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- To: eigen@xxxxxxxxxxxxxxxxxxx
- Subject: Re: [eigen] issue 77: map with stride
- From: Hauke Heibel <hauke.heibel@xxxxxxxxxxxxxx>
- Date: Mon, 18 Jan 2010 11:33:42 +0100
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On Mon, Jan 18, 2010 at 11:15 AM, Gael Guennebaud
<gael.guennebaud@xxxxxxxxx> wrote:
> On Mon, Jan 18, 2010 at 9:48 AM, Hauke Heibel
> <hauke.heibel@xxxxxxxxxxxxxx> wrote:
>> Personally, I don't like the notion of inner and outer stride. For 2D
>> objects, there should only be one stride function. Conversely, the
>> stride for 1D objects should either not be called at all or it should
>> be one. But as I said, maybe I am not yet understanding the problem
>> this causes within Eigen.
>
> As Benoit explain in his example, taking a row from a column major
> matrix yields a vector object having an "inner stride" greater than 1,
> so yes "inner stride" is an important notion.
Ok, now I do understand the issue.
> To make things clearer, perhaps we could borrow the terminology of BLAS:
> - "leading dimension" for matrices only (aka outer stride)
> - "increment" for vectors only (aka inner stride).
Since we already bound ourselves to rows() and cols(), yet another
option would be something like rowStepSize() and colStepSize() or just
stepSize() for vectors (similar to rows()/cols() and size()). For
'leading dimension', I do have right away to think about what it
means. Is it the dimension with the fastest or slowest running index?
The same holds for inner and outer. In case I have a for-loop over a
2D entity, there is a notion for inner and outer but nothing keeps me
from changing the loop order. Here, you are assuming that everybody
agrees that the inner loop is the one processing elements being
closest in memory.
By the way, do you plan to force strides to be multiples of
sizeof(Scalar) (basically counting elements) or is the plan to have
the strides in bytes?
- Hauke