All other Bits instances seem to inline well enough on their own
to enable constant folding, e.g.
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
}
With this patch, rotate gets inlined and folded too,
sumU . mapU (`rotate` 3) . replicateU
10000000 $ (7 :: Int)
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 it was left as a call to $wrotate before.
#ifdef __GLASGOW_HASKELL__
(I# x#) .&. (I# y#) = I# (word2Int# (int2Word# x# `and#` int2Word# y#))
+
(I# x#) .|. (I# y#) = I# (word2Int# (int2Word# x# `or#` int2Word# y#))
+
(I# x#) `xor` (I# y#) = I# (word2Int# (int2Word# x# `xor#` int2Word# y#))
+
complement (I# x#) = I# (word2Int# (int2Word# x# `xor#` int2Word# (-1#)))
+
(I# x#) `shift` (I# i#)
| i# >=# 0# = I# (x# `iShiftL#` i#)
| otherwise = I# (x# `iShiftRA#` negateInt# i#)
+
+ -- Important for constant folding (May 2008):
+ {-# INLINE rotate #-}
(I# x#) `rotate` (I# i#) =
I# (word2Int# ((x'# `uncheckedShiftL#` i'#) `or#`
(x'# `uncheckedShiftRL#` (wsib -# i'#))))
- where
+ where
x'# = int2Word# x#
i'# = word2Int# (int2Word# i# `and#` int2Word# (wsib -# 1#))
wsib = WORD_SIZE_IN_BITS# {- work around preprocessor problem (??) -}