Re: [eigen] nesting

[Gael Guennebaud <gael.guennebaud@xxxxxxxxx>]

On Thu, Feb 4, 2010 at 11:11 PM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx> wrote:
> Just one thing that I don't follow:
>
> 2010/2/4 Gael Guennebaud <gael.guennebaud@xxxxxxxxx>:
>> Our real problem is the following:
>>
>> A*B + A1 + A2 + A3 + A4 + A5
>>
>> creates 5 temporary matrices, the result of A*B is copied 4 times....
>
> Why is it so? After A*B has evaluated into a temporary matrix, isn't
> it the same as
>
> tmp + A1 + A2 + A3 + A4 + A5
>
> ? That doesn't evaluate at every step ...!

this is the problem that Hauke is talking about: since we nest by
value, tmp is stored in the expression of tmp + A1, so sizeof(tmp+A1)
= big, so does (tmp+A1)+A2, etc. I did not check but I think that the
way it works.

on the other hand if you write:

(A*B).eval() + A1 + A2 + A3 + A4 + A5

then it is fine fine because the temporary is explicitly created on
the stack and stored by reference by the binary expressions...

gael


> Benoit
>
>>
>> gael
>>
>>>
>>> Am I missing something? I am especially afraid of being missing
>>> something about the blas_traits and how you implemented that stuff ---
>>> you know better than me.
>>>
>>> Benoit
>>>
>>>
>>>
>>>> Such an analyzer/evaluator would look like the current ei_blas_traits...
>>>> Some examples of what could be done with such an approach:
>>>>
>>>> (A + B).block() => (A.block() + B.block())
>>>>
>>>> E.noalias() += A*B + C*D;
>>>>
>>>> =>
>>>>
>>>> E.noalias() += A*B;
>>>> E.noalias() += C*D;
>>>>
>>>> This also offers more parallelization opportunities.
>>>>
>>>> Sounds good, but of course I'm really scared about compilation times... This
>>>> is why I did not talk that much about that idea so far.
>>>>
>>>> gael.
>>>>
>>>>
>>>> On Thu, Feb 4, 2010 at 2:35 PM, Hauke Heibel <hauke.heibel@xxxxxxxxxxxxxx>
>>>> wrote:
>>>>>
>>>>> Hi,
>>>>>
>>>>> While looking into the "performance degradation" issue from the forum
>>>>> I found out that it is due to temporaries - as Benoit already guessed.
>>>>>
>>>>> I am a little bit afraid, that what I once proposed, namely copying
>>>>> expressions by value, is now backfiring. The reason is that initially
>>>>> I assumed expressions to be tiny little objects with close to no copy
>>>>> costs. The issue is related to those expressions holding temporaries.
>>>>> Copying them (e.g. a product expression) means copying all the data
>>>>> including the temporary and that will happen as many times as we nest
>>>>> expressions.
>>>>>
>>>>> The only solution I can think about at the moment is the
>>>>> specialization of ei_nested for those types and to go back to nesting
>>>>> by reference for these heavy weight guys.
>>>>>
>>>>> - Hauke
>>>>
>>>>
>>>>
>>>
>>>
>>>
>>
>>
>>
>
>
>