -funbox-strict-fields or StrictData pragma allow the code to compile correctly, but you do need to compile with -O to get the incorrect behaviour -ghci is fine loading the module.

Although I wouldn't expect them to interact here as I don't get the same behaviour if I explicitly write out what would be implied by the flag and pragma:

{{{ data V a = V {-# UNPACK #-} !a {-# UNPACK #-} !a deriving (Show) }}}

just warnings about the unused UNPACKs.

New description: The attached program {{{ {-# LANGUAGE ExplicitForAll #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE TemplateHaskell #-} {-# LANGUAGE TypeFamilies #-} module Lib where import qualified Data.Vector.Unboxed as V import Data.Vector.Unboxed.Deriving data V a = V {-# UNPACK #-} !a {-# UNPACK #-} !a deriving (Show) derivingUnbox "V" [t| forall a. V.Unbox a => V a -> (a, a) |] [| \(V a b) -> (a, b) |] [| \(a, b) -> V a b |] }}} fails to compile with the following error: {{{ ghc: panic! (the 'impossible' happened) (GHC version 8.6.3 for x86_64-unknown-linux): mkNewTyConRhs MV_V [Int#, MVector s_a7WC[sk:1] a_a7WA[sk:1], MVector s_a7WC[sk:1] a_a7WA[sk:1]] Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable pprPanic, called at compiler/iface/BuildTyCl.hs:65:27 in ghc:BuildTyCl }}} Removing either of the -funbox-strict-fields or StrictData pragma allow the code to compile correctly, but you do need to compile with -O to get the incorrect behaviour -ghci is fine loading the module. Although I wouldn't expect them to interact here as I don't get the same behaviour if I explicitly write out what would be implied by the flag and pragma: {{{ data V a = V {-# UNPACK #-} !a {-# UNPACK #-} !a deriving (Show) }}} just warnings about the unused UNPACKs. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16191#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#16191: GHC Panic deriving unbox vector (StrictData/unbox-strict-fields interaction) -------------------------------------+------------------------------------- Reporter: dbeacham | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: Operating System: Linux | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by dbeacham): Here's a much shorter example without TH or Vector. {{{ {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE StrictData #-} {-# OPTIONS_GHC -funbox-strict-fields #-} module Lib where data V a = V a a deriving (Show) data family Test a newtype instance Test (V a) = V_V (a, a) }}} with this error: {{{ ghc: panic! (the 'impossible' happened) (GHC version 8.6.3 for x86_64-unknown-linux): applyTypeToArgs Expression: V_V @ a_a1BO dt_a1R2 dt_a1R3 Type: forall a. (a, a) -> Test (V a) Args: [TYPE: a_a1BO, dt_a1R2, dt_a1R3] Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable pprPanic, called at compiler/coreSyn/CoreUtils.hs:234:14 in ghc:CoreUtils }}} Removing either StrictData or -funbox-strict-fields makes it compile. Maybe of interest that removing StrictData but adding in explicit strictness compiles OK: {{{ {-# LANGUAGE TypeFamilies #-} {-# OPTIONS_GHC -funbox-strict-fields #-} module Lib where data V a = V !a !a deriving (Show) data family Test a newtype instance Test (V a) = V_V (a, a) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16191#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#16191: GHC Panic deriving unbox vector (StrictData/unbox-strict-fields interaction) -------------------------------------+------------------------------------- Reporter: dbeacham | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.8.1 Component: Compiler | Version: 8.6.3 Resolution: duplicate | Keywords: Operating System: Linux | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #16141 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Marge Bot ): In [changeset:"a5373c1fe172dee31e07bcb7c7f6caff1035e6ba/ghc" a5373c1/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="a5373c1fe172dee31e07bcb7c7f6caff1035e6ba" Fix bogus worker for newtypes The "worker" for a newtype is actually a function with a small (compulsory) unfolding, namely a cast. But the construction of this function was plain wrong for newtype /instances/; it cast the arguemnt to the family type rather than the representation type. This never actually bit us because, in the case of a family instance, we immediately cast the result to the family type. So we get \x. (x |> co1) |> co2 where the compositio of co1 and co2 is ill-kinded. However the optimiser (even the simple optimiser) just collapsed those casts, ignoring the mis-match in the middle, so we never saw the problem. Trac #16191 is indeed a dup of #16141; but the resaon these tickets produce Lint errors is not the unnecessary forcing; it's because of the ill-typed casts. This patch fixes the ill-typed casts, properly. I can't see a way to trigger an actual failure prior to this patch, but it's still wrong wrong wrong to have ill-typed casts, so better to get rid of them. }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16191#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler