On Wednesday 28 May 2003 10:32, Simon Marlow wrote:
Even in GHC where we use garbage collected memory instead of malloc, we still make use of the IO monad to control the lifetime of the allocated memory.
Thanks for that info. I'm not to clear about what the intended implementation is (if there is one) from reading the FFI spec. It seems to me that if the IO monad is used to control the lifetime you could use stack allocation quite easily. (Dunno if that would be any more efficient for the ghc rts though.) Using the garbage collected heap but not actually using normal garbage collection mechanisms seems a bit strange to me :-)
I actually thought long and hard about how to implement allocaBytes efficiently in GHC, before eventually coming up with the current scheme. The problem with using stack allocation is that stacks move around in GHC, and you need stable storage for allocaBytes because it passes the pointer to a foreign call. You could use a separate, stable, stack, but then you also need to move the stack pointer back down at the end of the alloca, and that needs to be done in an exeception-safe way, which is also expensive. The current idea is to use a pinned, but garbage-collectable allocation. The pinned object is "touched" at the end of the alloca to ensure that it cannot be garbage collected until then. We don't need an exception handler: if an exception is raised in the middle of the alloca, then the pointer to the object will be simply dropped and the memory garbage collected. Cheers, Simon