Re: Double-Checked Locking pattern issue

"Igor Tandetnik" <>
Mon, 31 Dec 2007 09:07:53 -0500
"George" <> wrote in message

I think you mean we get performance benefits if we exchange the
instructions. I can understand that for performance consideration,
CPU may exchange the instructions. But I can not understand why in my
sample, we could have performance benefits if we exchange step 2
(constructor) and step 3 (assignment of singleton instance pointer)?
Do you have any ideas?

Note that CPU doesn't work at the level of "constructor" or "assignment
to pointer". It only sees low-level machine instructions. Since writing
to memory is expensive, it is conceivable the CPU may reorder the write
to the pointer as early as possible.

The point is not that the CPU will definitely do this because somehow it
is sure to confer some performance advantage. The point is that the CPU
_may_ decide to do this, and you have no control over it. So you have to
write code that still works correctly in the presence of such
rearrangements. Otherwise your code is buggy, and it's the worst kind of
bug - one that only manifests itself once in a blue moon, seemingly
randomly. Most of the time, your code will appear to work fine.
With best wishes,
    Igor Tandetnik

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necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
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