linux-kernel - Re: linux/bitops.h

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Date:	Fri, 6 May 2016 16:07:13 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	"H. Peter Anvin" <hpa@...or.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 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