linux-kernel - Re: [PATCH] bitops: add 8-bit and 16-bit rotation functions

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Message-Id: <1205270102.22317.40.camel@brick>
Date:	Tue, 11 Mar 2008 14:15:02 -0700
From:	Harvey Harrison <harvey.harrison@...il.com>
To:	Andreas Schwab <schwab@...e.de>
Cc:	Segher Boessenkool <segher@...nel.crashing.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] bitops: add 8-bit and 16-bit rotation functions

On Tue, 2008-03-11 at 21:59 +0100, Andreas Schwab wrote:
> Segher Boessenkool <segher@...nel.crashing.org> writes:
> 
> >> +/**
> >> + * rol16 - rotate a 16-bit value left
> >> + * @word: value to rotate
> >> + * @shift: bits to roll
> >> + */
> >> +static inline __u16 rol16(__u16 word, unsigned int shift)
> >> +{
> >> +	return (word << shift) | (word >> (16 - shift));
> >> +}
> >
> > This doesn't work for shift values of 0: you get word >> 16, and
> > shifts greater than or equal to the word size aren't valid C.  GCC
> > will warn about this, too.
> 
> On the other hand, a value narrower than int will always be promoted
> first, so this is not a problem in this case.
> 

It's the same way rol32/ror32 is done directly above this section, I saw
this as well, but figured that if checking for shift = 0 was wanted, it
would have been there.

So...don't do that ;-)

Harvey

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