Re: linux/bitops.h

From: Jeffrey Walton
Date: Wed May 04 2016 - 21:20:40 EST

On Wed, May 4, 2016 at 7:06 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> On Wed, May 04, 2016 at 03:06:04PM -0700, 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?
> I don't think Linux runs on a system where it would make a difference
> (like a VAX), and also gcc always converts it before it could.
> Even UBSan should not complain because it runs after the conversion
> to ROTATE.
>From what I understand, its a limitation in the barrel shifter and the
way the shift bits are handled.

Linux runs on a great number of devices, so its conceivable (likely?)
a low-cost board would have hardware limitations that not found in
modern desktops and servers or VAX.