linux-kernel - Re: Promoting Crusoe and Geode Processors to i686 Status

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Date:	Wed, 8 Sep 2010 23:14:11 +0100
From:	Nick Lowe <nick.lowe@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Promoting Crusoe and Geode Processors to i686 Status

Hi,

Here goes...

-mtune tunes the binary for given architecture. This as opposed to
defining the baseline architecture, and thus available instruction
set, which -march does.

I believe that -mtune is predominantly used for instruction
scheduling. GCC uses it when it's reordering instructions to optimise
for specific micro-architecture over others.

With -mtune=i686, I am of the opinion that you instruct the compiler,
and give it the opportunity, to deliver a better ordering of the
instructions. In this case, better ordering for i686 than you would
get with -mtune=generic32.

I'm probably wrong somehow, it's probably marginal, and I'll concede
to your infinitely better knowledge any day! That's why I phrased it
as a question! :)

Cheers,

Nick

On Wed, Sep 8, 2010 at 10:39 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 09/07/2010 05:27 PM, Nick Lowe wrote:
>>
>> 2) Should there be the special case that forces the suboptimal
>> -mtune=generic32 to work around the binutils NOPL bug or, as I think,
>> should people just update to a sane binutils version? (It's a special
>> case triggered where CONFIG_X86_P6_NOP is not set.)
>>
>
> What's suboptimal about it?
>
>        -hpa
>
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