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 11:58:41

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On 05/03/2010 07:39 AM, Stewart, Robert wrote:

>> If I read your example correctly, then that's about what I was
>> thinking of: algorithms that have to start with a copy of one or
>> more of the passed-in parameters, but then need to modify it/them.
>> The division algorithm requires this, just as one example off the
>> top of my head.
> 
> If your algorithm requires a copy, then COW should make it slower 
> because it will first manipulate the reference count and because you 
> must query in each mutating operation whether to make a unique copy.

That depends on how expensive a deep-copy operation is. Making a deep
copy of an xint::integer can be a very expensive proposition, compared
to just manipulating some pointers, enough to more than offset the
overhead of the reference counting stuff.

Deep-copying a 2048-bit integer, for example, requires copying something
like 76 bytes (the exact number will vary by platform), as well as an
allocation. And 2048-bit integers are small for a lot of uses that the
library will be put to.
- -- 
Chad Nelson
Oak Circle Software, Inc.
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