linux-kernel - Re: [patch 4/7] Immediate Values

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Message-ID: <20070919110846.GC15500@Krystal>
Date:	Wed, 19 Sep 2007 07:08:46 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Chuck Ebbert <cebbert@...hat.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [patch 4/7] Immediate Values - i386 Optimization

* Andi Kleen (andi@...stfloor.org) wrote:
> Jeremy Fitzhardinge <jeremy@...p.org> writes:
> > 
> > It's a pity that gas seems to generate plain 0x90 nops rather than
> > long-nop forms here.  I thought it could do that.
> 
> .p2align does it.
> 

Sadly, p2align does not apply well to my context. I have to align on 4
bytes boundaries - 1 for the 4 bytes mov, so, if I would use p2align, I
would end up aligning on 4 bytes with p2align and then add 3 bytes
(worse case: adding 3 + 3 = 6 bytes of nops).

However, with the .org arithmetic, I can simply add then quantity of
nops needed to make my alignment on 4 bytes - 1, so the worse case
becomes adding 3 bytes.

The example is:

originally: address & 3 = 1
* p2align
p2align adds 3 bytes to align on 4 bytes boundaries
we add 3 bytes to align on the next 4 bytes - 1, so the immediate value
within the instruction is aligned on 4 bytes boundaries

* org
we add 2 bytes to be aligned on the next 4 bytes - 1.

And yes, it's a pity there is no way to produce the long-nops there. :(

Mathieu

> -Andi

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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