#7542: GHC doesn't optimize (strict) composition with id --------------------------------------------+------------------------------ Reporter: shachaf | Owner: simonpj Type: bug | Status: new Priority: normal | Milestone: 7.8.1 Component: Compiler | Version: 7.6.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime performance bug | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Comment (by nomeata): Tracing this back I found the `Note [Dealing with bottom]` in `CoreArity`, where the arity of case branches is used to calculate the arity of a case. Quote:

We ignore this "problem" (unless -fpedantic-bottoms is on), because being scrupulous would lose an important transformation for many programs. (See Trac #5587 for an example.)

So the next step would be answering: Can we distinguish between cases where this dodgy optimization is useful and required, and cases where it hurts? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7542#comment:15 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler