linux-kernel - Re: [RFC][PATCH] A generic boolean

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Message-ID: <44D39A59.5070805@zytor.com>
Date:	Fri, 04 Aug 2006 12:04:57 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jes Sorensen <jes@....com>
CC:	Andreas Schwab <schwab@...e.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Jeff Garzik <jeff@...zik.org>, ricknu-0@...dent.ltu.se,
	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...l.org>
Subject: Re: [RFC][PATCH] A generic boolean

Jes Sorensen wrote:
>>
>> And what will break if you make that switch?
> 
> If we are lucky, some binary only modules? :-)
> 
> But you're right, it may just have to be documented as one of those
> nasty issues to watch out for.
> 

What is really poisonous is structures which get padded when all the 
members are naturally aligned.  Unfortunately gcc produces really crap 
code with __attribute__((packed)) on some architectures, so just using 
that isn't a good solution.  On the other hand, non-AEABI ARM sometimes 
needs it!

For the lack of a __attribute__((nopad)) that would throw a warning or 
error on excessive padding, I fear that our best option is an __abi 
annotation which would enforce certain rules using sparse, and 
presumably provide __attribute__((packed)) on ARM:

- All padding must be explicit.
- All members must be naturally aligned.
- No unportable constructs, like non-int-sized bitfields.

	-hpa
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