GHC's many flavours of command line flags make the code interpreting them rather involved. The following provides a brief overview of the processing of these options. Since the addition of the interactive front-end to GHC, there are two kinds of flags: static and dynamic. Static flags can only be set once on the command line. They remain the same throughout the whole GHC session (so for example you cannot change them within GHCi using :set or with OPTIONS_GHC pragma in the source code). Dynamic flags are the opposite: they can be changed in GHCi sessions using :set command or OPTIONS_GHC pragma in the source code. There are few static flags and it is likely that in the future there will be even less. Thus, you won't see many static flag references in the source code, but you will see a lot of functions that use dynamic flags.
Function parseStaticFlags :: is an entry point for parsing static flags. It is called by the main :: IO () function of GHC in ghc/Main.hs. Two global IORefs are used to parse static flags: v_opt_C_ready and v_opt_C. These are defined using GLOBAL_VAR macro from compiler/HsVersions.h. First IORef is a flag that checks whether the static flags are parsed at the right time. Initialized to False, it is set to True after the parsing is done. v_opt_C is a [String] used to store parsed flags (see addOpt and removeOpt functions).
In compiler/main/StaticFlags.hs, flagsStatic :: [Flag IO] defines a list of static flags and what actions should be taken when these flags are encountered (see Flag data type above). It also contains some helper functions to check whether particular flags have been set. Functions staticFlags :: [String] and packed_staticFlags :: [FastString] return a list of parsed command line static flags, provided that parsing has been done (checking the value of v_opt_C_ready).
They are managed by functions in compiler/main/DynFlags.hs file. Looking from the top you will find data types used to described enabled dynamic flags: DumpFlag, GeneralFlag, WarningFlag, Language, SafeHaskellMode, ExtensionFlag and finally DynFlags. Function defaultDynFlags :: Settings -> DynFlags initializes some of the flags to default values. Available dynamic flags and their respective actions are defined by dynamic_flags :: [Flag (CmdLineP DynFlags)]. Also, fWarningFlags :: [FlagSpec WarningFlag], fFlags :: [FlagSpec GeneralFlag], xFlags :: [FlagSpec ExtensionFlag] and a few more smaller functions define even more flags needed for example for language extensions, warnings and other things. These flags are descibred by the data type FlagSpec f:
type FlagSpec flag = ( String -- Flag in string form , flag -- Flag in internal form , TurnOnFlag -> DynP ()) -- Extra action to run when the flag is found -- Typically, emit a warning or error
Flags described by FlagSpec can be reversed, e.g. flags that start with -f prefix are reversed by using -fno- prefix instead.