Re: [boost] [xint] Third release is ready, requesting preliminary review

Subject: Re: [boost] [xint] Third release is ready, requesting preliminary review
From: Chad Nelson (chad.thecomfychair_at_[hidden])
Date: 2010-05-03 18:52:34

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On 05/03/2010 06:37 PM, Jeffrey Lee Hellrung, Jr. wrote:

>> [...] I just realized that, since I have the boost::move calls 
>> wrapped in a #define already, it would be easy to check on that, so
>> I did. The timings all indicate that without the boost::move() call
>> for the return values, XInt runs as if Boost.Move weren't even
>> there. So the boost::move call *is* required to get the benefits of
>> emulated move semantics, as I originally thought.
> 
> Sorry, I *definitely* should've added that, as far as I know, RVO 
> generally requires a single, simple return statement. If you have 
> multiple returns, or a return expression more complicated than
> "return foo;" or "return T(bar,zap);", then I don't think RVO
> typically kicks in. Can you confirm or deny this to be the case? If
> it is, then the move(...) calls or COW will almost certainly be
> necessary to get optimal performance.

In the cases I'm looking at, there are usually only three lines in the
function: a declaration of an integer to hold the return value, a call
to a function in the detail namespace to do the operation (with the
return value object as a parameter), and a return of the return value.
- From what you're saying, RVO should be easy for the compiler to use, but
the boost::move call on that return value is still required to get the
benefits of move semantics.

(That three-line function design is what is preventing the move
semantics from working as well as they could, I'm pretty sure. I haven't
had enough time to confirm that yet though.)
- -- 
Chad Nelson
Oak Circle Software, Inc.
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