
Ublas : 
Subject: Re: [ublas] I have a problem with expression templates
From: Oswin Krause (Oswin.Krause_at_[hidden])
Date: 20120926 16:13:55
Hi,
This issue is actually bugging me a lot.
While i can understand that one wants to prevent unneccessary copying,
i don't understand why this is applied to matrixmatrix multiplication.
The issue arises quite often and is in fact a FAQ.
I think, that the better default behavior should be that prod() should
always return an intermediate result for matrixmatrix multiplication,
thus making this point obsolete. Given a high enough data dimension, the
unneccessary copy does not actually matter and since C++11 move
semantics helps us in most cases. If the copy really matters, there is
always axpy_prod.
A change in this direction would also help for another issue:
actually prod() is the wrong choice of function in the first place,
because axpy_prod (the one with the worse syntax) is a lot faster given
problems of higher dimensionality (like 20 or so). Which is totally
counter intuitive. So whil the copy dos not matter most of the time, it
also nables us to use the optimized kernels of axpy_prod (and allows to
remove a lot of code...)
So I suggest a change along the following line (temporary_matrix_type
is some scary type traits I don't know by heart right now):
template<class E1, class E2>
temporary_matrix_type prod(blas::matrix_expression<E1> const& a,
blas::matrix_expression<E2> const& b){
temporary_matrix_type result;
axpy_prod(a,b,result);
return result;
}
There is a small disadvantage in edge cases like:
Matrix B(100,100);
Matrix C(100,100);
Matrix A(3,3);
A+=subrange(prod(B,C),0,3,0,3); //most of prods work is in vain and we
have a unneccessary intermediate result.
however, in almost all cases this change should do what most people
actually need.
Greetings,
Oswin
On 20120926 21:53, Gunter Winkler wrote:
> Hello,
>
> Am 26.09.2012 19:59, schrieb Oswin Krause:
>> Hi,
>>
>> it is a bad idea to nest calls to prod(), so ublas prevents it. Your
>> workaround with the intermediate result is fine.
>>
>> Also, try ublas::prod(trans(A), ublas::prod<ublas::matrix<double>
>> >(B,
>> A))
> Let me add some additional explanation:
>
> The expression prod(A, B) does not mean "compute the product of A and
> B". It means "remember to multiply A and B on demand". The actual
> computation is deferred until next assignment.
>
> What does that mean for a nested prod()?
>
> prod(A, prod(C,D)) means that during computation of the final result
> each element of B is computed (repeatedly on the fly) from the
> expression prod(C,D). The "performance" would be a bad surprise. Thus
> uBLAS explicitly forbids nested product expressions.
>
> The solution is to use a temporary (as George said) or to define the
> result type of prod as a real matrix instead of the default
> matrix_expression (as Oswin suggested).
>
> mfg
> Gunter
>
>
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