[commit: ghc] master: Check for singletons when creating Bag/OrdList from a list. (18cb4f5)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/18cb4f5e1b88aef7770446a354bfcc1e0a075e89/ghc

>---------------------------------------------------------------

commit 18cb4f5e1b88aef7770446a354bfcc1e0a075e89
Author: klebinger.andreas at gmx.at <klebinger.andreas at gmx.at>
Date:   Sat Jun 2 21:18:19 2018 -0400

    Check for singletons when creating Bag/OrdList from a list.
    
    This gives us `One x` instead of `Many (x : [])` reducing overhead.
    For compiling spectral/simple with -O0 difference was ~ -0.05%
    allocations.
    
    The only drawback is that something like toOL (x:panic "") will now
    panic.  But that seems like a reasonable tradeoff.
    
    Test Plan: ci, looking at +RTS -s
    
    Reviewers: bgamari, jmct
    
    Reviewed By: bgamari
    
    Subscribers: jmct, rwbarton, thomie, carter
    
    Differential Revision: https://phabricator.haskell.org/D4770


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18cb4f5e1b88aef7770446a354bfcc1e0a075e89
 compiler/utils/Bag.hs     | 1 +
 compiler/utils/OrdList.hs | 1 +
 2 files changed, 2 insertions(+)

diff --git a/compiler/utils/Bag.hs b/compiler/utils/Bag.hs
index 727d1c5..41c8039 100644
--- a/compiler/utils/Bag.hs
+++ b/compiler/utils/Bag.hs
@@ -328,6 +328,7 @@ mapAccumBagLM f s (ListBag xs)    = do { (s', xs') <- mapAccumLM f s xs
 
 listToBag :: [a] -> Bag a
 listToBag [] = EmptyBag
+listToBag [x] = UnitBag x
 listToBag vs = ListBag vs
 
 bagToList :: Bag a -> [a]
diff --git a/compiler/utils/OrdList.hs b/compiler/utils/OrdList.hs
index 081210a..a573976 100644
--- a/compiler/utils/OrdList.hs
+++ b/compiler/utils/OrdList.hs
@@ -122,4 +122,5 @@ foldlOL k z (Many xs)   = foldl k z xs
 
 toOL :: [a] -> OrdList a
 toOL [] = None
+toOL [x] = One x
 toOL xs = Many xs