Date: Sat, 13 Sep 2003 20:25:39 +0100
From: Jamie Lokier <jamie@shareable.org>
To: =?iso-8859-1?Q?J=F6rn?= Engel <joern@wohnheim.fh-wedel.de>
Cc: Ricardo Bugalho <ricardo.b@zmail.pt>, insecure@mail.od.ua,
       linux-kernel@vger.kernel.org
Subject: Re: nasm over gas?
Message-ID: <20030913192539.GE7404@mail.jlokier.co.uk>
References: <20030904104245.GA1823@leto2.endorphin.org> <200309100034.58742.insecure@mail.od.ua> <pan.2003.09.11.11.06.59.523742@zmail.pt> <200309121826.22936.insecure@mail.od.ua> <1063387648.15891.26.camel@ezquiel.nara.homeip.net> <20030912221717.GB11952@wohnheim.fh-wedel.de>
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J?rn Engel wrote:
> - Why has Alan measured faster kernels with -Os than with -O2?
> 
> Code size *does* matter.

That's not just i-cache pressure.  It is partly a GCC problem, and
it's possible -Os would run faster than -O2 even with no i-cache.

I've observed -Os emitting exactly the same code as -O2 for some
trivial functions, except that -O2 has a few extra redundant
instructions.

Obvious the _intent_ of -O2 is to compile for speed, but it's clear
that GCC often emits trivially redundant instructions (like stack
adjustments) that don't serve to speed up the program at all.

-- Jamie
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