Re: [boost] [review] Multiprecision review results

Subject: Re: [boost] [review] Multiprecision review results
From: John Maddock (boost.regex_at_[hidden])
Date: 2012-08-05 04:41:18

Many thanks Jeff for managing the review!

> Performance
> --------
> Phil, the lone NO vote, lamented that the scope of the library was
> evidently beyond simple fixed-precision (e.g., 128-bit or 256-bit) 
> integral
> types, which is his desired use case. Indeed, at least in an earlier
> version of MP, expression templates seemed to give a performance penalty
> relative to using the raw back-end types; in theory, expression templates
> should give no performance hit, but compilers just may not be there yet. 
> In
> any case, several reviewers requested that the scope of the library be
> clarified up front in the documentation.

As noted in a recent post - the cost of wrapping a native type inside 
mp_number should be close to zero now *when expression templates are off*.

When ET's are turned on, this will always IMO be slower, consider an 
operator returning an expression template - it's basically returning a pair 
of references (and larger objects still for more complex expressions), where 
as say a wrapped integer is actually returning a smaller cheaper to copy 
object in that case.  So for a wrapped integer, returning the result by 
value will always win out, and that's before you even consider the cost of 
unpacking the expression template.

So, basically reserve expression templates for objects that are either large 
and/or allocate memory: I'll try to make that clearer in the docs.

~~~~

I have an open question for you all: I was planning on working on cpp_int 
performance next, and trying to improve the computation geometry use cases. 
Obviously that pushes back a potential release.  The alternative is to 
address all the issues relating to the front end first, and aim for an 
earlier first release.  Which would folks prefer?

Regards, John.