GitLab

Cheng Shao pushed to branch wip/hi-atomic at Glasgow Haskell Compiler / GHC

Commits:

c740a860

by Cheng Shao at 2026-01-19T12:44:14+01:00

compiler: monomorphize & inline withAtomicRename

This commit monomorphizes withAtomicRename to IO and return type to
(), since all its call sites are in the IO monad and discards return
value, and also marks it as INLINE since it proves to be a hot spot
and there may be stat increases otherwise.

22ca4a67

by Cheng Shao at 2026-01-19T12:44:49+01:00

compiler: write interface files atomically

This patch makes the GHC driver write the interface files atomically.
This opens up the theoretical possibility for increased parallelism in
an external build system, since they can monitor the appearance of
upstream module's interface file and speculatively start downstream
module's build without needing to wait for upstream module's codegen
pipeline to finish.

2 changed files:

compiler/GHC/Iface/Load.hs
compiler/GHC/Utils/Misc.hs

Changes:

compiler/GHC/Iface/Load.hs

@@ -66,6 +66,7 @@ import GHC.Tc.Utils.Monad
  import GHC.Utils.Binary   ( BinData(..) )
  import GHC.Utils.Error
 +import GHC.Utils.Misc
  import GHC.Utils.Outputable as Outputable
  import GHC.Utils.Panic
  import GHC.Utils.Constants (debugIsOn)
@@ -989,7 +990,8 @@ writeIface :: Logger -> Profile -> CompressionIFace -> FilePath -> ModIface -> I
  writeIface logger profile compression_level hi_file_path new_iface
      = do createDirectoryIfMissing True (takeDirectory hi_file_path)
           let printer = TraceBinIFace (debugTraceMsg logger 3)
 -         writeBinIface profile printer compression_level hi_file_path new_iface
 +         withAtomicRename hi_file_path $ \temp_path ->
 +           writeBinIface profile printer compression_level temp_path new_iface
  flagsToIfCompression :: DynFlags -> CompressionIFace
  flagsToIfCompression dflags

compiler/GHC/Utils/Misc.hs

@@ -133,7 +133,6 @@ import GHC.Stack (HasCallStack)
  import GHC.Data.List
  import Control.Monad    ( guard )
 -import Control.Monad.IO.Class ( MonadIO, liftIO )
  import System.IO.Error as IO ( isDoesNotExistError )
  import System.Directory ( doesDirectoryExist, getModificationTime, renameFile )
  import qualified System.Directory.OsPath as OsPath
@@ -1273,8 +1272,8 @@ fileHashIfExists f =
  -- and uses their modification time to skip work later,
  -- as otherwise a partially written file (e.g. due to crash or Ctrl+C)
  -- also results in a skip.
+-
 -withAtomicRename :: (MonadIO m) => FilePath -> (FilePath -> m a) -> m a
 +{-# INLINE withAtomicRename #-}
 +withAtomicRename :: FilePath -> (FilePath -> IO ()) -> IO ()
  withAtomicRename targetFile f = do
    -- The temp file must be on the same file system (mount) as the target file
    -- to result in an atomic move on most platforms.
@@ -1282,9 +1281,8 @@ withAtomicRename targetFile f = do
    -- This can still be fooled when somebody mounts a different file system
    -- at just the right time, but that is not a case we aim to cover here.
    let temp = targetFile <.> "tmp"
 -  res <- f temp
 -  liftIO $ renameFile temp targetFile
 -  return res
 +  f temp
 +  renameFile temp targetFile
  -- --------------------------------------------------------------
  -- split a string at the last character where 'pred' is True,