[ppc] Update identifyReturn to identify based on IP == LNK

The identifyReturn was previously unused because the Macaw Discovery
performed this test inline, but some architectures have different
semantics so the identifyReturn is now used by the Discovery process.
This implements the return discovery that should be sufficient for the
PPC.

macaw-ppc/src/Data/Macaw/PPC/Identify.hs

@@ -10,6 +10,10 @@ import qualified Data.Sequence as Seq
 
 import qualified Data.Macaw.CFG as MC
 import qualified Data.Macaw.Memory as MM
+import           Data.Macaw.AbsDomain.AbsState ( AbsProcessorState
+                                               , AbsValue(..)
+                                               , transferValue
+                                               )
 
 import           Data.Macaw.SemMC.Simplify ( simplifyValue )
 import           Data.Macaw.PPC.Arch
@@ -37,9 +41,20 @@ identifyCall _ mem stmts0 rs
       Just (Seq.fromList stmts0, retAddr)
   | otherwise = Nothing
 
-identifyReturn :: (PPCArchConstraints ppc)
-               => proxy ppc
+
+-- | Called to determine if the instruction sequence contains a return
+-- from the current function.
+--
+-- An instruction executing a return from a function will place the
+-- Macaw 'ReturnAddr' value (placed in the LNK register by
+-- 'mkInitialAbsState') into the instruction pointer.
+identifyReturn :: (PPCArchConstraints ppc) =>
+                  proxy ppc
                -> [MC.Stmt ppc ids]
                -> MC.RegState (MC.ArchReg ppc) (MC.Value ppc ids)
-               -> Maybe [MC.Stmt ppc ids]
-identifyReturn _ = undefined
+               -> AbsProcessorState (MC.ArchReg ppc) ids
+               -> Maybe (Seq.Seq (MC.Stmt ppc ids))
+identifyReturn _ stmts s finalRegSt8 =
+    case transferValue finalRegSt8 (s^.MC.boundValue PPC_IP) of
+      ReturnAddr -> Just $ Seq.fromList stmts
+      _ -> Nothing