[commit: ghc] master: UniqSet: Implement unionManyUniqSets in terms of foldl' instead of foldr (deb75cb)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/deb75cbf6741d84859eb256f1773807b099ca12f/ghc

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

commit deb75cbf6741d84859eb256f1773807b099ca12f
Author: Ben Gamari <bgamari.foss at gmail.com>
Date:   Tue Jan 24 12:50:00 2017 -0500

    UniqSet: Implement unionManyUniqSets in terms of foldl' instead of foldr
    
    foldr generally isn't a good choice for folds where the result can't be
    consumed incrementally. This gives a very modest improvement in
    compiler allocations,
    ```
            -1 s.d.                -----          -0.182%
            +1 s.d.                -----          -0.050%
            Average                -----          -0.116%
    ```
    This is clearly semantics-preserving since we are constructing a set.
    
    Test Plan: Validate
    
    Reviewers: austin
    
    Subscribers: dfeuer, thomie
    
    Differential Revision: https://phabricator.haskell.org/D2965


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

deb75cbf6741d84859eb256f1773807b099ca12f
 compiler/utils/UniqSet.hs | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/compiler/utils/UniqSet.hs b/compiler/utils/UniqSet.hs
index f08fa86..6f58652 100644
--- a/compiler/utils/UniqSet.hs
+++ b/compiler/utils/UniqSet.hs
@@ -34,6 +34,7 @@ module UniqSet (
 
 import UniqFM
 import Unique
+import Data.Foldable (foldl')
 
 {-
 ************************************************************************
@@ -90,19 +91,18 @@ type UniqSet a = UniqFM a
 
 emptyUniqSet = emptyUFM
 unitUniqSet x = unitUFM x x
-mkUniqSet = foldl addOneToUniqSet emptyUniqSet
+mkUniqSet = foldl' addOneToUniqSet emptyUniqSet
 
 addOneToUniqSet set x = addToUFM set x x
 addOneToUniqSet_C f set x = addToUFM_C f set x x
-addListToUniqSet = foldl addOneToUniqSet
+addListToUniqSet = foldl' addOneToUniqSet
 
 delOneFromUniqSet = delFromUFM
 delOneFromUniqSet_Directly = delFromUFM_Directly
 delListFromUniqSet = delListFromUFM
 
 unionUniqSets = plusUFM
-unionManyUniqSets [] = emptyUniqSet
-unionManyUniqSets sets = foldr1 unionUniqSets sets
+unionManyUniqSets = foldl' (flip unionUniqSets) emptyUniqSet
 minusUniqSet = minusUFM
 intersectUniqSets = intersectUFM