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NNTP-Posting-Date: Thu, 30 Jun 2011 19:05:23 -0500
Date: Thu, 30 Jun 2011 17:05:12 -0700
From: Patricia Shanahan <pats@acm.org>
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Subject: Re: Why "lock" functionality is introduced for all the objects?
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On 6/30/2011 3:29 PM, KitKat wrote:
> On 30/06/2011 6:04 PM, Tom Anderson wrote:
...
>> The details are described quite clearly in the papers, but the upshot is
>> that an object is created with neither a lock nor a slot for a lock
>> pointer (and so only a two-word header), and the lock is allocated only
>> when needed, and then wired in. Some fancy footwork means that the
>> object doesn't need to grow a pointer when this happens; the header
>> remains two words, at the expense of some slight awkwardness elsewhere.
>
> Such as? I can think of only one possibility that could be even close to
> efficient: maintain an IdentityHashMap<Object,Lock> somewhere under the
> hood.

The obvious alternative is the make one of the existing words dual
purpose, either directly containing its data or containing an index to a
structure containing both the lock and the original use of the word.
That does require, in effect, a spare bit to indicate which mode the
object is in.

Patricia