linux-kernel - Re: linux/bitops.h

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Date:	Wed, 4 May 2016 17:30:47 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	John Denker <jsd@...n.com>, tytso@....edu, noloader@...il.com,
	linux-kernel@...r.kernel.org,
	Stephan Mueller <smueller@...onox.de>,
	Herbert Xu <herbert@...dor.apana.org.au>, andi@...stfloor.org,
	Sandy Harris <sandyinchina@...il.com>,
	cryptography@...edaemon.net, linux-crypto@...r.kernel.org
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: linux/bitops.h

On 05/04/16 15:06, John Denker wrote:
> On 05/04/2016 02:56 PM, H. Peter Anvin wrote:
>>> Beware that shifting by an amount >= the number of bits in the
>>> word remains Undefined Behavior.
>
>> This construct has been supported as a rotate since at least gcc2.
>
> How then should we understand the story told in commit d7e35dfa?
> Is the story wrong?
>
> At the very least, something inconsistent is going on.  There
> are 8 functions.  Why did d7e35dfa change one of them but
> not the other 7?

Yes. d7e35dfa is baloney IMNSHO.  All it does is produce worse code, and 
the description even says so.

As I said, gcc has treated the former code as idiomatic since gcc 2, so 
that support is beyond ancient.

	-hpa