Re: [eigen] resizing in ReturnByValue assignment

[Mark Borgerding <mark@xxxxxxxxxxxxxx>]

My concern is that there is no value added by the initial resize.  At 
best it is a NOOP.  At worst, it is a silent performance killer when 
resizes don't match exactly.

In the case of Inverse FFTs, the size of the output depend on the TYPE 
of output requested. This is NOT available during the construction of 
the proxy object.  It is not available until the evalTo is called.

I did run the regression tests with and without the initial resize. The 
same tests passed and failed in both cases.
This would seem to support my theory that most implementations of 
ReturnByValue<Derived>::evalTo resize the destination matrix.

Can anyone come up with a concrete example of why resizing is necessary 
*before* calling evalTo?


-- Mark


Benoit Jacob wrote:
> 2010/3/8 Hauke Heibel <hauke.heibel@xxxxxxxxxxxxxx>:
>   
> > Resize should be almost like a nullary op when the destination object
> > already has correct size - if it is not, this is a bug. The advantage
> > of having the assignment operator doing the resizing is that you
> > cannot forget about it when implementing ReturnByValue since you
> > simply don't need to take care of it.
> >     
I admit that the FFT ReturnByValue has a bug.  The problem is the bug 
compiles correctly and gives the right answer.  This is the hardest kind 
of bug to track down.
By removing that initial resize, the path to using ReturnByValue is less 
error-prone.

> > Regarding the confusion about rows()/cols() you have to think of
> > ReturnByValue as your actual result and thus this object should also
> > be reflecting your result's properties as e.g. rows() and cols().
> >     
>
> exactly... think for example of the case where you are using your
> returnByValue object in some expression that wants to evaluate it as a
> temporary (as is automatic when nesting ReturnByValue into a bigger
> expression)... then it needs to know the dimensions of the temporary
> matrix to create!
>   
Won't this created matrix then be the argument to evalTo?  It can assume 
responsibility for resizing.

> Benoit
>
>   
> > Hope that helps...
> >
> > - Hauke
> >
> > On Mon, Mar 8, 2010 at 2:33 PM, Mark Borgerding <mark@xxxxxxxxxxxxxx> wrote:
> >     
> > > First, the praise: I really like the ReturnByValue paradigm.  It is
> > > syntactically cleaner than using the destination-as-an-argument. It bends
> > > the rules of c++ somewhat and allows one to overload/specialize based on
> > > return type.  Very cool.
> > >
> > > Next, my confusion:
> > > Eigen::FFT fwd,inv now uses ReturnByValue but it elicits an unnecessary
> > > resize in some conditions (eliminating the advantage of the ReturnByValue
> > > proxy object). This is my bug -- I thought the rows() and cols() referred to
> > > the source matrix. They refer to the destination.   I can fix this by
> > > duplicating the logic required to decide the output size from the FFT fwd
> > > and inv functions, but I'm lazy and I don't like doing work I don't need to
> > > do.
> > > Why does DenseStorageBase::operator=( ReturnByValue ) call resize() first?
> > >   (defined in Eigen/src/Core/DenseStorageBase.h around line 300)
> > > Why not just require the ReturnByValue subclass evalTo method resize the
> > > destination as needed?
> > >
> > > Removing the resize also simplifies the implementation slightly by
> > > eliminating the need for rows() and cols() to be defined in the
> > > ReturnByValue subclass.
> > >
> > > Am I missing something?
> > >
> > > -- Mark Borgerding
> > >
> > >
> > >
> > >       
> >
> >     
>
>
>