#16284: Abortion of fixed-point iteration in Demand Analyser discards sound results -------------------------------------+------------------------------------- Reporter: sgraf | Owner: (none) Type: task | Status: new Priority: normal | Milestone: ⊥ Component: Compiler | Version: 8.6.3 Keywords: | Operating System: Unknown/Multiple DemandAnalysis | Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider the following program: {{{#!hs -- Extracted from T4903 module Foo where data Tree = Bin Tree Tree tree :: Tree tree = Bin tree tree eq :: Tree -> Bool eq (Bin l r) = eq l && eq r }}} `eq` amounts to the following Core: {{{#!hs eq = \ (ds_d20O :: Tree) -> case ds_d20O of { Bin l_aY8 r_aY9 -> case eq l_aY8 of { False -> False; True -> eq r_aY9 } } }}} It clearly diverges. That's also what pure strictness/termination/CPR analysis would find out. But because usage analysis can't find out in finite time (and that's fine) that `eq` uses its argument completely, we abort fixed-point iteration [1] and return `nopSig`, discarding useful and sound strictness information we found out along the way, like the fact that it diverges. This isn't hard to fix: Just track which parts of the signature were still unsound during abortion and only zap them. I'm only recording this for posterity and as an argument in a discussion I'll maybe start in a few months of time... [1] This is the ascending chain of demands on `ds_d20O` during fixed-point iteration: {{{ H U(1*H,1*H) U(1*U(1*H,1*H),1*U(1*H,1*H)) ... }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16284 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler