Re: [eigen] Indexes: why signed instead of unsigned?

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Is there any reason why sparse matrices backed by distributed storage need to use the same indexing integer data type as the more common dense storage in memory?

Beware bending a design to meet the edge case.

Benoit Jacob wrote:
Note: the distribute storage case is an example where one might want
64bit indices even if the CPU architecture is 32bit. Indeed, since the
array doesn't need anymore to fit in RAM...

So it's an example where ptrdiff_t may not be the right choice.


2010/5/11 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
grrr, i had forgotten but a while ago someone inquired about the
possibility to use eigen with distributed storage, and we took that
into account when doing the big class-hierarchy-refactorings, to
ensure it's possible to plug in a new storage system, see
StorageKind... so it would be a bit too bad to restrict the indices
types to int.

This actually suggests that no matter what the default is, the indices
type should be something that the Storage can control. This would also
allow Sparse matrices to use different index types. We could make it a
typedef in ei_traits, that is propagated to nested expressions, ...

Gael, what's your opinion?


2010/5/11 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
2010/5/11 Rui Maciel <rui.maciel@xxxxxxxxx>:
On 11 May 2010 16:47, Benoit Jacob <jacob.benoit.1@xxxxxxxxx> wrote:
I just had a diabolic idea, if we agree that bigger-than-2^31-support
is not a top priority and will at most be used by relatively few
people, then we can keep things unchanged for now, so keep 'int' in
Eigen 3.0, and later on, when actual people actually need it (perhaps
in eigen 3.45), add a non-default compile-time option
EIGEN_USE_LONG_INDICES. Such an option would then break the ABI of
Eigen classes, so it couldn't become default.

Question - is BLAS/LAPACK using int or ptrdiff_t ?

As this move will break the ABI then why not implement it right on the
3.0 release instead of pushing it to a point release in the future?
Because we've a lot on our plates already now.


Rui Maciel

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