diff options
| -rw-r--r-- | libraries/base/Data/Bits.hs | 30 |
1 files changed, 28 insertions, 2 deletions
diff --git a/libraries/base/Data/Bits.hs b/libraries/base/Data/Bits.hs index c0b52903bc..9a20b35385 100644 --- a/libraries/base/Data/Bits.hs +++ b/libraries/base/Data/Bits.hs @@ -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'#)))) @@ -328,3 +327,30 @@ 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 + } +-} + + |