Hi:
[EMAIL PROTECTED] wrote:
> <<On IA32 systems, how the code is aligned is also a factor. To compare
> accurately, you'd really need the separate code fragments to be in
> their own dedicated segments. This is not the way that un*x or Win32
> applications are usually coded.
>
> Not being up on the latest HLL compilers, I would still suspect that some
> compiler directives/options would be available to handle alignment properly
> without much headache, especially if it is as important as you suggest.>>
>
> I have the teensiest fraction of knowledge about C (I'm trying to learn it
> now), but I know a little about compiling C programs. With DJGPP, you can
> enable cool options like -malign-double which can really speed up some
> programs. Is this the "alignment" that you're speaking of?
>
On IA32 system, the alignment is one of the most important factors to
achieve a decent performance. Unfortunately, some compilers (GNU/gcc) do
not make the alignment correctly in a easy way. When writing Glucas, I
discovered than -malign-double only aligns double when they are
global-static variables. The local variables on the stack are not
aligned, there is only a 50% chance of that (because they are 4-byte
aligned). In a bad alignment scenery the performance can drop to the
half (or even more). To achieve good performance I had to try the the
same trick based in a built-in malloc than FFTW uses, i.e., allocate
conditionally 4 bytes in the calling routine and so the doubles on the
called are properly aligned, an ugly solution, you see.
Regards
Guillermo.
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