Fix #1709: do not expose the worker for a loop-breaker

The massive 'Uni' program produced a situation in which a function that
had a worker/wrapper split was chosen as a loop breaker.  If the worker
is exposed in the interface file, then an importing module may go into
an inlining loop: see comments on TidyPgm.tidyWorker.

This patch fixes the inlining bug.  The code that gives rise to this
bizarre case is still not good (it's a bunch of implication constraints
and we are choosing a bad loop breaker) but the first thing is to fix the
bug.

It's rather hard to produce a test case!

Please merge to the 6.8 branch.

diff --git a/compiler/main/TidyPgm.lhs b/compiler/main/TidyPgm.lhs
index b9dfa03..a1a049a 100644 (file)
--- a/compiler/main/TidyPgm.lhs
+++ b/compiler/main/TidyPgm.lhs
@@ -717,9 +717,10 @@ tidyTopPair ext_ids rhs_tidy_env caf_info name' (bndr, rhs)
     bndr'   = mkVanillaGlobal name' ty' idinfo'
     ty'            = tidyTopType (idType bndr)
     rhs'    = tidyExpr rhs_tidy_env rhs
     bndr'   = mkVanillaGlobal name' ty' idinfo'
     ty'            = tidyTopType (idType bndr)
     rhs'    = tidyExpr rhs_tidy_env rhs

+    idinfo  = idInfo bndr

     idinfo' = tidyTopIdInfo rhs_tidy_env (isJust maybe_external)
     idinfo' = tidyTopIdInfo rhs_tidy_env (isJust maybe_external)

-                           (idInfo bndr) unfold_info arity
-                           caf_info
+                           idinfo unfold_info worker_info
+                           arity caf_info

 
     -- Expose an unfolding if ext_ids tells us to
     -- Remember that ext_ids maps an Id to a Bool: 
 
     -- Expose an unfolding if ext_ids tells us to
     -- Remember that ext_ids maps an Id to a Bool: 

@@ -728,6 +729,7 @@ tidyTopPair ext_ids rhs_tidy_env caf_info name' (bndr, rhs)
     show_unfold = maybe_external `orElse` False
     unfold_info | show_unfold = mkTopUnfolding rhs'
                | otherwise   = noUnfolding
     show_unfold = maybe_external `orElse` False
     unfold_info | show_unfold = mkTopUnfolding rhs'
                | otherwise   = noUnfolding

+    worker_info = tidyWorker rhs_tidy_env show_unfold (workerInfo idinfo)

 
     -- Usually the Id will have an accurate arity on it, because
     -- the simplifier has just run, but not always. 
 
     -- Usually the Id will have an accurate arity on it, because
     -- the simplifier has just run, but not always. 

@@ -751,7 +753,7 @@ tidyTopPair ext_ids rhs_tidy_env caf_info name' (bndr, rhs)
 --     unfoldings, which are inside Ids imported by GHCi. Ditto RULES.
 --     CoreToStg makes use of this when constructing SRTs.
 
 --     unfoldings, which are inside Ids imported by GHCi. Ditto RULES.
 --     CoreToStg makes use of this when constructing SRTs.
 

-tidyTopIdInfo tidy_env is_external idinfo unfold_info arity caf_info
+tidyTopIdInfo tidy_env is_external idinfo unfold_info worker_info arity caf_info

   | not is_external    -- For internal Ids (not externally visible)
   = vanillaIdInfo      -- we only need enough info for code generation
                        -- Arity and strictness info are enough;
   | not is_external    -- For internal Ids (not externally visible)
   = vanillaIdInfo      -- we only need enough info for code generation
                        -- Arity and strictness info are enough;

@@ -767,17 +769,27 @@ tidyTopIdInfo tidy_env is_external idinfo unfold_info arity caf_info
        `setAllStrictnessInfo` newStrictnessInfo idinfo
        `setInlinePragInfo`    inlinePragInfo idinfo
        `setUnfoldingInfo`     unfold_info
        `setAllStrictnessInfo` newStrictnessInfo idinfo
        `setInlinePragInfo`    inlinePragInfo idinfo
        `setUnfoldingInfo`     unfold_info

-       `setWorkerInfo`        tidyWorker tidy_env (workerInfo idinfo)
+       `setWorkerInfo`        worker_info

                -- NB: we throw away the Rules
                -- They have already been extracted by findExternalRules
 
 
 
 ------------  Worker  --------------
                -- NB: we throw away the Rules
                -- They have already been extracted by findExternalRules
 
 
 
 ------------  Worker  --------------

-tidyWorker tidy_env (HasWorker work_id wrap_arity) 
-  = HasWorker (tidyVarOcc tidy_env work_id) wrap_arity
-tidyWorker tidy_env other
+tidyWorker tidy_env show_unfold NoWorker

   = NoWorker
   = NoWorker

+tidyWorker tidy_env show_unfold (HasWorker work_id wrap_arity) 
+  | show_unfold = HasWorker (tidyVarOcc tidy_env work_id) wrap_arity
+  | otherwise   = WARN( True, ppr work_id ) NoWorker
+    -- NB: do *not* expose the worker if show_unfold is off,
+    --     because that means this thing is a loop breaker or
+    --     marked NOINLINE or something like that
+    -- This is important: if you expose the worker for a loop-breaker
+    -- then you can make the simplifier go into an infinite loop, because
+    -- in effect the unfolding is exposed.  See Trac #1709
+    -- 
+    -- Mind you, it probably should not be w/w'd in the first place; 
+    -- hence the WARN

 \end{code}
 
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