FYI: I've found the reason why tests/ffi/Callback.hs fails on Intel platforms: The x86 part of mkThunk in src/builtin.c is broken. It simply squeezes an additional argument below the return address (the stable pointer for calling back later), but this word is never popped. We should probably include something like __obscure_ccall_ret_code from GHC's fptools/ghc/rts/Adjustor.c, but it is a bit late now for such severe stack juggling... Cheers, S.