[linux-audio-dev] Re: Re: Steve Harris' C++ v C benchmark

[Paul]

Steve Harris <S.W.Harris at ecs.soton.ac.uk>, on Mon Feb 17, 2003 [09:20:26 AM] said:
> On Mon, Feb 17, 2003 at 08:34:10 +0100, Roger Larsson wrote:
> > Testing that i have done suggest that you should ALWAYS define the
> > architecture of your target. (I have not checked if the example does this
> > but it is usually forgot...)
> > 
> >  -march=pentium3
> > or if you need it to run on older computers
> >  -mcpu=pentium3
> > (see man gcc search for "Intel 386")
> > 
> > And at least use optimization level -O1 (use -O3 to get automatic inlining)
> 
> Agreed. I generally use -O6, its not a good idea for general use, but for
> small fast plugin code its often the fastest. My full set of flags is:
> 
> -O6 -fomit-frame-pointer -fstrength-reduce -funroll-loops
> -fmove-all-movables -ffast-math ${MACHINE_SPECIFIC}
> 
> I'd be interested to see how this compares with other peoples. I've been
> meaning to write a test script that tries all the combinations to find
> which produces the fastest code.
> 
> The -march, -mcpu flags are the mostimportant (they go in
> MACHINE_SPECIFIC), but -ffast-math can help a lot too.
> 
> - Steve

	Hi;

	Here is an article and some discussion of these things:

http://freshmeat.net/articles/view/730/

	Unfortunatley, it is hard to make a hard and fast rule
as to what are the optimal compilation switchs; you cant be
sure until you actually test and compare on a specific piece
of software. (or maybe a class-- like a 'small fast plugin')
eg. -O3, loop unrolling, and alignment for architecture can
bloat up code and _might_ result in critical stuff not fitting
in cache. (Ive heard claims about stuff that ran faster with
-Os than -O3)  Perhaps a set of switchs appropriate to the
concerns of audio developers could be generalized.
	Im barely even a novice in this area, though.:)

Paul
set at pobox.com