linux-kernel - Re: [PATCH] remove abs64()

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Message-ID: <20110412211452.GA20135@p183>
Date:	Wed, 13 Apr 2011 00:14:53 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Randy Dunlap <rdunlap@...otime.net>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	behlendorf1@...l.gov, oleg@...hat.com
Subject: Re: [PATCH] remove abs64()

On Tue, Apr 12, 2011 at 02:07:26PM -0700, Randy Dunlap wrote:
> On Wed, 13 Apr 2011 00:00:45 +0300 Alexey Dobriyan wrote:
> 
> > We don't need no stinking abs64() given some GCC extensions
> > (especially __builtin_choose_expr()).
> > 
> > One abs() implementation is better than two abs() implementations.
> 
> questionable.

Come on, what's so special in 64-bit types?

> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h

> > -#define abs64(x) ({				\
> > -		s64 __x = (x);			\
> > -		(__x < 0) ? -__x : __x;		\
> > -	})
> > +#define abs(x)								\
> > +({									\
> > +	typeof(x) _x = (x);						\
> > +									\
> > +	__builtin_choose_expr(						\
> > +		__builtin_types_compatible_p(typeof(_x), signed char),	\
> > +		(unsigned char)({ _x < 0 ? -_x : _x; }),		\

> that is better?

Infinitely better.
sizeof(abs(x)) == sizeof(x) for one thing.

Implementation is ugly, but, hey, one can't do better in C.

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