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

Subject: Re: [boost] [proto] transforms && rvalue refs
From: troy d. straszheim (troy_at_[hidden])
Date: 2009-03-30 14:24:28

Eric Niebler wrote:
> 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?
> 

Yup.

>> 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?
> 

That's definitely where I'm looking to go... I've been using proto to 
organize things and putting off the fancier stuff until the basics are 
in place.  I just wanted to see if this rval thing would work.  Thanks 
for the help here.

-t