#12368: Demand Analyzer: Cunnig plan not adhered to with aborting fixpoint interation -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: bug | Status: patch Priority: low | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D2392 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Sorry to be slow. I've looked at this and I don't agree with it at all! In the current code, if we hit the iteration limit we do this: {{{ loop' n env pairs | n >= 10 = (env, lazy_fv, orig_pairs) -- Safe output }}} So we return an un-decorated binding `orig_pairs` but (wrongly) an extended environment `env`. We should just return the un-extended environment! (If a variable isn't in the environment it's treated as having `topSig`.) Simple. Well, not totally simple. There is a nasty corner case, when we have nested recursive bindings: {{{ f x = ...let g y = ...y... in ... }}} Suppose that * on the first `f` iteration we find a fixpoint for `g` which we attach to it. * But on the second `f` iteration we fail to find a fixpoint for `g`. Then we should revert to `topSig`, not to the result of the first iteration. Conclusion: if the fixpoint limit is reached: * return an environment that simply does not mention the new binders * set all the binders to no-strictness-at-all Finally, the `loop` function in `dmdFix` would be much better if it took a `SigEnv` rather than an `AnalEnv`, wouldn't it? Just a simple refactoring; but the `ae_sigs` field is the only bit that varies. OK Joachim? Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12368#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler