Re: [boost] Preview 2 of a template Geometry Library

From: Bruno Lalande (bruno.lalande_at_[hidden])
Date: 2008-03-26 05:42:19

Hello,

> > > To be honest I've done some experiments, also with a polygon with a
> > > compile-time number of vertices. Called it the template <size_t D> gon
> > > so a gon<3> for a triangle, etc. It disappointed me a bit because the
> > > compile-time area calculation routine which I drafted turned out to be
> > > slower than the runtime version...
>

For the pythagoras version I've given above, my tests show that the
performances are the same as with the original version with GCC 4, which is
not surprising. This is true with or without optimizations (but I don't care
about the "without" version, anyway).

>
> >
> > Do you mean the version generated using a compile time area calculation
> > turns out to be slower at runtime than the native runtime version? I
> don't
> > understand how this might be possible. What are you trying to achieve at
> > compile time?
>

Yep, it would be interesting to see your compile-time version because the
fact it's slower sounds very surprising, indeed.

> In the post, the compile-time version unrolls the loop during compilation;
> it doesn't compute a result based on values specified in the source that
> are
> defined at compilation. Thus, the compile-time function could have a
> higher
> cost. It will almost certainly have a higher cost in a debug build since
> many compilers do not inline in that situation. I'd have to think
> recursive
> function calls are going to be much more expensive than a simple for loop.
> In release, it really depends on how well a compiler inlines. I haven't
> done
> extensive testing on any current compiler, but I know MSVC 6.0 optimized
> very well when __forceinline was used judiciously, but was quite a bit
> more
> hit-or-miss when relying on its inlining decisions.

When I meta-program, I usually don't care about what will happen with a
non-optimizing compiler, since meta-programming entirely relies on the
compilers' inlining and optimizing skills, anyway. Thus, when it comes to
compare performances, I do it both with optimizations turned on and off, but
I only care about the results of the "on" version (see above).

Bruno