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GHC Flags

There are 2 types of GHC flags.

  1. Compiler flags
  2. RTS flags

The RTS flags are in ghc/rts/RtsFlags.c. To add an RTS flag one needs to edit this file and make some follow-up changes.

The Compiler flags are in ghc/compiler/main/DynFlags.hs. One needs to edit this file to add a dynamic flag.

FAQ:

Q: What is the difference between an RTS flag and a Compiler flag?

A Compiler flag is a flag that the compiler takes. ie. ghc. Whereas the RTS flag is the flag that the executable generated by the compiler takes.

Q: The Compiler flags seem to set some C Flags for a few options (for example "-DTRACING" when "-eventlog" is enabled, etc.). This is meta-programming. How does this work? Does a mini compilation happen everytime we invoke GHC?

We can describe what happens with eventlog which might answer the above question. WRT eventlog, the compiler has 2 versions of the RTS. One with eventlog enabled and one with eventlog disabled. The executable is linked with the RTS with the eventlog enabled when the -eventlog flag is enabled.

TODO:

We need to add 2 types of flags:

  1. A Compiler flag to enable perf counter structures.
  2. An RTS flag to enable the update of the counters.

This is similar to the eventlog approach.

  • -eventlog (Compiler flag) to enable the eventlog.
  • -l (RTS flag) to actually do the accounting.