#4306: UNPACK can lead to unnecessary copying and wasted stack space -------------------------------------+------------------------------------- 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: -------------------------------------+------------------------------------- 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 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/4306#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler