The full x86 effective address looks like displacement + base + index * scale
(where displacement
is a constant, base
and index
are registers, and scale
is 1, 2, 4 or 8).
It sounds like they call an address simple if only the displacement
is present (or maybe additionally the base
term), while having index * scale
would certainly fall under the complex category.
Update: Indeed, the intel optimization manual has this statement (for Sandy Bridge, though): The common load latency is five cycles. When using a simple addressing mode, base plus offset that is smaller than 2048, the load latency can be four cycles. See also Table 2-12. Effect of Addressing Modes on Load Latency.