[commit: base] master: Use IORef/atomicModifyIORef instead of STM (5cbb4db)

[Simon Marlow]

Repository : ssh://darcs.haskell.org//srv/darcs/packages/base

On branch  : master

http://hackage.haskell.org/trac/ghc/changeset/5cbb4dbf13e5a4fae922c4b5e3b6d2dd434102cb

>---------------------------------------------------------------

commit 5cbb4dbf13e5a4fae922c4b5e3b6d2dd434102cb
Author: Simon Marlow <marlowsd at gmail.com>
Date:   Tue May 1 11:57:56 2012 +0100

    Use IORef/atomicModifyIORef instead of STM
    
    Follows discussion on the libraries@ mailing list; see comments for
    rationale.

>---------------------------------------------------------------

 Data/Unique.hs |   22 ++++++++++++++--------
 1 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/Data/Unique.hs b/Data/Unique.hs
index 50bc40f..3443c9a 100644
--- a/Data/Unique.hs
+++ b/Data/Unique.hs
@@ -33,8 +33,8 @@ import System.IO.Unsafe (unsafePerformIO)
 #ifdef __GLASGOW_HASKELL__
 import GHC.Base
 import GHC.Num
-import GHC.Conc
 import Data.Typeable
+import Data.IORef
 #endif
 
 -- | An abstract unique object.  Objects of type 'Unique' may be
@@ -45,8 +45,8 @@ newtype Unique = Unique Integer deriving (Eq,Ord
 #endif
    )
 
-uniqSource :: TVar Integer
-uniqSource = unsafePerformIO (newTVarIO 0)
+uniqSource :: IORef Integer
+uniqSource = unsafePerformIO (newIORef 0)
 {-# NOINLINE uniqSource #-}
 
 -- | Creates a new object of type 'Unique'.  The value returned will
@@ -54,11 +54,9 @@ uniqSource = unsafePerformIO (newTVarIO 0)
 -- previous calls to 'newUnique'.  There is no limit on the number of
 -- times 'newUnique' may be called.
 newUnique :: IO Unique
-newUnique = atomically $ do
-  val <- readTVar uniqSource
-  let next = val+1
-  writeTVar uniqSource $! next
-  return (Unique next)
+newUnique = do
+  r <- atomicModifyIORef uniqSource $ \x -> let z = x+1 in (z,z)
+  r `seq` return (Unique r)
 
 -- SDM (18/3/2010): changed from MVar to STM.  This fixes
 --  1. there was no async exception protection
@@ -67,6 +65,14 @@ newUnique = atomically $ do
 --     suffer from adverse scheduling issues (see #3838)
 --  4. also, the STM version is faster.
 
+-- SDM (30/4/2012): changed to IORef using atomicModifyIORef.  Reasons:
+--  1. STM version could not be used inside unsafePerformIO, if it
+--     happened to be poked inside an STM transaction.
+--  2. IORef version can be used with unsafeIOToSTM inside STM,
+--     because if the transaction retries then we just get a new
+--     Unique.
+--  3. IORef version is very slightly faster.
+
 -- | Hashes a 'Unique' into an 'Int'.  Two 'Unique's may hash to the
 -- same value, although in practice this is unlikely.  The 'Int'
 -- returned makes a good hash key.