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From Marvin Humphrey <mar...@rectangular.com>
Subject Re: [lucy-dev] Have Clownfish::Obj extend Perl SV
Date Fri, 12 Apr 2013 00:22:54 GMT
On Thu, Apr 11, 2013 at 5:38 AM, Nick Wellnhofer <wellnhofer@aevum.de> wrote:
> Currently, we lazily allocate a Perl object when the refcount exceeds 4 even
> if we don't really need it. There's a common technique to store flags in the
> least significant bits of an aligned pointer that could be employed here.
> Assuming that SVs are always allocated on even memory addresses, we could
> use the LSB of the lucy_ref_t union to signify whether the value is a
> refcount or a pointer to a host object. So we'd have all the other bits to
> store refcounts, and we'd only have to allocate a Perl object if we really
> need it.

+1, this would be a nice improvement.

Similar to how Ruby and OCaml differentiate pointers and integers.

    http://stackoverflow.com/a/3774467

I wonder if we're going to need additional space in our objects for flags or
other purposes.  I've been thinking about dedicating a few bits to represent
whether the object was allocated using heap memory, stack memory, or some
other pool.  That would allow us to get rid of ZombieCharBuf and possibly
ZombieKeyedHash.  But then, maybe we should get rid of those some other way.

> If we allocate a Perl object then, I'd create the RV pointing to the inner
> object right away, store it in host_obj, and use it for refcounting. This
> saves a bit of memory and cycles if a Clownfish object is converted to a
> Perl object multiple times. Then we could also use sv_bless directly which
> is a bit slower than the code in S_lazy_init_host_obj but more
> forward-compatible (PL_sv_objcount will be deprecated in Perl 5.18, for
> example).

So let me see if I understand correctly.  If we store that RV, can we incref
it and pass it into Perl-space an arbitrary number of times?  If so, then it
seems worthwhile to accept the extra RAM cost of caching the RV in addition to
the inner object SV.

Marvin Humphrey

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