> From: Olivier Langlois [mailto:[EMAIL PROTECTED]]
> I've played a little bit FFTW and I've remarked that its performance can
> vary a lot depending on how good your compiler is at optimization.
>
> For instance, compiled FFTW code is far from optimal with MSVC++ 6. This
> compiler doesn't fully use the FPU stack like an ASM programmer could do
and
> I don't know why since I'm sure that writing a compiler that would make a
> good usage of the FPU registers is far from impossible.
>
> So, the compiler you use to compile FFTW is a major factor for the
> performance you'll get from it.
>
> I don't know if someone have done similar experiences and if there is a
> better compiler than MSVC for intensive FP code.
Actually, we at Microsoft Research in Cambridge have seen similar effects
when compiling and running FFTW code. Our discovery is that the alignment
of FP data values is critical. Get it wrong, and performance can plummet.
Unless you set the alignment explicitly, it will be wrong approximately half
the time.
Jonathan Hardwick investigated this effect as part of his research into
high-performance computing. He gave an internal seminar (which is where I
learned about it) and wrote it up in detail. The full details are at
http://www.research.microsoft.com/users/jch/fftw-performance.html
Paul
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