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From: Stephen Dolan <stephen.dolan@cl.cam.ac.uk>
To: jon@ffconsultancy.com
Cc: Gabriel Scherer <gabriel.scherer@gmail.com>, 
	Pierre Chambart <pierre.chambart@laposte.net>, Goswin von Brederlow <goswin-v-b@web.de>, 
	caml users <caml-list@inria.fr>
Content-Type: text/plain; charset=UTF-8
Subject: Re: [Caml-list] Why List.map does not be implemented
 tail-recursively?

On Mon, Jun 1, 2015 at 1:02 PM, Jon Harrop <jon@ffconsultancy.com> wrote:
> What happens when the first allocation in the creation of a cyclic immutable structure incurs a minor collection?
>
> let rec xs = 0::ys and ys = 1::xs

Such allocations are handled specially. Essentially, it boils down to
having a write barrier on the final mutation that ties the knot on the
cyclic data structure. This isn't multicore-specific: see
caml_alloc_dummy and caml_update_dummy in byterun/alloc.c for the gory
details.

In multicore, this mutation needs to promote (copy to the shared heap)
the cyclic data structure if the head node has already been collected.

Stephen