On Tue, Dec 7, 2021 at 12:28 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Argh.. __atomic_add_fetch() != __atomic_fetch_add(); much confusion for
> GCC having both. With the right primitive it becomes:
>
> movl $1, %eax
> lock xaddl %eax, (%rdi)
> testl %eax, %eax
> je .L5
> js .L6
>
> Which makes a whole lot more sense.
Note that the above misses the case where the old value was MAX_INT
and the result now became negative.
That isn't a _problem_, of course. I think it's fine. But if you cared
about it, you'd have to do something like
> movl $1, %eax
> lock xaddl %eax, (%rdi)
> jl .L6
> testl %eax, %eax
> je .L5
instead (I might have gotten that "jl" wrong, needs more testing.
But if you don't care about the MAX_INT overflow and make the overflow
boundary be the next increment, then just make it be one error case:
> movl $1, %eax
> lock xaddl %eax, (%rdi)
> testl %eax, %eax
> jle .L5
and then (if you absolutely have to distinguish them) you can test eax
again in the slow path.
Linus
