On Thu, Feb 27, 2003 at 08:38:00AM +0000, Bruce Cran wrote:
> On Wed, Feb 26, 2003 at 06:51:55PM -0800, Rhett Monteg Hollander wrote:
> > Nuno Teixeira wrote:
> > > 
> > > Hi,
> > > 
> > > Just a little question:
> > > 
> > > Does -march=k6-2 implies -m3dnow? Or -march=pentiumpro implies -mmmx?
> > Pentium Pro doesn't support MMX; -march=pentiumpro (aka -march=i686) enables
> > compiling with main i686 instruction set, no MMX\SSE or whatever.
> > 
> > > 
> > > I always thought that when I use -march it will enable other
> > > porcessor specific optimizations like mmx and 3dnow (if available).
> > No way. Besides, only a dozen of open-source UNIX applications can really
> > profit from MMX, so it's absurd to use -mmmx for day-by-day use. To be correct,
> > MMX is only useful for real-time video\audio processing.
> > 
> > A little example: consider some 16-bit pixel set that you need to increase
> > brightness, i.e. to add some value to every pixel. You can load a word into
> > integer register, process, and store. You can also load a quadword of four
> > 16-bit values into MMX register which is mapped onto FPU register, process them
> > at once, and store. Indeed to be faster.
> > 
> > Enabling MMX usually results in about 50% performance increase for true CISC
> > processors like P-MMX, and about 20% for PII and up.
> > 
> I'm afraid you're wrong - the V2SI datatype and MMX functions automatically
> become available after -march=pentium2, while with other processor types
> you've got to explicitly add -mmmx. -msse is presumed with -march=pentium3 
> and up.  It's far from absurd to use mmx for everyday applications - sure,
> only a few applications may take advantage of it, but I've seen code which
> runs 40x faster when compiled for athlon-xp than for i386, and I would guess
> that a lot of that is because of clever use of sse and mmx.   That wasn't
> an audio/video program, it was the libgmp arbitrary precision maths
> package.  Also, I'm sure
> most people wouldn't say no to 50% more processing speed for free!
> So, if you've got a pentium, k6 or pentiumpro which supports MMX, you _do_ 
> need to explicitly add -mmmx, but for other processors it's implied.
> Bruce Cran
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I searched gcc docs and didn't found info for what you say here. I'm
seeing a lots of people using e.g. athlon-xp with -mmmx and -m3dnow
included. I'm confused about if this optimizations are implied or not by
processores that supports it.

Thanks very much,

                Nuno Teixeira


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