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Subject: Re: [boost] Accelerating algorithms with SIMD - Segmented iterators and alternatives
From: Jeffrey Lee Hellrung, Jr. (jhellrung_at_[hidden])
Date: 2010-10-13 15:08:43

On 10/13/2010 08:04 AM, David Abrahams wrote:
> At Wed, 13 Oct 2010 03:44:37 -0700,
> Jeffrey Lee Hellrung, Jr. wrote:
>> Seems to me segmented iterators and a visitation-based/push-based
>> iteration address fundamentally different problems. At least, I don't
>> see how segmented iterators help here. The problem that
>> visitation-based iteration seems to try to solve is improving
>> iteration efficiency over join(t)_range-like ranges...
> That's the same problem (in essence) that segmented iterators are
> solving. Did you read the Austern paper?

Yes. But I'm not convinced yet that iterators exposing the segmentation
of the data structure are the right solution to presenting a flattened
view of that data structure. 1) It doesn't play nice with standard C++
looping constructs, specifically for loops. 2) I have yet to see a
benchmark comparison between segmented iterators and alternatives (other
than a side-note claim in the paper). I'm open to looking at some
specific comparisons if one has already done this, and this is something
I'm interested enough in that I might try to do myself. Seems to me a
more general solution is a "flat_multirange_iterator" templated on the
outer range and the "segmentation depth". I.e., a
flat_multirange_iterator< vector< vector<T> >, 2 > would iterate over
the T's.

To address your claim that "that's the same problem ... that segmented
iterators are solving", what I meant by a join_range or joint_range is,
essentially, a flattened out Boost.Fusion sequence of ranges of
*differing* types (but with compatible value_type's and references).
E.g., the thing that boost::join [1] returns. Segmented iterators can't
deal with such ranges (at least with a strict reading of Austern's
paper), and I think such ranges are closer to what Mathias has in mind
(to be verified......). But perhaps I'm just not seeing your
generalization beyond the immediate scope of the Austern paper?

- Jeff


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