Commit 9f29faa8 by Simon Marlow

### add foldr/build optimisation for take and replicate

```This allows take to be deforested, and improves performance of
replicate and replicateM/replicateM_.  We have a separate problem that
means expressions involving [n..m] aren't being completely optimised
because eftIntFB isn't being inlined but otherwise the results look
good.

Sadly this has invalidated a number of the nofib benchmarks which were
erroneously using take to duplicate work in a misguided attempt to
lengthen their runtimes (ToDo).```
parent fe2c41ba
 ... ... @@ -254,6 +254,7 @@ repeatFB c x = xs where xs = x `c` xs -- every element. -- It is an instance of the more general 'Data.List.genericReplicate', -- in which @n@ may be of any integral type. {-# INLINE replicate #-} replicate :: Int -> a -> [a] replicate n x = take n (repeat x) ... ... @@ -311,6 +312,23 @@ drop n (_:xs) = drop (n-1) xs splitAt n xs = (take n xs, drop n xs) #else /* hack away */ {-# RULES "take" [~1] forall n xs . take n xs = case n of I# n# -> build (\c nil -> foldr (takeFB c nil) (takeConst nil) xs n#) "takeList" [1] forall n xs . foldr (takeFB (:) []) (takeConst []) xs n = takeUInt n xs #-} {-# NOINLINE [0] takeConst #-} -- just a version of const that doesn't get inlined too early, so we -- can spot it in rules. Also we need a type sig due to the unboxed Int#. takeConst :: a -> Int# -> a takeConst x _ = x {-# NOINLINE [0] takeFB #-} takeFB :: (a -> b -> c) -> c -> a -> (Int# -> b) -> Int# -> c takeFB c n x xs m | m <=# 0# = n | otherwise = x `c` xs (m -# 1#) {-# INLINE [0] take #-} take (I# n#) xs = takeUInt n# xs -- The general code for take, below, checks n <= maxInt ... ...
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