On Mon, 25 Apr 2022, David Laight wrote: > From: Linus Torvalds > > Sent: 24 April 2022 22:42 > > > > On Sun, Apr 24, 2022 at 2:37 PM Linus Torvalds > > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > Finally, for the same reason - please don't use ">> 8". Because I do > > > not believe that bit 8 is well-defined in your arithmetic. The *sign* > > > bit will be, but I'm not convinced bit 8 is. > > > > Hmm.. I think it's ok. It can indeed overflow in 'char' and change the > > sign in bit #7, but I suspect bit #8 is always fine. > > > > Still, If you want to just extend the sign bit, ">> 31" _is_ the > > obvious thing to use (yeah, yeah, properly "sizeof(int)*8-1" or > > whatever, you get my drift). > > Except that right shifts of signed values are UB. > In particular it has always been valid to do an unsigned > shift right on a 2's compliment negative number. > > David Yes. All the standard versions (C89, C99, C11, C2X) say that right shift of a negative value is implementation-defined. So, we should cast it to "unsigned" before shifting it. Mikulas
