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Date: Fri, 22 Nov 2002 10:54:38 +0000
From: Dave Jones <davej@codemonkey.org.uk>
To: Denis Vlasenko <vda@port.imtp.ilyichevsk.odessa.ua>
Cc: Margit Schubert-While <margitsw@t-online.de>, linux-kernel@vger.kernel.org
Subject: Re: P4 compile options
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On Fri, Nov 22, 2002 at 01:04:39PM -0200, Denis Vlasenko wrote:

 > I consider 16-byte code alignment as way too big.
 > P4 zealots can demand even more I guess :(
 > I will happily change my mind when/if I'll see
 > favorable speed/kernel size benchmarks. Until then,

I think there's a misunderstanding here.
The march=pentium4 option is only used when you select
"build me a pentium 4 kernel" You do realise that right?
Generic kernels don't change 1 bit.

 > I think 4-byte alignment is closest to sanity.

You know where to find the Intel P4 optimisation manuals..

 > Not exactly P4 related but: if you tell gcc your
 > processor has cmov, gcc will try to use it.

So what ? Show me a P4 without cmov.

 > Results: 
 > * gcc code is worse with cmov than without
 > * some CPUs (Cyrix?) have slow cmovs (microcoded?)
 > * you lose whenever you try to use your code
 >   on cmov-less CPU.

  <------------ The point.
                              --------------> You.

Cmov is completely irrelevant here.
Sure its still an optional instruction which
should be tested for before use, but until Intel
make a P4 without CMOV, adding march=pentium4
is harmless.
 
 > Dave, I am absolutely sure _you_ do not compile
 > for P4 needlessly, but lots of ordinary people
 > do that just to be hip.

Those are probably the same folks who run Gentoo/Slackware/ or
some-other-compile-everything-myself-because-I've-too-much-time-on-my-hands-distro.
Fine, let them be happy.
If some loon wants a P4 optimised /bin/ls, that's his problem,
but optimisation of key components (like say, the kernel) _is_
important.

 > I wanted to point out why it may be undesirable.

All you've pointed out is that a P4 kernel won't run
optimally on a 486. Well surprise, it won't run at all.

		Dave

-- 
| Dave Jones.        http://www.codemonkey.org.uk
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