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Re: i486 base architecture



On Mon, Nov 29, 2004 at 01:30:11AM -0800, Nicholas Miell wrote:
> On Mon, 2004-11-29 at 10:16 +0100, Arjan van de Ven wrote:
> > On Mon, Nov 29, 2004 at 01:02:46AM -0800, Nicholas Miell wrote: 
> > > CMOVcc will use less space in the instruction cache than the Jcc/MOV
> > > pair, though.
> > 
> > only sometimes.... since cmov doens't work on all register/memory
> > combinations extra code might be needed to glue that together...
> > 
> > 
> > .... and we're suddenly talking about 0.01% performance ;)
> 
> Well, yeah. :)
> 
> There's also branch prediction and decode bandwidth issues that I didn't
> bother to mention.

branch prediction on p4 actually takes hints from the compiler now ;)

> 
> But, if you're going to optimize for i686 or better for other reasons,

we *already* optimize for i686 even in the i386 rpms


> there's no reason not to use CMOVcc instead of Jcc/MOV, where possible.

there is a reason... it keeps running on older hw and on via C3's :)


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