GHCi "Prelude.undefined" exceptions on ARM; ByteCodeItbls.mkJumpToAddr unimplemented
A number of non-trivial operations fail in ghci on ARM (specifically armhf on both Debian experimental and Ubuntu raring). I initially noticed this as conduit's doctests failing, but reduced this to:
$ ghci GHCi, version 7.6.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> data Type a = Nothing *** Exception: Prelude.undefined
This is a singularly unhelpful exception, coming as it does without context, but I eventually tracked it down to ByteCodeItbls.mkJumpToAddr having no specific implementation for ARM and thus falling back to the default of "undefined". I've tried adding this:
+#elif arm_TARGET_ARCH +type ItblCode = Word32 +mkJumpToAddr a + = [ 0xe51ff004 -- ldr pc, [pc, #-4] # pc reads as <current insn>+8 + , fromIntegral (ptrToInt a) ]
This definitely changes the nature of the problem, so I think I'm on the right track, and I'm reasonably confident that that's the correct branch implementation; but when I actually try it in practice I get crashes (SIGSEGV/SIGILL/SIGBUS) with trashed stack traces, so I'm clearly not done yet. Friends of mine suggest that I may need to
__clear_cache or equivalent, which sounds plausible, so my current line of attack is figuring out how to glue that in; but if any actual GHC hackers can spot something obviously missing in the meantime then that would be great.