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Subject: Re: [boost] New library in the Vault: ConstantTimeSize
From: Simonson, Lucanus J (lucanus.j.simonson_at_[hidden])
Date: 2008-10-16 17:20:58

>> Boost.ConstantTimeSize defines a configurable wrapper to the stl
>>container list
>> providing a size function with constant, linear or quasy_constant

Dave Abrahams wrote:
>Interesting, and probably useful.
>However, I would like any library using the proposed name to go much
>further: loop unrolling does not depend on random access, but on having
>an O(1) size operation, so I would like to see algorithm
>that take advantage of it.

Do we know that loop unrolling of iteration over a linked list is
profitable? I would think that a conscientious programmer interested in
loop unrolling of linked lists should pursue an analysis to measure the
potential benefits that might be realized with current compilers and
hardware before implementing a general solution along those lines. This
analysis would be somewhat challenging, because it is likely that a toy
testcase would fit in L1 cache and show promising results while
realistic usage of lists would lead to their elements being fragmented
in the heap and a large proportion of L1 and L2 cache misses that
dominates runtime for loop iteration. If the long latency cache miss
dominates the overhead of checking if itr == list.end() you will realize
no benefit from loop unrolling on a modern architecture which can mask
the latency of the comparison and overhead of the branch instruction
that depends upon it within the latency of the cache miss of the next
iteration of the loop under speculative execution. It is conceivable
that the benefit would be negligible in the common case. We won't know,
however, unless someone does the analysis.


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