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Date:      Wed, 26 Feb 2003 18:51:55 -0800 (PST)
From:      Rhett Monteg Hollander <victorysoldier@yahoo.com>
To:        current@freebsd.org
Subject:   Re: -O2 considered harmful
Message-ID:  <20030227025155.61529.qmail@web40310.mail.yahoo.com>

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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.

---
Regards,
 Rhett


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