sebb schrieb:
On 28/09/2009, sebb <seb...@gmail.com> wrote:
On 28/09/2009, Stephen Colebourne <scolebou...@btopenworld.com> wrote:
> sebb wrote:
>
> > Where is that described? The first reference I found says that the
> > local variable is a performance optimisation; it's the volatile
> > keyword that makes it thread-safe:
> >
> >
> http://technology.amis.nl/blog/4384/the-double-checked-locking-confusion
> > also
> >
> http://jeremymanson.blogspot.com/2008/05/double-checked-locking.html
> >
> > But of course these could be wrong...
> >
>
>
http://java.sun.com/developer/technicalArticles/Interviews/bloch_effective_08_qa.html
>
> "The idiom is very fast but also complicated and delicate, so don't be
> tempted to modify it in any way"
>
That does not necessarily mean that the version without the temporary
variable is wrong.
As I see it, the fact that the variable is volatile is enough to
ensure as-sequential operation, because the write of the volatile
variable happens-before the read.
I've just discovered that the book also says:
"This code may appear a bit convoluted.
In particular, the need for the local variable <result> may be unclear ...
While not strictly necessary this may improve performance ..."
So the local variable is indeed for performance reasons - it reduces
the number of reads of the volatile variable to once in the case where
the variable has already been initialised.
Yes, this was also my understanding.
Well, obviously it is easy to get this pattern wrong or at least with
reduced performance. This makes it a good candidate for a low-level
library like [lang].
Oliver
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