diff options
| -rw-r--r-- | libraries/base/GHC/List.lhs | 34 |
1 files changed, 20 insertions, 14 deletions
diff --git a/libraries/base/GHC/List.lhs b/libraries/base/GHC/List.lhs index 89c33d66f2..6a930333e0 100644 --- a/libraries/base/GHC/List.lhs +++ b/libraries/base/GHC/List.lhs @@ -533,23 +533,29 @@ take n _ | n <= 0 = [] take _ [] = [] take n (x:xs) = x : take (n-1) xs #else --- We always want to inline this to take advantage of a known --- length argument sign. -{-# INLINE take #-} + +{- We always want to inline this to take advantage of a known length argument +sign. Note, however, that it's important for the RULES to grab take, rather +than trying to INLINE take immediately and then letting the RULES grab +unsafeTake. Presumably the latter approach doesn't grab it early enough; it led +to an allocation regression in nofib/fft2. -} +{-# INLINE [1] take #-} take n xs | 0 < n = unsafeTake n xs | otherwise = [] -- A version of take that takes the whole list if it's given an argument less --- than 1. This does the same thing as the fold version. +-- than 1. {-# NOINLINE [1] unsafeTake #-} unsafeTake :: Int -> [a] -> [a] -unsafeTake _ [] = [] -unsafeTake 1 (x: _) = [x] -unsafeTake m (x:xs) = x : unsafeTake (m - 1) xs +unsafeTake !_ [] = [] +unsafeTake 1 (x: _) = [x] +unsafeTake m (x:xs) = x : unsafeTake (m - 1) xs {-# RULES -"unsafeTake" [~1] forall n xs . unsafeTake n xs = - build (\c nil -> foldr (takeFB c nil) (flipSeqTake nil) xs n) +"take" [~1] forall n xs . take n xs = + build (\c nil -> if 0 < n + then foldr (takeFB c nil) (flipSeqTake nil) xs n + else nil) "unsafeTakeList" [1] forall n xs . foldr (takeFB (:) []) (flipSeqTake []) xs n = unsafeTake n xs #-} @@ -558,8 +564,8 @@ unsafeTake m (x:xs) = x : unsafeTake (m - 1) xs -- Just flip seq, specialized to Int, but not inlined too early. -- It's important to force the numeric argument here, even though -- it's not used. Otherwise, take n [] doesn't force n. This is --- bad for strictness analysis and unboxing, and leads to test suite --- performance regressions. +-- bad for strictness analysis and unboxing, and leads to increased +-- allocation in T7257. flipSeqTake :: a -> Int -> a flipSeqTake x !_n = x @@ -602,9 +608,9 @@ drop n ls -- A version of drop that drops the whole list if given an argument -- less than 1 unsafeDrop :: Int -> [a] -> [a] - unsafeDrop _ [] = [] - unsafeDrop 1 (_:xs) = xs - unsafeDrop m (_:xs) = unsafeDrop (m - 1) xs + unsafeDrop !_ [] = [] + unsafeDrop 1 (_:xs) = xs + unsafeDrop m (_:xs) = unsafeDrop (m - 1) xs #endif -- | 'splitAt' @n xs@ returns a tuple where first element is @xs@ prefix of |