This kind of memory order speculatiom is basically required on x86 since the strong semantics would otherwise prevent many useful reorderings (especially L-L which is absolutely critical).

The basic way it works is that pretty much any reordered is allowed, speculatively, even if it violates the explicit or implicit barriers, but then the operations are tracked in a memory order buffer (MOB) until retirement which can detect whether the reordering was detectable and if so flush the pipeline (so-called "memory order nuke", visible in performance counters).