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two: Optimize computation of idx2 further #530

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10 changes: 4 additions & 6 deletions Data/HashMap/Internal.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -164,7 +164,7 @@ import Data.Hashable (Hashable)
import Data.Hashable.Lifted (Hashable1, Hashable2)
import Data.HashMap.Internal.List (isPermutationBy, unorderedCompare)
import Data.Semigroup (Semigroup (..), stimesIdempotentMonoid)
import GHC.Exts (Int (..), Int#, TYPE, (==#))
import GHC.Exts (Int (..), Int#, TYPE, Word (..), (==#))
import GHC.Stack (HasCallStack)
import Prelude hiding (Foldable (..), filter, lookup, map,
pred)
Expand Down Expand Up @@ -978,11 +978,9 @@ two = go
ary <- A.unsafeFreeze mary
return $ BitmapIndexed (bp1 .|. bp2) ary
where
bp1 = mask h1 s
bp2 = mask h2 s
!(I# i1) = index h1 s
!(I# i2) = index h2 s
idx2 = I# (i1 Exts.<# i2)
!bp1@(W# bp1#) = mask h1 s
!bp2@(W# bp2#) = mask h2 s
idx2 = I# (bp1# `Exts.ltWord#` bp2#)
-- This way of computing idx2 saves us a branch compared to the previous approach:
--
-- idx2 | index h1 s < index h2 s = 1
Expand Down