#14068: Loopification using join points -------------------------------------+------------------------------------- Reporter: nomeata | Owner: nomeata Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: JoinPoints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #13966 #14067 | Differential Rev(s): Phab:D3811 #14827 | Wiki Page: | -------------------------------------+------------------------------------- Comment (by nomeata):
Without loopification we get a specialization for drop, with loopification we do not.
I wonder why. Are you happy to pursue, or would you like help?
It seems rather clear to me *why*: We get this code for `drop`, which does not satisfy the conditions for !SpecConstr; in particular, `drop` is not recursive (!SpecConstr only creates specializations for recursive functions): {{{ drop n xs = case xs of [] -> [] (y:ys) -> case n of I# n# -> case n# of 0# -> [] _ -> joinrec j xs n# = case xs of [] -> [] (y:ys) -> case n# of 0# -> [] _ -> call j ys (n# -# 1#) in j ys (n# -# 1#) }}} But I think I need your guidance as to what we should do about it! Should we simply enable !SpecConstr even for all non-recursive functions? (My gut feeling is that this will blow up code size and compilation times too much.) Maybe we need to float the `joinrec … in …` out out of `drop`, so that the remaining code in `drop` (which does the case analysis on `n`) is small enough to be always inlined? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14068#comment:33 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler