Re: [boost] [proto] transforms && rvalue refs

Subject: Re: [boost] [proto] transforms && rvalue refs
From: Eric Niebler (eric_at_[hidden])
Date: 2009-03-30 13:51:27

troy d. straszheim wrote:
> Eric Niebler wrote:
>>
>> Using expression templates to eliminate unnecessary temporaries is a 
>> well understood idiom; it's why expression templates were invented in 
>> the first place. You don't need rvalue references at all.
>>
>> Could you say more about what you are trying to do and why you think 
>> rvalue references are needed?
>>
> 
> Maybe I've missed something stupid or have organized things so as to 
> cause myself trouble.  In an expression like
> 
>    // these contain pointers to memory out on the GPU
>    array<float> A, B(10,10), C(10,10);
> 
>    A = exp(B) * C;
> 
> When it is time to evaluate the expression, the grammar fires the 
> transform for exp(B) first.  It creates a temporary T with the same 
> dimensions as B and and a function is called to compute the result into 
> T.  Now 'T * C' happens.  In this case there is an available temporary, 
> 'T', so the result can go right in to T.  T is returned and moved in to 
> A.  Done...  at least, that's the idea.

This is how the expression would be evaluated eagerly -- that is, in the 
absence of expression templates -- right?

> So how should these transforms know whether their arguments are 
> overwritable or not?    I didn't see how to get the 'slicing through the 
> expression' approach, as in the calc examples, with contexts and 
> overloaded operator[], to work here:  each step of the calculation has 
> to get done all at once, via a function call.   But maybe it is time to 
> go back and look again.

You use expression templates when you can take advantage of 
domain-specific knowledge combined with a complete description of the 
expression tree you're trying to evaluate to perform domain-specific 
optimizations like loop unrolling. You need to ask yourself, "What 
special features of my domain abstraction can I use to optimize the 
evaluation of this expression?" If the answer is "rvalues," then that's 
not an answer that justifies the use of expression templates. If you can 
find a way to use the full expression to precompute the size of the 
result and compute it in-place, *now* you're talking. See?

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com