Commit c940e3b9 authored by Ben Gamari's avatar Ben Gamari Committed by Ben Gamari

dmdAnal: Ensure that ExnStr flag isn't dropped inappropriately

This fixes #13977 and consequently #13615. Here an optimization in the
demand analyser was too liberal, causing us to drop the ExnStr flag and
consequently resulting in incorrect demand signatures. This manifested
as a segmentation fault in #13615 as we incorrectly concluded that an
application of catchRetry# would bottom.

Specifically, we had

    orElse' :: STM a -> STM a -> STM a
    orElse' x = catchRetry# x y
      where y = {- some action -}

Where the catchRetry# primop places a demand of <xC(S),1*C1(U)> on its
first argument. However, due to #13977 the demand analyser would assign
a demand of <C(S),1*C1(U)> on the first argument of orElse'. Note the
missing `x`.

    case orElse' bottomingAction anotherAction of { x -> Just x }

being transformed to,

    case orElse' bottomingAction anotherAction of {}

by the simplifier. This would naturally blow up when orElse' returned at
runtime, causing the segmentation fault described in #13615.

Test Plan: Validate, perhaps add a testcase

Reviewers: austin, simonpj

Reviewed By: simonpj

Subscribers: rwbarton, thomie

GHC Trac Issues: #13977, #13615

Differential Revision: https://phabricator.haskell.org/D3756
parent 0b89b2de
......@@ -1442,8 +1442,11 @@ postProcessDmdResult _ res = res
postProcessDmdEnv :: DmdShell -> DmdEnv -> DmdEnv
postProcessDmdEnv ds@(JD { sd = ss, ud = us }) env
| Abs <- us = emptyDmdEnv
| Str _ _ <- ss
, Use One _ <- us = env -- Shell is a no-op
-- In this case (postProcessDmd ds) == id; avoid a redundant rebuild
-- of the environment. Be careful, bad things will happen if this doesn't
-- match postProcessDmd (see #13977).
| Str VanStr _ <- ss
, Use One _ <- us = env
| otherwise = mapVarEnv (postProcessDmd ds) env
-- For the Absent case just discard all usage information
-- We only processed the thing at all to analyse the body
......
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