Re: [boost] [proto] RValue reference support?

Subject: Re: [boost] [proto] RValue reference support?
From: John Maddock (boost.regex_at_[hidden])
Date: 2011-08-12 04:35:19

>> If proto supported rvalue references then a could reuse foo()'s temporary
>> resources and maybe avoid dynamic allocation for a.
>
> I'm not actually sure that's true in the general case. foo()'s temporary
> is holding a value that is used in the computation. You can't reuse the
> memory until you are done with the value, and in the general case,
> that's not until you are done evaluating the expression. Consider how it
> works for vectors. You want to evaluate the expression as:
>
> tmp1 = foo();
> tmp2 = vector of right size
> for (int i in 0 to tmp2.size()-1)
>  tmp2[i] = x[i]*y[i]+tmp1[i]*z[i];
> swap(a, tmp2);
>
> You're not ready to throw out tmp1 until after the for loop, so you need
> tmp2.
>
> OK, in this isolated case, you actually /could/ use tmp1 as your scratch
> space:
>
>  tmp1[i] = x[i]*y[i]+tmp1[i]*z[i];
>
> But in the general case, you can't know that it's ok to stomp a value
> (tmp1) that's still in use like this. And for bigint, I'm guessing
> that's not the case.

For sure in the general case this may not be a viable optimization, so I 
would probably expect proto to ignore revalue refs in the default evaluation 
context.  Likewise the kind of fused multiply-add that you show in the 
vector case isn't available to me either.  Imagine that each arithmetic op 
is a black box atomic operation (usually an external C routine - the 
vector/matrix equivalent would be to shell out to BLAS for example), so the 
only control I have is:

* the order of operation.
*how the expression is decomposed into the available atomics.
* how many temporaries are needed to service those atomics.

Also in the real-number case, functions returning result-by-value are as 
common as gloomy bankers (!) so the example I gave is not all that unusual - 
though for sure there are better gains to be made by proto-icing the most 
common functions (the usual transcendentals etc).

I'm also able to figure out exactly when a value is used and when it's no 
longer needed, so if I also knew that it was a temporary already I could 
reuse it.

Hope this explains things better, John.