[Git][ghc/ghc][wip/torsten.schmits/parallel-depanal-downsweep] 2 commits: Make addToFinderCache monotonic

[Sjoerd Visscher (@trac-sjoerd_visscher)]

Sjoerd Visscher pushed to branch wip/torsten.schmits/parallel-depanal-downsweep at Glasgow Haskell Compiler / GHC


Commits:
d3ee9c49 by Sjoerd Visscher at 2024-08-29T17:00:27+02:00
Make addToFinderCache monotonic

- - - - -
04a3a9e6 by Sjoerd Visscher at 2024-08-29T17:00:28+02:00
Fix tests that call downsweep

- - - - -


3 changed files:

- compiler/GHC/Unit/Finder.hs
- testsuite/tests/ghc-api/downsweep/OldModLocation.hs
- testsuite/tests/ghc-api/downsweep/PartialDownsweep.hs


Changes:

=====================================
compiler/GHC/Unit/Finder.hs
=====================================
@@ -104,9 +104,29 @@ flushFinderCaches (FinderCache ref file_ref) ue = do
  where
   is_ext mod _ = not (isUnitEnvInstalledModule ue mod)
 
+{-
+[Note: Monotonic addToFinderCache]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+addToFinderCache is only used by functions that return the cached value
+if there is one, or by functions that always write an InstalledFound value.
+Without multithreading it is then safe to always directly write the value
+without checking the previously cached value.
+
+However, with multithreading, it is possible that another function has
+written a value into cache between the lookup and the addToFinderCache call.
+in this case we should check to not overwrite an InstalledFound with an
+InstalledNotFound.
+-}
+
 addToFinderCache :: FinderCache -> InstalledModule -> InstalledFindResult -> IO ()
 addToFinderCache (FinderCache ref _) key val =
-  atomicModifyIORef' ref $ \c -> (extendInstalledModuleEnv c key val, ())
+  atomicModifyIORef' ref $ \c ->
+    case (lookupInstalledModuleEnv c key, val) of
+      -- Don't overwrite an InstalledFound with an InstalledNotFound
+      -- See [Note Monotonic addToFinderCache]
+      (Just InstalledFound{}, InstalledNotFound{}) -> (c, ())
+      _ -> (extendInstalledModuleEnv c key val, ())
 
 removeFromFinderCache :: FinderCache -> InstalledModule -> IO ()
 removeFromFinderCache (FinderCache ref _) key =


=====================================
testsuite/tests/ghc-api/downsweep/OldModLocation.hs
=====================================
@@ -6,6 +6,7 @@ import GHC
 import GHC.Driver.Make
 import GHC.Driver.Session
 import GHC.Driver.Env
+import GHC.Types.Error (mkUnknownDiagnostic)
 import GHC.Unit.Module.Graph
 import GHC.Unit.Finder
 
@@ -47,13 +48,13 @@ main = do
 
     liftIO $ do
 
-    _emss <- downsweep hsc_env [] [] False
+    _emss <- downsweep hsc_env mkUnknownDiagnostic Nothing [] [] False
 
     flushFinderCaches (hsc_FC hsc_env) (hsc_unit_env hsc_env)
     createDirectoryIfMissing False "mydir"
     renameFile "B.hs" "mydir/B.hs"
 
-    (_, nodes) <- downsweep hsc_env [] [] False
+    (_, nodes) <- downsweep hsc_env mkUnknownDiagnostic Nothing [] [] False
 
     -- If 'checkSummaryTimestamp' were to call 'addHomeModuleToFinder' with
     -- (ms_location old_summary) like summariseFile used to instead of


=====================================
testsuite/tests/ghc-api/downsweep/PartialDownsweep.hs
=====================================
@@ -6,6 +6,7 @@
 import GHC
 import GHC.Driver.Make
 import GHC.Driver.Session
+import GHC.Types.Error (mkUnknownDiagnostic)
 import GHC.Utils.Outputable
 import GHC.Utils.Exception (ExceptionMonad)
 import GHC.Data.Bag
@@ -168,7 +169,7 @@ go label mods cnd =
     setTargets [tgt]
 
     hsc_env <- getSession
-    (_, nodes) <- liftIO $ downsweep hsc_env [] [] False
+    (_, nodes) <- liftIO $ downsweep hsc_env mkUnknownDiagnostic Nothing [] [] False
 
     it label $ cnd (mapMaybe moduleGraphNodeModSum nodes)
 



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/7dff0159f196ad7d5ac8f188d79f148b0d80676a...04a3a9e61d0cc14865b445af3220d93e15a1e29f

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/7dff0159f196ad7d5ac8f188d79f148b0d80676a...04a3a9e61d0cc14865b445af3220d93e15a1e29f
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