#4306: UNPACK can lead to unnecessary copying and wasted stack space
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Reporter: Olathe | Owner: (none)
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler | Version: 6.12.3
Resolution: fixed | Keywords:
Operating System: Unknown/Multiple | Architecture: Unknown/Multiple
Type of failure: Runtime | Test Case:
performance bug | simplCore/should_compile/T4306
Blocked By: | Blocking:
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Comment (by Krzysztof Gogolewski ):
In [changeset:"448b77b93b369745e9bfbc8b46a5b87bb73dd379/ghc"
448b77b9/ghc]:
{{{
#!CommitTicketReference repository="ghc"
revision="448b77b93b369745e9bfbc8b46a5b87bb73dd379"
Add RubbishLit for absent bindings of UnliftedRep
Summary:
Trac #9279 reminded us that the worker wrapper transformation copes
really badly with absent unlifted boxed bindings.
As `Note [Absent errors]` in WwLib.hs points out, we can't just use
`absentError` for unlifted bindings because there is no bottom to hide
the error in.
So instead, we synthesise a new `RubbishLit` of type
`forall (a :: TYPE 'UnliftedRep). a`, which code-gen may subsitute for
any boxed value. We choose `()`, so that there is a good chance that
the program crashes instead instead of leading to corrupt data, should
absence analysis have been too optimistic (#11126).
Reviewers: simonpj, hvr, goldfire, bgamari, simonmar
Reviewed By: simonpj
Subscribers: osa1, rwbarton, carter
GHC Trac Issues: #15627, #9279, #4306, #11126
Differential Revision: https://phabricator.haskell.org/D5153
}}}
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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/4306#comment:3
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