#11731: Simplifier: Inlining trivial let can lose sharing -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 8.1 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:D2064 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I agreed comment:18 is odd, but first why does "demand information become something we should try to keep hold on"? What, specifically, goes wrong? The reasoning here is as follows. Consider {{{ let f = \xy. x+y in ...map (f expensive).... }}} Now, in `f`'s definition `x` and `y` are both marked * `OneOcc not-in-lambda` (syntactically occurring once not inside a lambda; added by the occurrence analyser) * Demanded (added by the demand analyser) These annotations assume that `f` is fully applied But when we inline `f` at this partial application site, we get {{{ ...map (let x = expensive in \y. x+y)... }}} and now `x` jolly well shouldn't be marked as occurring not inside a lambda; nor should it be marked as strictly demanded. Hence the `zapLamIdInfo`. What I had forgotten (and has somehow never arisen) is that this ''also'' happens at `f` definition site. Maybe it doesn't matter. Back to the quesion at the beginning: why does it matter? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11731#comment:20 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler