[Git][ghc/ghc][master] Avoid allocations in `splitAtList` (#18535)

[Marge Bot]

 Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC


Commits:
77398b67 by Sylvain Henry at 2020-08-09T21:18:34-04:00
Avoid allocations in `splitAtList` (#18535)

As suspected by @simonpj in #18535, avoiding allocations in
`GHC.Utils.Misc.splitAtList` when there are no leftover arguments is
beneficial for performance:

   On CI validate-x86_64-linux-deb9-hadrian:
    T12227 -7%
    T12545 -12.3%
    T5030  -10%
    T9872a -2%
    T9872b -2.1%
    T9872c -2.5%

Metric Decrease:
    T12227
    T12545
    T5030
    T9872a
    T9872b
    T9872c

- - - - -


1 changed file:

- compiler/GHC/Utils/Misc.hs


Changes:

=====================================
compiler/GHC/Utils/Misc.hs
=====================================
@@ -774,12 +774,15 @@ dropList _  xs@[] = xs
 dropList (_:xs) (_:ys) = dropList xs ys
 
 
+-- | Given two lists xs=x0..xn and ys=y0..ym, return `splitAt n ys`.
 splitAtList :: [b] -> [a] -> ([a], [a])
-splitAtList [] xs     = ([], xs)
-splitAtList _ xs@[]   = (xs, xs)
-splitAtList (_:xs) (y:ys) = (y:ys', ys'')
-    where
-      (ys', ys'') = splitAtList 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
 
 -- drop from the end of a list
 dropTail :: Int -> [a] -> [a]



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/77398b678aba45ba25932a39b7e8a7a31d0dd6f3

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/77398b678aba45ba25932a39b7e8a7a31d0dd6f3
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