On Wednesday, 6 June 2012 at 01:44:24 UTC, bearophile wrote:
Writing specialized versions without any language help is not
nice, and I think the gain is significant, it's not just tiny
optimizations. My D programs contain lot of stuff known at
compile-time. I think such simple poor's man hand-made version
of partial compilation is able to do things like (done by true
partial compilation):
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.31.5469&rep=rep1&type=pdf
Thanks for the link. The paper is interesting.
One interesting point to note is that they bloat the executable
massively by specialising (30x larger in a few cases).
Fortunately for them, their program is small enough to fit in the
I-cache anyway, so it has no effect on performance, however, in a
large program that size blowup would cause much more I-cache
capacity misses, which would then negatively affect runtime.
This effect has already been seen in the games industry with
std::sort. As you probably know, std::sort specialises the sort
routine for the types being sorted, so you end up with N
specialised sort routines for N different types and each one adds
a few KB to the executable.
If I want to sort a small array (maybe just 8 integers or so)
then using std::sort is often worse than qsort because std::sort
takes up more of the I-cache. It is now common in games to use
qsort (or a small type-safe equivalent) instead of std::sort for
small sorts to reduce bloat and I-cache pollution.
See slide 14-15 of this DICE presentation:
http://www.slideshare.net/DICEStudio/executable-bloat-how-it-happens-and-how-we-can-ght-it
I'm sure your D programs do contain a lot of stuff known at
compile-time, but I'm also sure that in any non-trivial program,
95% of your code is not performance sensitive and would be better
to be small than fast. The sample ray-tracer is not
representative of a real program.
I hold my position that this would be counterproductive 95% of
the time. In the 5% that is highly performance sensitive, we can
use metaprogramming techniques (in D, or using external codegen)
to make a workable solution.
