Re: linux/bitops.h

From: H. Peter Anvin
Date: Fri May 06 2016 - 16:30:54 EST

On May 6, 2016 1:07:13 PM PDT, Sasha Levin <sasha.levin@xxxxxxxxxx> 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.

When the compiler in question is our flagship target and our reference compiler, then yes, it matters.
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