#1600: Optimisation: CPR the results of IO -------------------------------------+------------------------------------- Reporter: simonmar | Owner: Type: task | Status: new Priority: lowest | Milestone: 7.6.2 Component: Compiler | Version: 6.6.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: Runtime | Difficulty: Moderate (less performance bug | than a day) Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------- Comment (by nomeata): Analysing the `typecheck` divergence yields a very interesting example of how nested CPR can go wrong, which I’d like to document here. It completely wreak havoc with this innocent function: {{{ repeat :: x -> [x] repeat x = x : repeat x }}} (This requires nesting of sum types, but just imagine for this example that `[]` was a stream data type with only one constructor, if you want). The analyizer figurs out that its demand signature is `DmdType <L,U>tm2(d,tm2(d,tm2(d,d)))`, i.e. it lazily uses its argument and will, with guaranteed convergence, produce a `:` constructor, and evaluating the second parameter thereof will also converge to a `:`, and so on. The signature could actually be infinite; my code cuts them off a a certain depth. This signature is certainly correct. So it seems this is eligible to a worker-wrapper-transformation. But when we do it we end up with (using non-core patterns for clarity): {{{ $wrepeat :: x -> (# a, a, a, [a] #) $wrepeat x = case x : repeat x of (x1:x2:x3:r) -> (# x1, x2, x3, r#) repeat x :: -> [x] repeat x = case $wrepeat x of (# x1, x2, x3, r#) -> (x1:x2:x3:r) }}} and now this diverges. I’m not entirely sure who is at fault here. Since the analysis yields a correct result, most likely the w/w transformation. But that does not seem to have enough information to prevent this. Needs more thinking. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/1600#comment:20> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler