GHC releases 8.2.1 through 8.4.3 exhibit various crashes when invoked with '-jn' where n > 1. GHCHQ's binary releases have this behavior, as well as GHCs I've cross-built on my own.

In order to reproduce this issue there must be some parallelism in the module dependency graph; I've attached a test package for easily reproducing this. Use of deriving in the test modules isn't necessary to trigger this; it merely gives the compiler some work to do. The 'hscolour' package also triggers this issue reliably.

To trigger the bad behavior, simply run:

ghc --make -jn Main.hs -o test

with n > 1 in the test-package. Running this repeatedly (removing .hi and .o files in between runs, of course), I've observed these outcomes with varying frequencies:

• Segmentation fault.
• Bus fault.
• Compiler process sleeps indefinitely.

<no location info>: error:
    ghc: panic! (the 'impossible' happened)
  (GHC version 8.4.3 for aarch64-unknown-linux):
        Binary.UserData: no put_binding_name

   ghc: internal error: MUT_VAR_CLEAN object entered!
       (GHC version 8.4.3 for aarch64_unknown_linux)
       Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
   Aborted (core dumped)

• And most strangely:

A.hs:3:1: error:
    • Kind signature on data type declaration has non-* return kind *
    • In the data declaration for ‘A’
  |
3 | data A = A
  | ^^^^^^^^^^...

I have not noticed issues with any other concurrent Haskell programs on aarch64. I'm using the NVIDIA Jetson TX2 for these tests. I have not yet tried GHC 8.0.x, 8.6.x, or HEAD. I'm having trouble reproducing this in GDB; I'll report back when I've got that working.