On Wed, Dec 08, 2021 at 10:50:10AM -0800, Linus Torvalds wrote:
> On Wed, Dec 8, 2021 at 10:44 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > From testing xadd had different flags from add; I've not yet looked at
> > the SDM to see what it said on the matter.
>
> That should not be the case. Just checked, and it just says
>
> "The CF, PF, AF, SF, ZF, and OF flags are set according to the
> result of the addition, which is stored in the destination operand"
>
> which shows that I was confused about 'xadd' - I thought it returned
> the old value in the register ("fetch_add"). It doesn't. It returns
> the new one ("add_fetch"). And then 'fetch_add' ends up undoing it by
> doing a sub or whatever.
>
> So the actual returned value and the flags should match on x86.
>
> Other architectures have the "return old value" model, which does mean
> that my "different architectures can have different preferences for
> which one to test" argument was right, even if I got xadd wrong.
I think XADD does return old too; SDM states:
"Exchanges the first operand (destination operand) with the second
operand (source operand), then loads the sum of the two values into the
destination operand. The destination operand can be a register or a
memory location; the source operand is a register."
So it first exchanges and then adds. Which is why the flags are set for
add, not exchange.
