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Re: [RFC] Optimize swab operations on mips_r2 cpu

To: "Kevin D. Kissell" <kevink@mips.com>
Subject: Re: [RFC] Optimize swab operations on mips_r2 cpu
From: Franck <vagabon.xyz@gmail.com>
Date: Fri, 27 Jan 2006 11:45:34 +0100
Cc: Nigel Stephens <nigel@mips.com>, linux-mips@linux-mips.org
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2006/1/27, Kevin D. Kissell <kevink@mips.com>:
> > > You could, but why not stick with -march=4ksd if that's your CPU of
> > > choice? It appears to result in  marginally smaller code even when using
> > > -Os, and should have (slightly) better performance than a generic
> > > mips32r2 kernel?
> >
> > Just to avoid a new CPU_4KSD definition in the kernel code as
> > suggested by Kevin. Basically all mips32r2 specific code is the same
> > as 4ksd specific code (except the code that deals with SmartMIPS
> > extension). So it can use CONFIG_CPU_MIPS32_R2 macro. But I was not
> > aware of -march=4ksd and -march=mips32r2 differences. Maybe now it is
> > needed to have a new CPU_4KSD definition ?
>
> Configuration hacks that are specific to a single core create cruft and
> maintenence problems.  As I said yesterday, I think we'd be much better
> off to have a CONFIG_CPU_MIPS_SMALL or some such option
> that could cause -Os to be used, allow branch-likelies, etc. The optimizations
> under discussion aren't at all specific to the 4KSd,

no some are. As we said previously:

        1/ sizeof(vmlinux-mips32r2-Os) > sizeof(vmlinux-4ksd-Os)
        2/ with -march=4ksd can do (slightly) better optimizations

> for every 4KSd embedded Linux platform there are several 4KEc platforms
> that would benefit from a smaller kernel footprint.
>

-Os can already be choosen by user in kernel configuration. Your
CONFIG_CPU_MIPS_SMALL option brings nothing more except that the user
is stuck with -Os option. BTW, I think it may be the default option in
the near future for mainline...

Thanks
--
               Franck

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