linux-kernel - Re: Specific support for Intel Atom architecture

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Date:	Sun, 3 May 2009 07:53:46 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Willy Tarreau <w@....eu>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Tobias Doerffel <tobias.doerffel@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Specific support for Intel Atom architecture

On Sun, 3 May 2009 07:38:23 +0200
Willy Tarreau <w@....eu> wrote:

> On Thu, Apr 30, 2009 at 10:10:08AM -0700, H. Peter Anvin wrote:
> > Ingo Molnar wrote:
> > >> diff --git a/arch/x86/Makefile_32.cpu b/arch/x86/Makefile_32.cpu
> > >> index 80177ec..07a11b0 100644
> > >> --- a/arch/x86/Makefile_32.cpu
> > >> +++ b/arch/x86/Makefile_32.cpu
> > >> @@ -33,6 +33,7 @@ cflags-$(CONFIG_MCYRIXIII)	+= $(call
> > >> cc-option,-march=c3,-march=i486) $(align)-f
> > >> cflags-$(CONFIG_MVIAC3_2)	+= $(call
> > >> cc-option,-march=c3-2,-march=i686)
> > >> cflags-$(CONFIG_MVIAC7)		+= -march=i686
> > >> cflags-$(CONFIG_MCORE2)		+= -march=i686 $(call
> > >> tune,core2) +cflags-$(CONFIG_MATOM)		+=
> > >> -march=atom $(call tune,atom) 
> > 
> > There should be a fallback option used here rather than requiring a
> > new gcc, e.g. something like:
> > 
> > $(call cc-option,-march=atom,-march=i686)
> 
> if it's an in-order architecture, wouldn't it be better to tune for
> i386 or i486 instead ?

-march isn't about tuning, it's about supported instructions.
The right line is
$(call cc-option,-march=atom,-march=core2)

For tuning, our experience is that currently -mtune=generic works best.
Not sure about the gcc's that have complete atom tuning support yet.

Please don't do something like "oh it's in order, so was the Pentium,
so lets use that"; it actually gives really really bad results.


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
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