From acf537f9fefa31883b7cb28ff61b837ab7f8a44a Mon Sep 17 00:00:00 2001 From: Sylvain Henry Date: Mon, 10 Aug 2020 09:20:03 +0200 Subject: Make splitAtList strict in its arguments Also fix its slightly wrong comment Metric Decrease: T5030 T12227 T12545 --- compiler/GHC/Utils/Misc.hs | 14 +++++++++----- 1 file changed, 9 insertions(+), 5 deletions(-) diff --git a/compiler/GHC/Utils/Misc.hs b/compiler/GHC/Utils/Misc.hs index 1d827e7902..50ac5dc9e8 100644 --- a/compiler/GHC/Utils/Misc.hs +++ b/compiler/GHC/Utils/Misc.hs @@ -6,6 +6,7 @@ {-# LANGUAGE BangPatterns #-} {-# LANGUAGE TupleSections #-} {-# LANGUAGE ScopedTypeVariables #-} +{-# LANGUAGE MagicHash #-} {-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-} @@ -774,15 +775,18 @@ dropList _ xs@[] = xs dropList (_:xs) (_:ys) = dropList xs ys --- | Given two lists xs=x0..xn and ys=y0..ym, return `splitAt n ys`. +-- | Given two lists xs and ys, return `splitAt (length xs) ys`. splitAtList :: [b] -> [a] -> ([a], [a]) -splitAtList xs ys = go 0 xs ys +splitAtList xs ys = go 0# xs ys where -- we are careful to avoid allocating when there are no leftover -- arguments: in this case we can return "ys" directly (cf #18535) - go _ _ [] = (ys, []) -- len(ys) <= len(xs) - go n [] bs = (take n ys, bs) -- = splitAt n ys - go n (_:as) (_:bs) = go (n+1) as bs + -- + -- We make `xs` strict because in the general case `ys` isn't `[]` so we + -- will have to evaluate `xs` anyway. + go _ !_ [] = (ys, []) -- length ys <= length xs + go n [] bs = (take (I# n) ys, bs) -- = splitAt n ys + go n (_:as) (_:bs) = go (n +# 1#) as bs -- drop from the end of a list dropTail :: Int -> [a] -> [a] -- cgit v1.2.1