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

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Message-Id: <5116e96e09f6fc82587283a2a0cdd8ff@kernel.crashing.org>
Date:	Tue, 11 Mar 2008 22:18:27 +0100
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Andreas Schwab <schwab@...e.de>
Cc:	Harvey Harrison <harvey.harrison@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] bitops: add 8-bit and 16-bit rotation functions

>>> +/**
>>> + * 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,

Erm, yes of course.  It is promoted to _signed_ int though, but
that works okay here.

> so this is not a problem in this case.

It still needs documentation for the valid values of "shift".


Segher

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