Comment by toast0

> It would be weird to have 2 sets of different adders.

Not really. CPUs often have limited address math available separately from the ALU. On simple cores, it looks like a separate incrementer for the Program Counter, on x86 you have a lot of addressing modes that need a little bit of math; having address units for these kinds of things allows more effective pipelining.

> Do we really know that LEA is using the hardware memory address computation units?

There are ways to confirm. You need an instruction stream that fully loads the ALUs, without fully loading dispatch/commit, so that ALU throughput is the limit on your loop; then if you add an LEA into that instruction stream, it shouldn't increase the cycle count because you're still bottlenecked on ALU throughput and the LEA does address math separately.

You might be able to determine if LEAs can be dispatched to the general purpose ALUs if your instruction stream is something like all LEAs... if the throughput is higher than what could be managed with only address units, it must also use ALUs. But you may end up bottlenecked on instruction commit rather than math.