Re: [Caml-list] reference cells

[Michael Ekstrand <michael@elehack.net>]

On 01/31/2011 10:29 AM, Nicolas Ojeda Bar wrote:
> I am translating an imperative language into Ocaml. Right now I am
> translating mutable variables into ref cells. Will they be optimized
> when I compile the corresponding Ocaml program? Or will they be heap
> allocated? If this is the case, I will have to translate my language
> into some sort of SSA form before, and I would like to avoid that.

Most of them will be heap-allocated.  They may stay on the minor heap if
they're short-lived, so they may never hit the major heap (and thus
incur higher GC overhead), but they'll be on the heap.

I believe the compiler has optimizations to unbox some refs,
particularly for ints and floats, but for heap-allocated structures I
believe it will produce heap allocated references.

Further, mutating references can be a relatively expensive operation. 
In order to support the major-minor heap scheme, modifying a reference
that points to a non-integral value (anything that's heap allocated)
involves checking some tables to see where the reference and referenced
object are.  This is to keep track of all references in the major heap
which have been changed to point to objects on the minor heap.  Int refs
can avoid this problem, as integers are not stored on the heap, but for
anything more complex (including boxed floats), you incur the
caml_modify overhead.  This is true not only for references, but also
for mutable fields and array cells.

- Michael