DTrace enabled GHC does not work as a bootstrap compiler on FreeBSD
GHC 8.4.1 enables DTrace probes on FreeBSD. This currently causes issues with the using such compilers as a bootstrap compiler for building GHC itself due to the way linking of the RTS is handled. I didn't encounter any issues regarding daily use on FreeBSD but suspect it might cause problems with user defined hooks.
Current workaround is to build a version of GHC with DTrace disabled and use that as a bootstrap compiler.
A patch has been merged into HEAD. I've already tested it in combination with 8.4.2-rc1 and it worked fine.
I have an idea on how to fix this but haven't had the time to put that plan into action.