linux-kernel - Re: linux/bitops.h

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Message-ID: <33C7EF5D-096B-4578-A0D8-E4B94D516F9A@zytor.com>
Date:	Fri, 06 May 2016 13:25:07 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Sasha Levin <sasha.levin@...cle.com>, 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:	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: linux/bitops.h

On May 6, 2016 1:07:13 PM PDT, Sasha Levin <sasha.levin@...cle.com> wrote:
>On 05/04/2016 08:30 PM, H. Peter Anvin wrote:
>> 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.
>
>No, the description says that it produces worse code for *really
>really* ancient
>GCC versions.
>
>> As I said, gcc has treated the former code as idiomatic since gcc 2,
>so that support is beyond ancient.
>
>Because something works in a specific way on one compiler isn't a
>reason to
>ignore this noncompliance with the standard.
>
>
>Thanks,
>Sasha

4.6.2 is not "really, really ancient."
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