On Fri, Mar 27, 2009 at 4:50 PM, Phil Endecott <spam_from_boost_dev@chezphil.org> wrote:
Kornel Kisielewicz wrote:
- Although I doubt that wasting a word on a size field has much effect on speed, I would want to avoid it when dealing with large collections of points because of the memory overhead.
I'm not too much worried about the memory, as I am about data alignment.
Why? Do you have some motivating application where memory doesn't matter but unaligned accesses are expensive? If you have a motivating application please tell us about it. My comment that _I_ would want to avoid wasting memory is because I have a memory-constrained application in front of me. Code for a library like Boost will end up in lots of different applications with different constraints, and it really ought to be "good all round" to satisfy all potential users.
You misunderstood me. What I meant that out of the two problems, having 25% more memory usage, and having to deal with structures that don't align to architecture standards, the latter is more disturbing to me. Not that I like the former, memory is also an important issue here.
Also, in the case of 4d vectors, you'd have 5 values, which doesn't scale well ( 4*32 bits = 128bits which is nice -- an addition would ruin this niceness ).
Multiplication by 4 faster than multiplication by 5? Maybe, but probably premature optimisation :-)
This comes specifically from the architectural point of view. In this case it's about alignment for SIMD instructions, and cache/bus size issues. 128bits is nice, more without a serious reason is not. -- regards, Kornel Kisielewicz