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Re: [PATCH] fix dir2 shortform structures on ARM old ABI

To: Timothy Shimmin <tes@xxxxxxx>
Subject: Re: [PATCH] fix dir2 shortform structures on ARM old ABI
From: "Josef 'Jeff' Sipek" <jeffpc@xxxxxxxxxxxxxx>
Date: Mon, 17 Mar 2008 19:42:19 -0400
Cc: Eric Sandeen <sandeen@xxxxxxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
In-reply-to: <47DF0044.6080704@xxxxxxx>
References: <47DB4181.7040603@xxxxxxxxxxx> <20080315041722.GA25621@xxxxxxxxxxxxxx> <47DB4F4F.8030407@xxxxxxxxxxx> <20080315042703.GA28242@xxxxxxxxxxxxxx> <47DB51A3.70200@xxxxxxxxxxx> <20080315045147.GB28242@xxxxxxxxxxxxxx> <47DEB930.7020108@xxxxxxxxxxx> <47DF0044.6080704@xxxxxxx>
Sender: xfs-bounce@xxxxxxxxxxx
User-agent: Mutt/1.5.16 (2007-06-11)
On Tue, Mar 18, 2008 at 10:35:32AM +1100, Timothy Shimmin wrote:
> Eric Sandeen wrote:
>> Josef 'Jeff' Sipek wrote:
>>> Josef 'Jeff' Sipek, wondering exactly how passionate one can get about
>>> structure member alignment :)
>> Very.  ;)
>> Tossing packed at all the ondisk stuctures bloats things badly on ia64.
>> cvs/linux-2.6-xfs> wc -l before.dis
>> 166688 before.dis
>> cvs/linux-2.6-xfs> wc -l after.dis
>> 182294 after.dis
>> That's +15606 lines.
>> http://digitalvampire.org/blog/index.php/2006/07/31/why-you-shouldnt-use-__attribute__packed/
> Interesting.
> So the problem there is that gcc is doing the wrong thing
> on some arches (the example being ia64, sparc64).

Actually, it's not doing the wrong thing...

__attribute__((packed)) means:

1) condense the members of the struct leaving NO padding bytes

2) do NOT assume the entire structure is aligned on any boundary

This means, that even if you have a member that'd be nicely aligned without
the packed attribute (see below), the compiler will generate worst case
alignment code.

struct foo {
        u64 a;
} __attribute__((packed));

You can put struct foo anywhere in memory, and the code accessing ->a will
_always_ work.

Using __attribute((packed,aligned(4))), tells it that the structure as a
whole will be aligned on a 4-byte boundary, but there should be no padding
bytes inserted.

Josef 'Jeff' Sipek.

-- 
Penguin : Linux version 2.6.23.1 on an i386 machine (6135.23 BogoMips).


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