On May 4, 2016 6:20:32 PM PDT, Jeffrey Walton <noloader@xxxxxxxxx> wrote: >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. > >Jeff This is a compiler feature, though. And if icc or clang don't support the idiom they would never have compiled a working kernel. -- Sent from my Android device with K-9 Mail. Please excuse brevity and formatting. -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
