Commit f0c1eb8b authored by Ben Gamari's avatar Ben Gamari Committed by Ben Gamari

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
parent c983a1db
......@@ -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]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
......
......@@ -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'])
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