Date: Sat, 29 Jan 2000 14:32:45 +0100
From: Marc Lehmann <marc AT gimp DOT org>
To: pgcc AT delorie DOT com
Subject: Re: pgcc and egcs alignment -- function, basic block and string
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In-Reply-To: <20000129032101.A25630@atrey.karlin.mff.cuni.cz>; from hubicka@atrey.karlin.mff.cuni.cz on Sat, Jan 29, 2000 at 03:21:01AM +0100
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On Sat, Jan 29, 2000 at 03:21:01AM +0100, Jan Hubicka <hubicka AT atrey DOT karlin DOT mff DOT cuni DOT cz> wrote:
> It is. Consider memset/memcpy/strlen expanders. These can work
> much better when they know that destination is word size aligned.

source you meant ;)

> > documentation has a bug in the .align description saying that the
> I will verify this tommorow and in case you are correct, I will fix this bug.
> (in both gas and gcc).

AFAIK, the problem is that gas acts differently depending on wether
the target is ELF or a.out/COFF. The current gcc (AFAIK again) makes a
test at configure time to find out about that (or maybe it just probes
wether .p2align is available? In the latter case align shouldn't be used,
obviously).

> Again Intel Optimizing Manual recommends this. I believe Intel did some experiments

Well, doing some experiments yourself (as you said) does not hurt
either (if you have a pentium). While intel recommends relatively large
alignments, "common knowledge" (linus for example ;) recommends no
alignments at all.

In _my_ tests large alignment is a very very slight win, but in the real
world, the increased code size might not be worth it (cache effect, long
nops, AGI because of lea-nops).

It's a must on 486, though, and a bit better on ppro and later.

-- 
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