Proto for Boost mailing list page: Re: [proto] Transform result_of nightmare and preserving terminal identity

Subject: Re: [proto] Transform result_of nightmare and preserving terminal identity
From: Agustín K-ballo Bergé (kaballo86_at_[hidden])
Date: 2012-10-31 16:28:47

On 31/10/2012 04:59 p.m., Eric Niebler wrote:
>> Proto expressions build and optimization times are not taken into
>> >account. There is a considerable number of expression copies made by the
>> >expression optimization that cannot be avoided by the compiler.
 >
> Expression copies ... during expression evaluation? I wonder why that's
> necessary.

Not during expression evaluation, but during expression optimization. My 
optimization transform unrolls vector expressions into their 
component-wise expression equivalents; that generates a number of 
expressions that have to be held by value and hence copied.

>> >I will
>> >continue my research by implementing a custom evaluation context that
>> >does this optimization 'on the fly', without actually modifying the
>> >expression.
 >
> Evaluation contexts are weaker than transforms. If it can't be done with
> a transform, it can't be done with a context. I can't tell from the code
> fragment what exactly you're doing with the transforms you've written,
> or whether they can be improved.

Quoting from my original mail, what I want is to replace the following 
geometric vector expression:

     p = q + r;

by

     p[0] = q[0] + r[0],
     p[1] = q[1] + r[1],
     ...,
     p[N] = q[N] + r[N],
     p;

It **can** be done with a transform and that's what I did. The optimized 
proto expression evaluates ~33% faster than the original proto expression.

I believe it can also be done with an evaluation context, that evaluates 
an expression N times with a component-wise evaluation context. A 
component-wise evaluation context would be a default context evaluation 
except that vector terminals are evaluated to one of its scalar components.

Now that my transform finally works as intended, I will clean up the 
code and upload it somewhere for you and anyone interested to see it.

Agustín K-ballo Bergé.-
http://fusionfenix.com