Re: [eigen] nesting

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

2010/2/4 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...

Ah OK and can't we make an exception to the rule that we nest
expressions by value? Couldn't we say: we nest by value EXCEPT plain
matrices which we nest by reference? Then nesting by value an
expression referring it would just copy a reference.

I usually say stupid things when I'm talking about references and
lifetime of temporaries :)

Benoit



>
> 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
>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>>
>>
>>
>>
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