#13458: Panic with unsafeCoerce and -dcore-lint -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by dfeuer: Old description:

{{{#!hs {-# LANGUAGE MagicHash, TypeInType, ScopedTypeVariables #-} module UIOST where import GHC.Exts import Data.Kind import Unsafe.Coerce

unsafeCoerce' :: forall (r :: RuntimeRep) (a :: TYPE r) (b :: TYPE r). a -> b unsafeCoerce' = unsafeCoerce unsafeCoerce }}}

Compiling this with optimization and `-dcore-lint` produces a panic:

{{{ ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.1.20170308 for x86_64-unknown-linux): runtimeRepPrimRep typePrimRep (a_aW7 :: TYPE r_aW6) r_aW6 Call stack: CallStack (from HasCallStack): prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1191:58 in ghc:Outputable callStackDoc, called at compiler/utils/Outputable.hs:1195:37 in ghc:Outputable pprPanic, called at compiler/simplStg/RepType.hs:360:5 in ghc:RepType }}}

Using `unsafeCoerce# unsafeCoerce` produces the panic without optimization. I have not been able to reproduce the problem without `-dcore-lint`.

New description: {{{#!hs {-# LANGUAGE MagicHash, TypeInType, ScopedTypeVariables #-} module UIOST where import GHC.Exts import Data.Kind import Unsafe.Coerce unsafeCoerce' :: forall (r :: RuntimeRep) (a :: TYPE r) (b :: TYPE r). a -> b unsafeCoerce' = unsafeCoerce id }}} Compiling this with optimization and `-dcore-lint` produces a panic: {{{ ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.1.20170308 for x86_64-unknown-linux): runtimeRepPrimRep typePrimRep (a_aW7 :: TYPE r_aW6) r_aW6 Call stack: CallStack (from HasCallStack): prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1191:58 in ghc:Outputable callStackDoc, called at compiler/utils/Outputable.hs:1195:37 in ghc:Outputable pprPanic, called at compiler/simplStg/RepType.hs:360:5 in ghc:RepType }}} Using `unsafeCoerce# id` produces the panic without optimization. I have not been able to reproduce the problem without `-dcore-lint`. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13458#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13458: Panic with unsafeCoerce and -dcore-lint -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by dfeuer): Another possible approach would be to find a way to avoid the crash and just compile the bogus code like the fool said.... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13458#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13458: Panic with unsafeCoerce and -dcore-lint -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by dfeuer): One thought: we currently have a rule that no variable can have a levity- polymorphic type, but that doesn't seem quite strong enough in the presence of `unsafeCoerce`. One option might be to decree that a levity- polymorphic type can't occur in negative position in the signature of a binding. That is, I think we should likely reject the ''signature'' of `unsafeCoerce'` altogether. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13458#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13458: Panic with unsafeCoerce and -dcore-lint -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by rwbarton): If you change the type signature to `unsafeCoerce' :: Int# -> Int#` then the generated core is {{{ unsafeCoerce' = (id @ Any) `cast` (UnsafeCo representational Any Int# -> UnsafeCo representational Any Int# :: ((Any -> Any) :: *) ~R# ((Int# -> Int#) :: *)) }}} Optimizing the coercion like this seems dubious when the new coercions are ill-kinded. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13458#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13458: Panic with unsafeCoerce and -dcore-lint -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Keywords: Resolution: | LevityPolymorphism Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): I'm on it. Indeed, I believe I've fixed this, but need to rebuild to be sure. The problem is the checks from #9122 that try to make sure we're not `unsafeCoerce`ing between types with incompatible representations. But when we look to see about the representations involved here, the types don't have representations, so GHC falls over. What's annoying is that the user-written program is safe in this regard. It's the coercion optimizer that rewrites an `unsafeCoerce` from a lifted type to a lifted type (`Any -> Any` to `a -> b`) to two `unsafeCoerce`s involving `a` and `b` separately. Thus, an alternate fix would be to have the coercion optimizer look for precisely this scenario and avoid pushing the unsafe coercion through the `->`. I haven't done this for two reasons: 1. Failing the checks from #9122 (which I've never fully agreed with) is a sometimes-unavoidable consequence of a sketchy `unsafeCoerce`. The workaround if you're in this scenario is not to use `-dcore-lint`. The original program is pretty sketchy here, and so requiring that user to avoid `-dcore-lint` may be reasonable. 2. It's quite annoying (and potentially computationally expensive) to do this check in the coercion optimizer. The check is necessary only when the user is willfully misbehaving, and the better citizens shouldn't have to pay for it every time. Regardless, GHC shouldn't panic. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13458#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13458: Panic with unsafeCoerce and -dcore-lint -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Keywords: Resolution: | LevityPolymorphism Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): I'm uncomfortable with restricting the type of `unsafeCoerce#`. What if someone wants to do something really dirty, like cast a `Float#` into a `Word#` just to see the bits? This would be prohibited by comment:10. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13458#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13458: Panic with unsafeCoerce and -dcore-lint -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: goldfire Type: bug | Status: merge Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Keywords: Resolution: | LevityPolymorphism Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Compile-time | Test Case: crash or panic | typecheck/should_compile/T13458 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by goldfire): * status: new => merge * testcase: => typecheck/should_compile/T13458 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13458#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13458: Panic with unsafeCoerce and -dcore-lint -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: goldfire Type: bug | Status: closed Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Keywords: Resolution: fixed | LevityPolymorphism Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Compile-time | Test Case: crash or panic | typecheck/should_compile/T13458 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * status: merge => closed * resolution: => fixed Comment: Merged comment:12 to `ghc-8.2` as 662c64226e302009175abfa7ed196ac905990486. I'll look into the warnings. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13458#comment:15 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13458: Panic with unsafeCoerce and -dcore-lint -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Keywords: Resolution: | LevityPolymorphism Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Compile-time | Test Case: crash or panic | typecheck/should_compile/T13458 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Richard, it's your commit. Might you look? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13458#comment:17 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13458: Panic with unsafeCoerce and -dcore-lint -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Keywords: Resolution: | LevityPolymorphism Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Compile-time | Test Case: crash or panic | typecheck/should_compile/T13458 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by bgamari): I have reverted comment:12 in `ghc-8.2` in 3ebbc387e3207dc7b5743a1dc6e20df6d4152282. We'll need to re-assess this situation prior to the release. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13458#comment:19 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler