> As said above maybe it is here for "symmetry reasons". Zen design is highly symmetric with each
> execution unit coming in pairs.

Not so highly symmetric. There is only one integer multiplier, and only one integer divider.

And there is quite a lot of asymmetricity on the FP pipes, there are not two similar add pipes and two similar mul pipes, almost all other fp/vector instructions go quite differently to the pipelines.

> I suspect that AMD is not enabling full SMT on Zen, but a partial
> CSMT implementation. If this is true Zen is a simple clustered design and the fourth ALU is here
> because both clusters have to be the identical for simplifying thread selection and scheduling.

:D

Does not work that way due only one multiplier and one divider.