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

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Message-ID: <4C880B12.3040200@zytor.com>
Date:	Wed, 08 Sep 2010 15:15:46 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Nick Lowe <nick.lowe@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Promoting Crusoe and Geode Processors to i686 Status

On 09/08/2010 03:14 PM, Nick Lowe wrote:
> 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! :)
> 

We don't pass -mtune=generic32 to gcc.  We pass -Wa,-mtune=generic32 to
gcc, which affects binutils only.

	-hpa
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