diff --git a/Pm/PmDriver.icl b/Pm/PmDriver.icl
index 261d8978fe5fcba9e97c3060223b98929c8fcfbd..f102cf6a2ae2565a0a67da387ccaee8c005a4794 100644
--- a/Pm/PmDriver.icl
+++ b/Pm/PmDriver.icl
@@ -1673,6 +1673,11 @@ OptimiseABC mdn abc_path ps
 	  abcopt = startupdir +++ DirSeparatorString +++ abcopt
 	# abcopt_path = abc_path % (0,size abc_path-4) +++ "opt.abc"
 
+	# (abc_date,ps) = accFiles (FModified abc_path) ps
+	# (abcopt_date,ps) = accFiles (FModified abcopt_path) ps
+	| abcopt_date.exists && Older_Date abc_date abcopt_date
+		= (True, ps)
+
 	# ps = showInfo (Level2 ("Optimising ABC for '" +++ mdn.mdn_name +++ "'.")) ps
 	# (exitcode,err,ps) = RunExternalCommand abcopt [abc_path,"-o",abcopt_path] startupdir ps
 	# err = if (exitcode == 0)
@@ -1686,11 +1691,16 @@ ByteCodeGen use_optimised_abc mdn abc_path ps
 	# (startupdir,ps) = getStup ps
 	  (bcgen,ps) = getCurrentBCgen ps
 	  bcgen = startupdir +++ DirSeparatorString +++ bcgen
-	# abcopt_path = abc_path % (0,size abc_path-4) +++ "opt.abc"
-	  bc_path = abc_path % (0,size abc_path-4) +++ "bc"
+	# bc_path = abc_path % (0,size abc_path-4) +++ "bc"
+	  abc_path = if use_optimised_abc (abc_path % (0,size abc_path-4) +++ "opt.abc") abc_path
+
+	# (abc_date,ps) = accFiles (FModified abc_path) ps
+	  (bc_date,ps) = accFiles (FModified bc_path) ps
+	| bc_date.exists && Older_Date abc_date bc_date
+		= (True, ps)
 
 	# ps = showInfo (Level2 ("Generating bytecode for '" +++ mdn.mdn_name +++ "'.")) ps
-	# (exitcode,err,ps) = RunExternalCommand bcgen [if use_optimised_abc abcopt_path abc_path,"-o",bc_path] startupdir ps
+	# (exitcode,err,ps) = RunExternalCommand bcgen [abc_path,"-o",bc_path] startupdir ps
 	# err = if (exitcode == 0)
 		err
 		(err ++ ["Bytecode generator failed for " +++ mdn.mdn_name +++ " (exit code: " +++ toString exitcode +++ ")"])