Re: [eigen] still the solve() API debate

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oh, actually we can even write a single
SolveReturnType<DecomositionType, DerivedB> class inheriting
ReturnByValue which would be used for all x = solve(b) functions.


On Thu, Sep 10, 2009 at 11:37 PM, Gael Guennebaud
<gael.guennebaud@xxxxxxxxx> wrote:
> you forgot the main advantage of sol 3 and 4 that is to be able to use
> temporary proxy object without having to declare them, e.g.:
> opt 3:  x.start(n) = A.solve(b);        // works without temporary
> opt 4: A.solve(b, x.start(n).ref());
> vs:
> VectorBlock<TypeOfVectorX, Dynamic> start_of_x = x.start(n);
> A.solve(b, &start_of_x);
> The advantage of opt 4 versus opt 3, is that it is much simpler to implement.
> Now, honestly I don't really like to use pointers for return types,
> unless they are optional. Also I don't really like to have two
> different API to do the same job.
> So why not giving a try to opt. 3 ? Indeed, actually the multiple
> matrix products are now implemented using something very very similar
> to opt 3 with a ReturnByValue inheriting MatrixBase, and they are not
> that heavy. What I mean is that all the limitations of opt.3 with a
> ReturnByValue inheriting MatrixBase already exists with the matrix
> products...  (the limitation is tha the NestByValue flag *must* be
> honored, otherwise you have a weird (currently) compilation error).
> Moreover, writing a solver using opt 3 should be less verbose than for
> the products. Basically:
> template<typename DerivedB>  LU::solve(const MatrixBase<DerivedB>& b)
> would return a LUSolver<LUType,DerivedB> class inheriting
> NestByValue<LuSolver<LUType,DerivedB>, Matrix<Scalar,XXX,XXX> > where
> the second template argument is the default evaluation matrix type.
> Then you only have to implement 3 trivial functions:
> - the ctor copying a ref to a LU and DerivedB objects,
> - the functions rows()  and cols()
> and the main template<DerivedX> evalTo(DerivedX& x) function which
> does all the job or call a function of LU, e.g.:
> return m_lu.solve(m_b, &x);
> ;)
> gael.
> On Thu, Sep 10, 2009 at 10:27 PM, Keir Mierle <mierle@xxxxxxxxx> wrote:
>> Nice writeup.
>> On Thu, Sep 10, 2009 at 12:04 PM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx>
>> wrote:
>>> Hi,
>>> we really need to make a decision here to be able to move on...
>>> *** The options ***
>>> Here are the options on the table:
>>> 1.
>>> A.solve(b, &result)
>>> Pro: simple, functional, predictable
>>> Con: not very nice API; some object that passing ptrs isn't C++-ish.
>>> 2.
>>> result = A.solve(b)  // returning a plain Matrix
>>> Pro: most natural, simple, nice API
>>> Con: the result may be copied redundantly which hurts performance.
>>> 3.
>>> result = A.solve(b) // returning a proxy object
>>> Pro: nice, natural API in this case; no performance problem
>>> Con: Unpredictable since A.solve(b) can't be used in an expression;
>>> adds more code complexity
>>> 4.
>>> A.solve(b, result.ref())
>>> Pro: combines the advantages of 1. while pleasing people who don't like
>>> pointers
>>> Con: ref() is more wordy than &; and It's still not as convenient as 2.
>>> *** My proposal ***
>>> I propose that we keep 1., where we actually put the implementation
>>> (this is how we currently do), and alongside it, also offer 2. in the
>>> API, as a trivial wrapper around 1.
>> +1
>> Simple. Avoids complicated code. Supports both use cases. RVO may be
>> solveable; can you make a small case where RVO doesn't happen? Rather
>> specific conditions must apply for RVO to happen.
>> Keir
>>> Like this:
>>> template<typename RightHandSideType, typename ResultType>
>>> void Solver::solve(const RightHandSideType& b, ResultType *result)
>>> {
>>>  /// bla bla, put the solver implementation there
>>> }
>>> template<typename RightHandSideType>
>>> PlainMatrixType solve(const RightHandSideType& b)
>>> {
>>>  PlainMatrixType m;
>>>  solve(b, &m);
>>>  return m;
>>> }
>>> Here are my arguments. Sorry if this contradicts opinions that i
>>> expressed before.
>>> I see 2 nontrivial points that I need to justify:
>>> a) why plain pointer and not .ref()
>>> b) why return by value and not ReturnByValue
>>> Let's start with b). The advantage of returning a plain matrix is
>>> obvious; the drawback is the potential performance problem. What
>>> happens in practice? There are two cases, either the result is a
>>> fixed-size matrix or it is dynamic-size. If the result is a fixed-size
>>> matrix then (i did some experiments...) it seems that the RVO works
>>> perfectly, so there's no performance issue at all. If the matrix is
>>> dynamic-size (array on the heap) then indeed the RVO doesn't seem to
>>> happen (by the way that's mysterious to me, why is that?) so we indeed
>>> pay for a redundant malloc/copy/free. But does it matter? We're
>>> talking about dynamic size, so we optimize for large sizes, where this
>>> overhead is relatively small (the solving itself has cubic complexity
>>> for matrix solve and quadratic complexity for a vector solve, so it's
>>> always bigger!) Thus, I don't really believe anymore in the
>>> performance argument against 2.
>>> Plus, we still offer 1. in the API for the user who's concerned about
>>> explicitly avoiding this potential performance issue.
>>> Now let's discuss a). Really my only argument for &result instead of
>>> result.ref() is that it's shorter to type and the result is more
>>> obvious to everybody. What would be arguments for ref() in this case?
>>> If I remember well the arguments for ref() were to honor constness in
>>> swap() and have a single, uniform syntax for output args everywhere.
>>> But there's a problem: it's heavy to write, especially in swap().
>>> We're not going to force the user to write
>>> mat.row(i).ref().swap(mat.row(j).ref());
>>> So at least swap would have to escape that uniformization... so after
>>> all, sorry, i'm afraid that the ref() idea is blocked by how heavy it
>>> makes the API :( internally, I agree that it was a very good idea.
>>> Anyway, in this particular case: A.solve(b,&result), there's no const
>>> correctness issue, and the only intrinsic advantage of ref() over a
>>> pointer is that many people don't like pointers in C++. But, here i
>>> need to be honest: i don't really understand why. They're almost the
>>> same thing as references (at least, _const_ pointers are. do you like
>>> it better if we make it a const pointer)? I understand that they don't
>>> have the same nice properties in some cases, but does any of that
>>> matter in the present use case?
>>> If you would prefer us to standardize on references instead of
>>> pointers, speak up, i don't want to impose a decision onto the rest of
>>> people here. I just thought that the advantage of using pointers here,
>>> namely that in A.solve(b, &result) it is immediately clear which one
>>> is the result, would outweigh the drawbacks (tell me if i'm missing
>>> something).
>>> Benoit

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