Re: linux/bitops.h

[Sasha Levin]

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