CoreToDo is effectively a data type that describes all the kinds of optimization passes GHC does on Core. There are passes for simplification, CSE, vectorisation, etc. There is a specific case for plugins, CoreDoPluginPass :: String -> PluginPass -> CoreToDo which should be what you always use when inserting your own pass into the pipeline. The first parameter is the name of the plugin, and the second is the pass you wish to insert.

CoreM is a monad that all of the Core optimizations live and operate inside of.

A plugin's installation function (install in the above example) takes a list of CoreToDos and returns a list of CoreToDo. Before GHC begins compiling modules, it enumerates all the needed plugins you tell it to load, and runs all of their installation functions, initially on a list of passes that GHC specifies itself. After doing this for every plugin, the final list of passes is given to the optimizer, and are run by simply going over the list in order.

You should be careful with your installation function, because the list of passes you give back isn't questioned or double checked by GHC at the time of this writing. An installation function like the following:

install :: [CommandLineOption] -> [CoreToDo] -> CoreM [CoreToDo]
install _ _ = return []

is certainly valid, but also certainly not what anyone really wants.

In the last section we saw that besides a name, a CoreDoPluginPass takes a pass of type PluginPass. A PluginPass is a synonym for (ModGuts -> CoreM ModGuts). ModGuts is a type that represents the one module being compiled by GHC at any given time.

A ModGuts holds all of the module's top level bindings which we can examine. These bindings are of type CoreBind and effectively represent the binding of a name to body of code. Top-level module bindings are part of a ModGuts in the field mg_binds. Implementing a pass that manipulates the top level bindings merely needs to iterate over this field, and return a new ModGuts with an updated mg_binds field. Because this is such a common case, there is a function provided named bindsOnlyPass which lifts a function of type ([CoreBind] -> CoreM [CoreBind]) to type (ModGuts -> CoreM ModGuts).

Continuing with our example from the last section, we can write a simple plugin that just prints out the name of all the non-recursive bindings in a module it compiles:

module SayNames.Plugin (plugin) where
import GhcPlugins

plugin :: Plugin
plugin = defaultPlugin {
  installCoreToDos = install
  }

install :: [CommandLineOption] -> [CoreToDo] -> CoreM [CoreToDo]
install _ todo = do
  reinitializeGlobals
  return (CoreDoPluginPass "Say name" pass : todo)

pass :: ModGuts -> CoreM ModGuts
pass = do dflags <- getDynFlags
          bindsOnlyPass (mapM (printBind dflags))
  where printBind :: DynFlags -> CoreBind -> CoreM CoreBind
        printBind dflags bndr@(NonRec b _) = do
          putMsgS $ "Non-recursive binding named " ++ showSDoc dflags (ppr b)
          return bndr 
        printBind _ bndr = return bndr

Previously we discussed annotation pragmas (Section 9.1, “Source annotations”), which we mentioned could be used to give compiler plugins extra guidance or information. Annotations for a module can be retrieved by a plugin, but you must go through the modules ModGuts in order to get it. Because annotations can be arbitrary instances of Data and Typeable, you need to give a type annotation specifying the proper type of data to retrieve from the interface file, and you need to make sure the annotation type used by your users is the same one your plugin uses. For this reason, we advise distributing annotations as part of the package which also provides compiler plugins if possible.

To get the annotations of a single binder, you can use `getAnnotations` and specify the proper type. Here's an example that will print out the name of any top-level non-recursive binding with the SomeAnn annotation:

{-# LANGUAGE DeriveDataTypeable #-}
module SayAnnNames.Plugin (plugin, SomeAnn(..)) where
import GhcPlugins
import Control.Monad (unless)
import Data.Data

data SomeAnn = SomeAnn deriving (Data, Typeable)

plugin :: Plugin
plugin = defaultPlugin {
  installCoreToDos = install
  }

install :: [CommandLineOption] -> [CoreToDo] -> CoreM [CoreToDo]
install _ todo = do
  reinitializeGlobals
  return (CoreDoPluginPass "Say name" pass : todo)

pass :: ModGuts -> CoreM ModGuts
pass g = do
          dflags <- getDynFlags
          mapM_ (printAnn dflags g) (mg_binds g) >> return g
  where printAnn :: DynFlags -> ModGuts -> CoreBind -> CoreM CoreBind
        printAnn dflags guts bndr@(NonRec b _) = do
          anns <- annotationsOn guts b :: CoreM [SomeAnn]
          unless (null anns) $ putMsgS $ "Annotated binding found: " ++  showSDoc dflags (ppr b)
          return bndr
        printAnn _ _ bndr = return bndr

annotationsOn :: Data a => ModGuts -> CoreBndr -> CoreM [a]
annotationsOn guts bndr = do
  anns <- getAnnotations deserializeWithData guts
  return $ lookupWithDefaultUFM anns [] (varUnique bndr)

Please see the GHC API documentation for more about how to use internal APIs, etc.