Adds documentation on skipTypes and mapType

compiler/ast.nim

@@ -1313,6 +1313,10 @@ proc newSons(father: PNode, length: int) =
     setLen(father.sons, length)
 
 proc skipTypes*(t: PType, kinds: TTypeKinds): PType =
+  ## Used throughout the compiler code to test whether a type tree contains or
+  ## doesn't contain a specific type/types - it is often the case that only the
+  ## last child nodes of a type tree need to be searched. This is a really hot
+  ## path within the compiler!
   result = t
   while result.kind in kinds: result = lastSon(result)
 

compiler/ccgtypes.nim

@@ -122,6 +122,7 @@ proc mapSetType(typ: PType): TCTypeKind =
   else: result = ctArray
 
 proc mapType(typ: PType): TCTypeKind = 
+  ## Maps a nimrod type to a C type
   case typ.kind
   of tyNone, tyStmt: result = ctVoid
   of tyBool: result = ctBool