Proposal #2875: remove StringRep and StringConstr

[ghc-base.git] / Data / Bits.hs

diff --git a/Data/Bits.hs b/Data/Bits.hs
index c0b5290..18c1f6d 100644 (file)
--- a/Data/Bits.hs
+++ b/Data/Bits.hs
@@ -1,4 +1,4 @@
-{-# OPTIONS_GHC -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -XNoImplicitPrelude #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Bits
@@ -95,9 +95,9 @@ class Num a => Bits a where
         question. -}
     shift             :: a -> Int -> a
 
-    x `shift`   i | i<0  = x `shiftR` (-i)
-                  | i==0 = x
-                  | i>0  = x `shiftL` i
+    x `shift`   i | i<0       = x `shiftR` (-i)
+                  | i>0       = x `shiftL` i
+                  | otherwise = x
 
     {-| @'rotate' x i@ rotates @x@ left by @i@ bits if @i@ is positive,
         or right by @-i@ bits otherwise.
@@ -109,9 +109,9 @@ class Num a => Bits a where
         question. -}
     rotate            :: a -> Int -> a
 
-    x `rotate`  i | i<0  = x `rotateR` (-i)
-                  | i==0 = x
-                  | i>0  = x `rotateL` i
+    x `rotate`  i | i<0       = x `rotateR` (-i)
+                  | i>0       = x `rotateL` i
+                  | otherwise = x
 
     {-
     -- Rotation can be implemented in terms of two shifts, but care is
@@ -213,8 +213,7 @@ instance Bits Int where
         | i# >=# 0#        = I# (x# `iShiftL#` i#)
         | otherwise        = I# (x# `iShiftRA#` negateInt# i#)
 
-    -- Important for constant folding (May 2008):
-    {-# INLINE rotate #-}
+    {-# INLINE rotate #-}      -- See Note [Constant folding for rotate]
     (I# x#) `rotate` (I# i#) =
         I# (word2Int# ((x'# `uncheckedShiftL#` i'#) `or#`
                        (x'# `uncheckedShiftRL#` (wsib -# i'#))))
@@ -223,6 +222,10 @@ instance Bits Int where
         i'# = word2Int# (int2Word# i# `and#` int2Word# (wsib -# 1#))
         wsib = WORD_SIZE_IN_BITS#   {- work around preprocessor problem (??) -}
     bitSize  _             = WORD_SIZE_IN_BITS
+
+    {-# INLINE shiftR #-}
+    -- same as the default definition, but we want it inlined (#2376)
+    x `shiftR`  i = x `shift`  (-i)
 #else /* !__GLASGOW_HASKELL__ */
 
 #ifdef __HUGS__
@@ -328,3 +331,30 @@ fromInts = foldr catInt 0
 
 numInts = toInteger (maxBound::Int) - toInteger (minBound::Int) + 1
 #endif /* !__GLASGOW_HASKELL__ */
+
+{-     Note [Constant folding for rotate]
+       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The INLINE on the Int instance of rotate enables it to be constant
+folded.  For example:
+     sumU . mapU (`rotate` 3) . replicateU 10000000 $ (7 :: Int)
+goes to:
+   Main.$wfold =
+     \ (ww_sO7 :: Int#) (ww1_sOb :: Int#) ->
+       case ww1_sOb of wild_XM {
+         __DEFAULT -> Main.$wfold (+# ww_sO7 56) (+# wild_XM 1);
+         10000000 -> ww_sO7
+whereas before it was left as a call to $wrotate.
+
+All other Bits instances seem to inline well enough on their
+own to enable constant folding; for example 'shift':
+     sumU . mapU (`shift` 3) . replicateU 10000000 $ (7 :: Int)
+ goes to:
+     Main.$wfold =
+       \ (ww_sOb :: Int#) (ww1_sOf :: Int#) ->
+         case ww1_sOf of wild_XM {
+           __DEFAULT -> Main.$wfold (+# ww_sOb 56) (+# wild_XM 1);
+           10000000 -> ww_sOb
+         }
+-} 
+     
+