Not once ever. Once per toolchain version and unique argument set. Perhaps the toolchain starts by attempting to never offer arguments, and perhaps it has lofty goals to release on day 1 with a perfect compilation model. All of my experience suggests these constraints are untenable, so ever is somewhat of an embellishment.

The model also precludes cross-unit optimizations so there's a performance upper bound on compilation model that leaves it in the middle tier of performance without the model being broken (given things we've seen before).