On Sun, Apr 24, 2022 at 2:37 PM Linus Torvalds
<[email protected]> 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).
Linus
From: Linus Torvalds
> Sent: 24 April 2022 22:42
>
> On Sun, Apr 24, 2022 at 2:37 PM Linus Torvalds
> <[email protected]> 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
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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
> > <[email protected]> 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