linux-kernel - Re: [PATCH-mm 1/8] ntfs: remove private wrapper around endian helpers

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Message-Id: <72F61C56-4613-4369-9319-09A0A8C90F7C@cam.ac.uk>
Date:	Mon, 1 Dec 2008 08:50:08 +0000
From:	Anton Altaparmakov <aia21@....ac.uk>
To:	Harvey Harrison <harvey.harrison@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH-mm 1/8] ntfs: remove private wrapper around endian helpers

Hi,

On 27 Nov 2008, at 19:35, Harvey Harrison wrote:
> On Thu, 2008-11-27 at 08:22 +0000, Anton Altaparmakov wrote:
>> On 26 Nov 2008, at 21:12, Harvey Harrison wrote:
>>> The base versions handle constant folding just fine.
>>> Done with sed other than the mft.c and inode.c one-liners.
>>
>> Have you checked the compiler output?  And on all architectures?  And
>> on all gcc versions?
>
> Yes I've checked the compiler output for the new byteorder helpers
> for gcc 3.4, 4, 4.2 on x86.
>
> Notice this depends on the byteorder bits in -mm.  The failure of
> the old bits to properly compile-time swap I've only seen at -O0,
> which the new bits do properly.  If you can cite a particular
> version/arch that you know of where the bug arose, please let me
> know.

Ah, I failed to notice this required some new byteorder code!  Sorry.

>> The reason I introduced the constant version are precisely that at
>> least some versions of gcc at least on some architectures do NOT
>> handle the constant folding and actually put in endianness conversion
>> code for the constants instead of converting them at compile time.
>
> Which ones....-O0 builds perhaps?
>
>> Also you are breaking sparse checking with this patch as you remove
>> some of the necessary endianness conversions (e.g. your one liner in
>> mft.c which causes us to apply a binary 'not' operator on a little
>> endian which sparse complains about - in fact that is the reason the
>> endianness conversions are in there in the first place - I added them
>> because sparse was complaining about them).
>
> No, sparse does not complain about it.  Bitwise operations are just
> fine on types such as le16 etc.  Try running sparse again with
> my changes, there are no new warnings.

Interesting.  Sparse must have grown up then.  It used to complain  
(although I always thought it was complaining incorrectly given that  
binary bit operations are endianness agnostic).

>> Andrew, please do not apply this patch.

> I hope you change you mind.

Yes, you have convinced me.  (-:

Andrew, please feel free to apply the patch.

Best regards,

	Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer, http://www.linux-ntfs.org/

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