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From: Dave Chinner <david@fromorbit.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Eric Sandeen <sandeen@sandeen.net>, xfs@oss.sgi.com
Subject: Re: [PATCH 3/3] xfs: remove __arch_pack
Date: Wed, 20 Jul 2016 09:46:34 +1000	[thread overview]
Message-ID: <20160719234633.GI16044@dastard> (raw)
In-Reply-To: <20160719085208.GA21785@lst.de>

On Tue, Jul 19, 2016 at 10:52:08AM +0200, Christoph Hellwig wrote:
> 
> On Mon, Jul 18, 2016 at 03:37:46PM +1000, Dave Chinner wrote:
> > > The reason I did this in the first place was a vague notion that unconditional
> > > packing was harmful.
> > > 
> > > http://digitalvampire.org/blog/index.php/2006/07/31/why-you-shouldnt-use-__attribute__packed/
> > > 
> > > "However, it's actively harmful to add the attribute to a structure that's
> > > already going to be laid out with no padding."
> > > ...
> > > "gcc gets scared about unaligned accesses and generates six times as much code
> > > (96 bytes vs. 16 bytes)! sparc64 goes similarly crazy, bloating from 12 bytes
> > > to 52 bytes"
> > > 
> > > I don't know if that's (still) correct or not, but that was the reason
> > > for the selective __pack application way back when.  Might be worth
> > > investigating?
> > 
> > Christoph? The first two ptches are fine, but more info is needed
> > for this one...
> 
> I don't have a sparc64 compiler to test unfortunately.  But I can confirm
> that on x86-64 xfs.o is bit to bit identical with or without the patch.

OK, that's probably good enough to go with for now. Thanks for
following up, Christoph.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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      reply	other threads:[~2016-07-19 23:46 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-06-24  7:52 get rid of unaligned embedded structs in on-disk structures Christoph Hellwig
2016-06-24  7:52 ` [PATCH 1/3] xfs: kill xfs_dir2_sf_off_t Christoph Hellwig
2016-06-24  7:52 ` [PATCH 2/3] xfs: kill xfs_dir2_inou_t Christoph Hellwig
2016-06-24  7:52 ` [PATCH 3/3] xfs: remove __arch_pack Christoph Hellwig
2016-06-24 14:55   ` Eric Sandeen
2016-07-18  5:37     ` Dave Chinner
2016-07-19  8:52       ` Christoph Hellwig
2016-07-19 23:46         ` Dave Chinner [this message]

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