[commit: ghc] master: Conservatively estimate levity in worker/wrapper (f0c1eb8)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/f0c1eb8b5640b0ec86b9fabb465ea5b841808d56/ghc

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

commit f0c1eb8b5640b0ec86b9fabb465ea5b841808d56
Author: Ben Gamari <bgamari.foss at gmail.com>
Date:   Thu May 31 07:49:55 2018 -0400

    Conservatively estimate levity in worker/wrapper
    
    The worker/wrapper transform needs to determine the levity of the result to
    determine whether it needs to introduce a lambda to preserve laziness of the
    result. For this is previously used isUnliftedType. However, this may fail in
    the presence of levity polymorphism.
    
    We now instead use isLiftedType_maybe, assuming that a lambda is needed if the
    levity of the result cannot be determined.
    
    Fixes #15186.
    
    Test Plan: make test=T15186
    
    Reviewers: simonpj, goldfire, tdammers
    
    Reviewed By: simonpj
    
    Subscribers: rwbarton, thomie, carter
    
    GHC Trac Issues: #15186
    
    Differential Revision: https://phabricator.haskell.org/D4755


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

f0c1eb8b5640b0ec86b9fabb465ea5b841808d56
 compiler/stranal/WwLib.hs                      | 12 +++++++++++-
 testsuite/tests/simplCore/should_compile/all.T |  2 +-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/compiler/stranal/WwLib.hs b/compiler/stranal/WwLib.hs
index ab0a4d1..040a6d7 100644
--- a/compiler/stranal/WwLib.hs
+++ b/compiler/stranal/WwLib.hs
@@ -269,11 +269,21 @@ mkWorkerArgs dflags args res_ty
     | otherwise
     = (args ++ [voidArgId], args ++ [voidPrimId])
     where
+      -- See "Making wrapper args" section above
       needsAValueLambda =
-        isUnliftedType res_ty
+        lifted
+        -- We may encounter a levity-polymorphic result, in which case we
+        -- conservatively assume that we have laziness that needs preservation.
+        -- See #15186.
         || not (gopt Opt_FunToThunk dflags)
            -- see Note [Protecting the last value argument]
 
+      -- Might the result be lifted?
+      lifted =
+        case isLiftedType_maybe res_ty of
+          Just lifted -> lifted
+          Nothing     -> True
+
 {-
 Note [Protecting the last value argument]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/testsuite/tests/simplCore/should_compile/all.T b/testsuite/tests/simplCore/should_compile/all.T
index a430521..5ad7dba 100644
--- a/testsuite/tests/simplCore/should_compile/all.T
+++ b/testsuite/tests/simplCore/should_compile/all.T
@@ -315,4 +315,4 @@ test('T15002', [ req_profiling ], compile, ['-O -fprof-auto -prof'])
 test('T15005', normal, compile, ['-O'])
 # we omit profiling because it affects the optimiser and makes the test fail
 test('T15056', [extra_files(['T15056a.hs']), omit_ways(['profasm'])], multimod_compile, ['T15056', '-O -v0 -ddump-rule-firings'])
-test('T15186', expect_broken(15186), multimod_compile, ['T15186', '-v0'])
+test('T15186', normal, multimod_compile, ['T15186', '-v0'])