Only overwrite GHC/Prim.hs and GHC/Primopwrappers.hs if they change

This avoids make doing unnecessary work after 'setup makefile'.

1 files changed, 18 insertions, 2 deletions

diff --git a/libraries/base/Setup.hs b/libraries/base/Setup.hs
index 79c60e9024..40033bdcd0 100644
--- a/libraries/base/Setup.hs
+++ b/libraries/base/Setup.hs
@@ -13,6 +13,8 @@ import Distribution.Simple.LocalBuildInfo
 import Distribution.Simple.Utils
 import System.Cmd
 import System.FilePath
+import System.Exit
+import System.Directory
 
 main :: IO ()
 main = do let hooks = defaultUserHooks {
@@ -40,12 +42,26 @@ build_primitive_sources f pd lbi uhs x
                                   "primops.txt"]
               primhs = joinPath ["GHC", "Prim.hs"]
               primopwrappers = joinPath ["GHC", "PrimopWrappers.hs"]
+              primhs_tmp = addExtension primhs "tmp"
+              primopwrappers_tmp = addExtension primopwrappers "tmp"
           maybeExit $ system (genprimopcode ++ " --make-haskell-source < "
-                           ++ primops ++ " > " ++ primhs)
+                           ++ primops ++ " > " ++ primhs_tmp)
+          maybeUpdateFile primhs_tmp primhs
           maybeExit $ system (genprimopcode ++ " --make-haskell-wrappers < "
-                           ++ primops ++ " > " ++ primopwrappers)
+                           ++ primops ++ " > " ++ primopwrappers_tmp)
+          maybeUpdateFile primopwrappers_tmp primopwrappers
       f pd lbi uhs x
 
+-- Replace a file only if the new version is different from the old.
+-- This prevents make from doing unnecessary work after we run 'setup makefile'
+maybeUpdateFile :: FilePath -> FilePath -> IO ()
+maybeUpdateFile source target = do
+  r <- rawSystem "cmp" ["-s" {-quiet-}, source, target]
+  case r of 
+    ExitSuccess   -> return ()
+    ExitFailure _ -> copyFile source target
+  removeFile source
+
 filter_modules_hook :: Hook a -> Hook a
 filter_modules_hook f pd lbi uhs x
  = let lib' = case library pd of