Optimise common cases of GHC.setProgramDynFlags

* If the package flags haven't changed, don't do initPackages (which
  might take multiple seconds in extreme cases)

* Provide a way to change the log_action without invalidating the
  summary cache.

Test Plan: validate

Reviewers: niteria, bgamari, austin, erikd, ezyang

Reviewed By: bgamari

Subscribers: mpickering, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3392

1 files changed, 20 insertions, 4 deletions

diff --git a/compiler/main/Packages.hs b/compiler/main/Packages.hs
index f938bbbda2..5db198be4b 100644
--- a/compiler/main/Packages.hs
+++ b/compiler/main/Packages.hs
@@ -500,10 +500,26 @@ getPackageConfRefs dflags = do
          | otherwise
          -> map PkgConfFile (splitSearchPath path)
 
-  return $ reverse (extraPkgConfs dflags base_conf_refs)
-  -- later packages shadow earlier ones.  extraPkgConfs
-  -- is in the opposite order to the flags on the
-  -- command line.
+  -- Apply the package DB-related flags from the command line to get the
+  -- final list of package DBs.
+  --
+  -- Notes on ordering:
+  --  * The list of flags is reversed (later ones first)
+  --  * We work with the package DB list in "left shadows right" order
+  --  * and finally reverse it at the end, to get "right shadows left"
+  --
+  return $ reverse (foldr doFlag base_conf_refs (packageDBFlags dflags))
+ where
+  doFlag (PackageDB p) dbs = p : dbs
+  doFlag NoUserPackageDB dbs = filter isNotUser dbs
+  doFlag NoGlobalPackageDB dbs = filter isNotGlobal dbs
+  doFlag ClearPackageDBs _ = []
+
+  isNotUser UserPkgConf = False
+  isNotUser _ = True
+
+  isNotGlobal GlobalPkgConf = False
+  isNotGlobal _ = True
 
 resolvePackageConfig :: DynFlags -> PkgConfRef -> IO (Maybe FilePath)
 resolvePackageConfig dflags GlobalPkgConf = return $ Just (systemPackageConfig dflags)