Re: [boost] [xint] Boost.XInt formal review (concrete complaint)

Subject: Re: [boost] [xint] Boost.XInt formal review (concrete complaint)
From: Chad Nelson (chad.thecomfychair_at_[hidden])
Date: 2011-03-07 00:07:34

On Sun, 6 Mar 2011 17:43:15 -0800
"Simonson, Lucanus J" <lucanus.j.simonson_at_[hidden]> wrote:

> Chad Nelson wrote:
>
>> The only concrete complaint that people could give me about CoW was
>> that code using it couldn't be made thread-safe, which is true. [...]
>
> This is not true, I made a concrete complaint about why I don't like
> COW or move-emulation for performance reasons. [...]

Sorry, but that isn't really a complaint about CoW. It's a complaint
about pass-by-value, an admitted mistake (which I'm correcting now)
which CoW makes cheaper.

> Rather that the false dicotomy between between COW and move lets
> answer the more fundamental question, why either?  Just use
> expression templates.  Don't bother to use proto for your expression
> templates, by the way, it is very easy to do yourself in this simple
> case.

I looked into it briefly yesterday (I've used the concept behind it
before, but I didn't realize that that the name referred to it), and
that's almost certainly the way to go in the long run. Apologies if I
missed answering your message; I did see it, but some may have slipped
through unanswered.

> This whole argument misses the point of optimizing code that uses a
> bigint class.  You want to reuse the memory allocated in a bigint
> variable over and over instead of reallocating every time.

That requires writing at least a minimal memory manager. As I
understand it, the managers that come with compiler libraries are
optimized for that sort of thing these days, since C++ is geared toward
so many small heap-allocated objects. I'd rather let the compiler's
library deal with it than write more non-core code for something that
the built-in code will almost certainly be able to do better most of the
time.

> Whether you make one unneeded allocation or two is a less important
> consideration than whether you make one or zero since 2/1 is a lot
> smaller than 1/0.  If a temporary is generated by the user's code they
> have already pessimized their code.  I am sorry, but you cannot avoid
> allocation with a return by value semantic, only expression templates
> allows you to reuse the storage on the left hand side of the
> assignment operator. [...]

I'm not familiar with a use of expression templates that would allow
that. Can you point me to something explaining it?
-- 
Chad Nelson
Oak Circle Software, Inc.
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