#15560: Full laziness destroys opportunities for join points -------------------------------------+------------------------------------- Reporter: AndreasK | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 (CodeGen) | Resolution: | Keywords: JoinPoints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #14287 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
I was under the impression that join points never allocate and that they rather re-use the closure of their enclosing scope
Correct. But consider {{{ f x = let j y = Just (y,y) in case x of A -> j (True, False) B -> j (False, True) C -> j (True, True) }}} Suppose branch `A` is taken. Then GHC must allocate the object `(True, False)` and jump to `j`. Then `j` must allocate `(y,y)` and return it. I suspect that there's a heap check at the beginning of the `A` branch; and then a second heap check at the start of the body of `j`. But instead we could make `j` not do a heap check (ever) and instead put on the caller (of `j`) the responsibility for making sure that there's enough heap space for `j` to do its allocation. That way we'd get one heap check, at the beginning of the `A` branch, which would ensure there was enough space for both pairs. (As you say, no closure is allocated for `j` itself, but that's an entirely different matter.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15560#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler