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

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First of all I don't like the use of pointers here, because it is not
a standard c++ practice, and therefore users we'll start wondering: "
why do they use a pointer here ? maybe because this parameter is
optional ? hm does not make sense... unless if x==0, then the
computation is performed in place overwritten b ? or the solve
function creates the matrix object and then I have to delete it myself
? etc. ". OK, maybe I'm exaggerating a bit, but that's the idea.

ok, now let's summarize, ideally we want an API which satisfies the
following constraints:

a - concise (not too verbose)
b - readable
c - simple behavior
d - it should return the result so that we can directly use it in an expression
e - does not create unnecessary temporary
f - directly works with temporary proxy object
g - compliant with standard c++ practices

since I changed my mind about ReturnByValue inheriting MatrixBase, I
think opt 3 is the one which best satisfies all these constraints.

However, I (we) forgot to include in the discussion the API of the
in-place solve functions which should be as close as possible to the
generic solve().  Currently this only apply to the triangular solver,
LLT and LDLT, but I think this could also apply to PartialLU. Here the
problem is a bit different because the result is also and input and so
only option 1 and 4 could work:

opt 1: A.solveInPlace(&b_and_x);

opt 4: A.solveInPlace(b_and_x.ref());

Since opt 1 does not satisfy the constraint f and that this constraint
is particularly important for the in-place triangular solver, it
currently remains the opt 4 only.

In order to satisfy the constraint d, the solveInPlace functions could
return a reference to the inout argument. So at that point one could
do either:

Z = 2 * M * A.solveInPlace(b.ref());    // no temporary, and b is overwritten

Z = 2 * M * A.solve(b);    // one temporary but b is preserved

So yes you got me right, I'm suggesting to rely on both the
ReturnByValue<> and Reference<> mechanisms ! and since they have not
been extensively tested yet I cannot promise that we won't discover
any issue with them (especially the Reference<> one).

Also, note that a variant of opt 4, is to replace .ref() by the (more
explicit ?) pair .out() and .inout(), in which case the "InPlace"
suffix may be removed, e.g.:

Z = 2 * M * A.solve(b.inout())

but note that how subtle the difference is with:

Z = 2 * M * A.solve(b);

so maybe it is better to stuck with .ref() and the InPlace suffix.


On Fri, Sep 11, 2009 at 1:10 AM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx> wrote:
> 2009/9/10 Keir Mierle <mierle@xxxxxxxxx>:
>> On Thu, Sep 10, 2009 at 2:47 PM, Gael Guennebaud <gael.guennebaud@xxxxxxxxx>
>> wrote:
>>> oh, actually we can even write a single
>>> SolveReturnType<DecomositionType, DerivedB> class inheriting
>>> ReturnByValue which would be used for all x = solve(b) functions.
>> I am not convinced the added complexity justifies the benefits. What about
>> using the result in an expression?
>> y = M*A.solve(b); // Parts of my code could use this today.
> My understanding is that Gael is now proposing a ReturnByValue class
> that inherits MatrixBase, so that one could use that in any
> expression. That's a departure from my assumption that he was
> proposing ReturnByValue not inheriting MatrixBase.
> Gael : is that right?
>> x.start(3) = A.solve(b) // Never used this.
>> Not saying that the 2nd use case isn't justified, just that I personally
>> haven't needed it. Can others speak to this?
> Same here, I don't think use cases are common. I can think of one:
> when implementing a blocked algorithm (e.g. block LU), sometimes some
> of the blocks can be obtained as a matrix solve.
> However, for such complex use cases, I think that it wasn't too bad
> (although not wonderful either) to force using named xpr objects
> instead of temporaries.
> As Keir notes, here we're giving a lot of weight to a quite unusual
> use case. I don't think it's worth going for option 4 just for that
> use case, as that would make the API heavier for everybody just to
> make it lighter in that unusual use case.
>>> 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
> You're right, I was forgetting about that unusual use case, but again,
> i've never met it outside of the implementation of blocked algorithms.
> Now it's true that it has intrinsic value, and that it's a good goal
> for our API design to fully support the seamless implementation of
> blocked algorithms.
> My main concern, like Keir, was that this option 3 wouldn't allow the
> result of solve() to be used in expressions, because you earlier said
> that ReturnByValue shouldn't inherit MatrixBase. Now I didn't think as
> much as you did about this, but now you say,
>>> >
>>> > 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.
> Ah OK, so now we're again discussing a ReturnByValue that inherits
> MatrixBase. So option 3 indeed looks pretty good, can be used in any
> expression.
>>> > 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).
> OK, that's not too bad, especially as, as you said, this issue already
> exists with all the products (and there it really is the best solution
> anyway).
> At least we'll have some uniformity. What's needed then is a good dox
> page on writing functions that take MatrixBase as argument.
>>> > Now, honestly I don't really like to use pointers for return types,
>>> > unless they are optional.
> Do you mean: as opposed to natural syntax, result = function() ? As in option 3.
> Then I can understand. Me too, i prefer natural syntax!
> Or do you mean: as opposed to function(result.ref()) or passing by c++
> reference? Then I need to ask, why? What is bad about pointer, as
> opposed to references or custom structs? OK custom structs allow
> passing temporaries, but are you also saying that you prefer c++
> references over pointers? Then I really would like to know why.
> Benoit

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