linux-kernel - Re: [COMPAT] Add compat

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Message-ID: <20071003121211.GA14628@linux-mips.org>
Date:	Wed, 3 Oct 2007 13:12:11 +0100
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Kyle McMartin <kyle@...artin.ca>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [COMPAT] Add compat_merge64 helper

On Sat, Sep 29, 2007 at 11:48:52AM +0200, Heiko Carstens wrote:

> > +static inline u64 compat_merge64(u32 left, u32 right)
> > +{
> > +#if defined(__BIG_ENDIAN)
> > +	return ((u64)left << 32) | right;
> > +#else /* defined (__LITTLE_ENDIAN) */
> 
> Could you change that to an #elif please and #error out if none is defined?
> Should safe us from subtle bugs caused by missing includes.

This funny macro gets away without any extra headers or #ifdef messiness:

#define merge_64(r1,r2)							\
({									\
	union {								\
		int __words[2];						\
		long long	__dword;				\
	} __u = {							\
		.__words = { (r1), (r2) }				\
	};								\
									\
	__u.__dword;							\
})

Thanks to gcc doing bogus sign and zero extensions it compiles into
slightly larger code for MIPS but that may not be an issue on other
architectures.

   Ralf
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