[commit: ghc] wip/T13479: Zap Call Arity info in the simplifier (8471634)

[git at git.haskell.org]

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

On branch  : wip/T13479
Link       : http://ghc.haskell.org/trac/ghc/changeset/84716345c856667ac4aef455bc6f254289dafb39/ghc

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

commit 84716345c856667ac4aef455bc6f254289dafb39
Author: Joachim Breitner <mail at joachim-breitner.de>
Date:   Tue Mar 28 16:19:16 2017 -0400

    Zap Call Arity info in the simplifier
    
    As #13479 shows, there are corner cases where the simplifier decides to
    not eta-expand a function as much as its call arity would suggest, but
    instead transforms the code that the call arity annotation becomes a
    lie.
    
    As the call arity information is only meant to be used by the
    immediatelly following simplifier run, it makes sense to simply zap the
    information there.
    
    Differential Revision: https://phabricator.haskell.org/D3390


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

84716345c856667ac4aef455bc6f254289dafb39
 compiler/basicTypes/IdInfo.hs  | 5 ++++-
 compiler/simplCore/Simplify.hs | 7 ++++++-
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/compiler/basicTypes/IdInfo.hs b/compiler/basicTypes/IdInfo.hs
index f29fba7..bd6ec8f 100644
--- a/compiler/basicTypes/IdInfo.hs
+++ b/compiler/basicTypes/IdInfo.hs
@@ -29,7 +29,7 @@ module IdInfo (
         -- ** Zapping various forms of Info
         zapLamInfo, zapFragileInfo,
         zapDemandInfo, zapUsageInfo, zapUsageEnvInfo, zapUsedOnceInfo,
-        zapTailCallInfo,
+        zapTailCallInfo, zapCallArityInfo,
 
         -- ** The ArityInfo type
         ArityInfo,
@@ -553,6 +553,9 @@ zapTailCallInfo info
         where
           safe_occ = occ { occ_tail = NoTailCallInfo }
 
+zapCallArityInfo :: IdInfo -> IdInfo
+zapCallArityInfo info = setCallArityInfo info 0
+
 {-
 ************************************************************************
 *                                                                      *
diff --git a/compiler/simplCore/Simplify.hs b/compiler/simplCore/Simplify.hs
index e78714d..dd9b01b 100644
--- a/compiler/simplCore/Simplify.hs
+++ b/compiler/simplCore/Simplify.hs
@@ -803,7 +803,12 @@ completeBind env top_lvl is_rec mb_cont old_bndr new_bndr new_rhs
                   | otherwise
                   = info2
 
-            final_id = new_bndr `setIdInfo` info3
+              -- Zap call arity info. We have used it by now (via
+              -- `tryEtaExpandRhs`), and the simplifier can invalidate this
+              -- information, leading to broken code later (e.g. #13479)
+            info4 = zapCallArityInfo info3
+
+            final_id = new_bndr `setIdInfo` info4
 
       ; -- pprTrace "Binding" (ppr final_id <+> ppr new_unfolding) $
         return (addNonRec env final_id final_rhs) } }