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diff --git a/src/Way.hs b/src/Way.hs
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+++ b/src/Way.hs
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+module Way (
+    WayUnit (..), Way, wayUnit, removeWayUnit, wayFromUnits, allWays,
+
+    vanilla, profiling, dynamic, profilingDynamic, threaded, debug, logging,
+    threadedDebug, threadedProfiling, threadedLogging, threadedDynamic,
+    threadedDebugProfiling, threadedDebugDynamic, threadedProfilingDynamic,
+    threadedLoggingDynamic, debugProfiling, debugDynamic, loggingDynamic,
+
+    wayPrefix, waySuffix, hisuf, osuf, hcsuf, obootsuf, hibootsuf, ssuf
+    ) where
+
+import Data.IntSet (IntSet)
+import qualified Data.IntSet as Set
+import Data.List
+import Data.Maybe
+import Development.Shake.Classes
+import Hadrian.Utilities
+
+-- Note: order of constructors is important for compatibility with the old build
+-- system, e.g. we want "thr_p", not "p_thr" (see instance Show Way).
+-- | A 'WayUnit' is a single way of building source code, for example with
+-- profiling enabled, or dynamically linked.
+data WayUnit = Threaded
+             | Debug
+             | Profiling
+             | Logging
+             | Dynamic
+             deriving (Bounded, Enum, Eq, Ord)
+
+-- TODO: get rid of non-derived Show instances
+instance Show WayUnit where
+    show unit = case unit of
+        Threaded  -> "thr"
+        Debug     -> "debug"
+        Profiling -> "p"
+        Logging   -> "l"
+        Dynamic   -> "dyn"
+
+instance Read WayUnit where
+    readsPrec _ s = [(unit, "") | unit <- [minBound ..], show unit == s]
+
+-- | Collection of 'WayUnit's that stands for the different ways source code
+-- is to be built.
+newtype Way = Way IntSet
+
+instance Binary Way where
+    put = put . show
+    get = fmap read get
+
+instance Hashable Way where
+    hashWithSalt salt = hashWithSalt salt . show
+
+instance NFData Way where
+    rnf (Way s) = s `seq` ()
+
+-- | Construct a 'Way' from multiple 'WayUnit's. Inverse of 'wayToUnits'.
+wayFromUnits :: [WayUnit] -> Way
+wayFromUnits = Way . Set.fromList . map fromEnum
+
+-- | Split a 'Way' into its 'WayUnit' building blocks.
+-- Inverse of 'wayFromUnits'.
+wayToUnits :: Way -> [WayUnit]
+wayToUnits (Way set) = map toEnum . Set.elems $ set
+
+-- | Check whether a 'Way' contains a certain 'WayUnit'.
+wayUnit :: WayUnit -> Way -> Bool
+wayUnit unit (Way set) = fromEnum unit `Set.member` set
+
+-- | Remove a 'WayUnit' from 'Way'.
+removeWayUnit :: WayUnit -> Way -> Way
+removeWayUnit unit (Way set) = Way . Set.delete (fromEnum unit) $ set
+
+instance Show Way where
+    show way = if null tag then "v" else tag
+      where
+        tag = intercalate "_" . map show . wayToUnits $ way
+
+instance Read Way where
+    readsPrec _ s = if s == "v" then [(vanilla, "")] else result
+      where
+        uniqueReads token = case reads token of
+            [(unit, "")] -> Just unit
+            _            -> Nothing
+        units  = map uniqueReads . words . replaceEq '_' ' ' $ s
+        result = if Nothing `elem` units
+                 then []
+                 else [(wayFromUnits . map fromJust $ units, "")]
+
+instance Eq Way where
+    Way a == Way b = a == b
+
+instance Ord Way where
+    compare (Way a) (Way b) = compare a b
+
+-- | Build default _vanilla_ way.
+vanilla :: Way
+vanilla = wayFromUnits []
+
+-- | Build with profiling.
+profiling :: Way
+profiling = wayFromUnits [Profiling]
+
+-- | Build with dynamic linking.
+dynamic :: Way
+dynamic = wayFromUnits [Dynamic]
+
+-- | Build with profiling and dynamic linking.
+profilingDynamic :: Way
+profilingDynamic = wayFromUnits [Profiling, Dynamic]
+
+-- RTS only ways below. See compiler/main/DynFlags.hs.
+-- | Build RTS with event logging.
+logging :: Way
+logging = wayFromUnits [Logging]
+
+-- | Build multithreaded RTS.
+threaded :: Way
+threaded = wayFromUnits [Threaded]
+
+-- | Build RTS with debug information.
+debug :: Way
+debug = wayFromUnits [Debug]
+
+-- | Various combinations of RTS only ways.
+threadedDebug, threadedProfiling, threadedLogging, threadedDynamic,
+    threadedDebugProfiling, threadedDebugDynamic, threadedProfilingDynamic,
+    threadedLoggingDynamic, debugProfiling, debugDynamic, loggingDynamic :: Way
+threadedDebug            = wayFromUnits [Threaded, Debug]
+threadedProfiling        = wayFromUnits [Threaded, Profiling]
+threadedLogging          = wayFromUnits [Threaded, Logging]
+threadedDynamic          = wayFromUnits [Threaded, Dynamic]
+threadedDebugProfiling   = wayFromUnits [Threaded, Debug, Profiling]
+threadedDebugDynamic     = wayFromUnits [Threaded, Debug, Dynamic]
+threadedProfilingDynamic = wayFromUnits [Threaded, Profiling, Dynamic]
+threadedLoggingDynamic   = wayFromUnits [Threaded, Logging, Dynamic]
+debugProfiling           = wayFromUnits [Debug, Profiling]
+debugDynamic             = wayFromUnits [Debug, Dynamic]
+loggingDynamic           = wayFromUnits [Logging, Dynamic]
+
+-- | All ways supported by the build system.
+allWays :: [Way]
+allWays =
+    [ vanilla, profiling, dynamic, profilingDynamic, threaded, debug, logging
+    , threadedDebug, threadedProfiling, threadedLogging, threadedDynamic
+    , threadedDebugProfiling, threadedDebugDynamic, threadedProfilingDynamic
+    , threadedLoggingDynamic, debugProfiling, debugDynamic, loggingDynamic ]
+
+wayPrefix :: Way -> String
+wayPrefix way | way == vanilla = ""
+              | otherwise      = show way ++ "_"
+
+waySuffix :: Way -> String
+waySuffix way | way == vanilla = ""
+              | otherwise      = "_" ++ show way
+
+osuf, ssuf, hisuf, hcsuf, obootsuf, hibootsuf :: Way -> String
+osuf      = (++ "o"      ) . wayPrefix
+ssuf      = (++ "s"      ) . wayPrefix
+hisuf     = (++ "hi"     ) . wayPrefix
+hcsuf     = (++ "hc"     ) . wayPrefix
+obootsuf  = (++ "o-boot" ) . wayPrefix
+hibootsuf = (++ "hi-boot") . wayPrefix