Add a note on GHC Flags

Author: adithyaov

dev/GhcFlags.md

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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.