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From: Lewis Hyatt (lhyatt_at_[hidden])
Date: 2007-03-17 12:13:37

> Are you sure about the quicksort speed difference? That seems like a huge
> difference. Did you have inlining enabled? Also, the current versions are
> not the final forms. I am implementing random-access iterators and
> user-provided functions.

Yes, this was on cygwin compiled with g++ -O3. One thing to keep in mind is that
your routines are competing with the standard library, which can be highly
optimized for whatever target platform the compiler was written for. Unless you
have a dramatically improved algorithm, it is unlikely that your sorting
routines will beat std::sort in general, even if they do on some platforms.

As an example, suppose your goal was to write an equivalent of std::bitset which
beat the standard implementation. It is unlikely that you could exceed the
performance of std::bitset::count(), for instance, because most implementations
use native assembly instructions to do this whenever possible. (e.g., gcc uses
its built-in popcount function on platforms where it is available.) On the other
hand, if you hand-coded a version with the best algorithm you could find, it
might beat a few poor implementations, but in general it would be inferior. The
only way to make it better in general would be to implement all these
optimizations yourself for each target platform, but at that point, it's just
not worth the time.

I don't think it's true that std::sort, std::stable_sort, and std::partial_sort
are the end of the story, but I think any work in a sorting library should focus
on needs which are not already met. So far, only radix sort appears to fall in
that category, but you would also need to demonstrate a case in which radix sort
is preferable to std::sort.


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