#8900: Strictness analysis regression --------------------------------------------+------------------------------ Reporter: tibbe | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.1-rc2 Resolution: | Keywords: Operating System: MacOS X | Architecture: x86_64 Type of failure: Runtime performance bug | (amd64) Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Comment (by simonpj): Johan, the "extra case" comes from commit 28d9a03253e8fd613667526a170b684f2017d299, whose comment says {{{ Make CaseElim a bit less aggressive See Note [Case elimination: lifted case]: We used to do case elimination if (c) the scrutinee is a variable and 'x' is used strictly But that changes case x of { _ -> error "bad" } --> error "bad" which is very puzzling if 'x' is later bound to (error "good"). Where the order of evaluation is specified (via seq or case) we should respect it. }}} And indeed that's a good point. `Note [Case binder next]` in `Simplify.lhs` is relevant here. You point out that `$wpoly_go` is strict in its last arg, so it really is "next". I wonder how picky we want to be. Consider {{{ case x of y -> g (error "uk") y }}} and suppose that g is strict in both arguments. Would it be ok to drop the case? Then we might get `error "uk"` (if `g` evaluated its first arg first) rather than any error arising from `x`? Probably yes, I guess. Indeed maybe the change is just wrong, or at least over-conservative. According to our paper "A semantics of imprecise exceptions" it is ok. I don't think anyone actually reported a bug. So maybe we should revert to the ghc 7.6 behaviour? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8900#comment:15 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler