Does anyone know what the speculative execution notes in processor.h are talking about? The code seems to be trying to set up spec execution "barriers." What d

Does anyone know what the speculative execution notes in processor.h are talking about? The code seems to be trying to set up spec execution "barriers." What does that mean? I thought speculative execution was a micro-architecture optimization (and the speculatively executed instructions won't be retired until the processor knows that it wants the side-effects). In contrast, I've only seen barriers come up for x86 at the ISA-level (and even then, only for multiprocessors setups).