Data.Macaw.PPC: finish block when link register equals the IP

To ensure that PC-relative reads are classified properly, the disassembler
needs to (prematurely) end a block when the IP is the same as the link register.
Normally, a `bl` instruction to the next address is not considered
a terminal statement (contrary to what is described in the previous commit).

Without this change, a pc-relative load followed by a direct jump
would be erroneously classifed as a function call (that returns to
where the pc-relative load happens), since the call classifier defines
a call as any jump where the link register has been modified.

This change handles this exact pattern by ensuring that the link
register update ends the block, preventing subsequent direct jumps from
deciding that the link register has been modified and classifying them
as calls.

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

@@ -43,6 +43,7 @@ import qualified SemMC.Architecture.PPC as SP
 import           Data.Macaw.SemMC.Generator
 import           Data.Macaw.SemMC.Simplify ( simplifyValue )
 import           Data.Macaw.PPC.Arch ( PPCArchConstraints )
+import           Data.Macaw.PPC.PPCReg ( PPCReg(..) )
 
 -- | Read one instruction from the 'MM.Memory' at the given segmented offset.
 --
@@ -152,16 +153,20 @@ disassembleBlock lookupSemantics gs curIPAddr blockOff maxOffset = do
                   , Just simplifiedIP <- simplifyValue v
                   , simplifiedIP == nextIPVal
                   , nextIPOffset < maxOffset
-                  , Just nextIPSegAddr <- MM.incSegmentOff curIPAddr (fromIntegral bytesRead) -> do
-                      let preBlock' = (pBlockState . curIP .~ simplifiedIP) preBlock
-                      let gs2 = GenState { assignIdGen = assignIdGen gs
-                                         , _blockState = preBlock'
-                                         , genAddr = nextIPSegAddr
-                                         , genRegUpdates = MapF.empty
-                                         , appCache = appCache gs
-                                         , _blockStateSnapshot = preBlock' ^. pBlockState
-                                         }
-                      disassembleBlock lookupSemantics gs2 nextIPSegAddr (blockOff + 4) maxOffset
+                  , Just nextIPSegAddr <- MM.incSegmentOff curIPAddr (fromIntegral bytesRead) -> 
+                      case simplifyValue (preBlock ^. (pBlockState . boundValue PPC_LNK )) of
+                        Just simplifiedLNK | simplifiedLNK == simplifiedIP -> 
+                          return (nextIPOffset, finishBlock' preBlock FetchAndExecute)
+                        _ -> do
+                          let preBlock' = (pBlockState . curIP .~ simplifiedIP) preBlock
+                          let gs2 = GenState { assignIdGen = assignIdGen gs
+                                            , _blockState = preBlock'
+                                            , genAddr = nextIPSegAddr
+                                            , genRegUpdates = MapF.empty
+                                            , appCache = appCache gs
+                                            , _blockStateSnapshot = preBlock' ^. pBlockState
+                                            }
+                          disassembleBlock lookupSemantics gs2 nextIPSegAddr (blockOff + 4) maxOffset
 
                   | otherwise -> return (nextIPOffset, finishBlock' preBlock FetchAndExecute)
                 FinishedPartialBlock b -> return (nextIPOffset, b)