[commit: ghc] master: Document determinism in shortOutIndirections (dc94914)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/dc94914eb0da985a2f006e2bd390fa1fdbafcc33/ghc

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

commit dc94914eb0da985a2f006e2bd390fa1fdbafcc33
Author: Bartosz Nitka <niteria at gmail.com>
Date:   Tue May 17 02:46:30 2016 -0700

    Document determinism in shortOutIndirections
    
    varEnvElts didn't introduce nondeterminism here. This makes it
    obvious that it could and explains why it doesn't.
    
    Test Plan: ./validate
    
    Reviewers: bgamari, simonmar, austin, simonpj
    
    Reviewed By: simonpj
    
    Subscribers: thomie
    
    Differential Revision: https://phabricator.haskell.org/D2228
    
    GHC Trac Issues: #4012


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dc94914eb0da985a2f006e2bd390fa1fdbafcc33
 compiler/simplCore/SimplCore.hs | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/compiler/simplCore/SimplCore.hs b/compiler/simplCore/SimplCore.hs
index 86eadc7..6884696 100644
--- a/compiler/simplCore/SimplCore.hs
+++ b/compiler/simplCore/SimplCore.hs
@@ -51,6 +51,7 @@ import Module
 
 import Maybes
 import UniqSupply       ( UniqSupply, mkSplitUniqSupply, splitUniqSupply )
+import UniqFM
 import Outputable
 import Control.Monad
 import qualified GHC.LanguageExtensions as LangExt
@@ -901,7 +902,10 @@ shortOutIndirections binds
   where
     ind_env            = makeIndEnv binds
     -- These exported Ids are the subjects  of the indirection-elimination
-    exp_ids            = map fst $ varEnvElts ind_env
+    exp_ids            = map fst $ nonDetEltsUFM ind_env
+      -- It's OK to use nonDetEltsUFM here because we forget the ordering
+      -- by immediately converting to a set or check if all the elements
+      -- satisfy a predicate.
     exp_id_set         = mkVarSet exp_ids
     no_need_to_flatten = all (null . ruleInfoRules . idSpecialisation) exp_ids
     binds'             = concatMap zap binds