#8281: The impossible happened: primRepToFFIType ---------------------------------------+----------------------------------- Reporter: tibbe | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time crash | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ---------------------------------------+----------------------------------- Changes (by simonpj): * cc: simonmar (added) Comment: I see what is happening here, but need help from Simon M or anyone else. Here's what I learned: * `ThreadId#` is represented by a heap pointer; it's "rep" is `PtrRep`. But it is a primitive type; i.e. not implemented in Haskell. * The totally-undocumented flag `-XUnliftedFFITypes` seems to allow all of GHC's primitive types to be marshalled across the FFI. * These primitive types include `Array#`, `MutableArray#`, `ByteArray#` and `MutableByteArray#`, all of which also have `PtrRep`. * Marshalling these pointer types seems utterly wrong for a '''safe''' call, where GC might occur during the call, thereby moving the array. (Or maybe arrays never move? If so, this is un-documented.) * The actual crash comes in `LibFFI.hsc`, in `primRepToFFIType` which is used (via `prepForeignCall`) only by the bytecode generator, to prepare the foreign call arguments. `primRepToFFIType` barfs if it gets a `PtrRep`. * How does it work for `Array#` and friends? Because have a special shim in `ByteCodeGen.generateCCall`, which adds an offset to get past the array header, and then says `AddrRep`! Hence `primRepToFFIType` sees an `AddrRep`. * There is no such special treatment for `ThreadId#`, hence the crash. * The base-library module `GHC.Conc.Sync` has precisely the `rts_getThreadId` import as in the tiny test case above, but works(just) because it is compiled. My conclusion: * These `PtrRep` things should not be allowed in '''safe''' foreign calls. * In `primRepToFFIType` we should allow `PtrRep` However I'm not certain about the last of these because I don't understand how foreign calls work in the bytecode interpreter. Simon M: do you agree? Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8281#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler