OoO cores have load stalls too after preemptions. Even something like a Apple M core basically has to stall if the load has to go out to the memory controller. The goal is to move far enough ahead in the stream that you can at least issue the next load, and advaned loads lets you do that. Some Intel cores will occasionally speculatively predict zero for a load, but AFAIK that got turned off as part of the Spectre mitigations.

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eigenform 20 hours ago | parent [-]

The difference is that this is not the programmer's responsibility in modern machines: instead we bake-in some hardware that watches the online state of the machine and then actively decides on what to do.

If you're trying to create a compiler that approaches the effectiveness of this statically, you're condemned to do a ton of extra work (you're essentially writing an emulator for your CPU core, and then a compiler that uses that model to produce optimal code - even then, you might not be accounting for nondeterminism on the actual target machine, and some information is simply not accessible to you when you are not on the target machine)

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monocasa 20 hours ago | parent [-]

Once again, Itanium isn't a normal, simple in order core that stalls on loads. It has a big table called the ALAT to allow you to start loads as soon as you know the address, and then finalize them later when you're out of other work to do.

It doesn't require determinism to work. And was designed by people that were quite aware of what the instruction stream looks like to an OoO core as it issues out of the rob.

Itanium had other sins than "magic compiler" woes, or even the inherent unpredictability of memory accesses. Mostly that it, like Cell, and Netburst was designed for a world where dennard scaling didn't end like a brick wall. As well as internal politics of Intel making it so that they were a bit loose and fast with die area.

	▲	eigenform 10 hours ago \| parent [-]
		Yes, I'm pointing out the fact that the programmer is expected to manage the ALAT, and that these machines do not automatically recover from cases where your advanced loads are incorrect. That process is expected to be part of the instruction stream, and [we have collectively learned that] that's an expensive feedback loop. Yes, it doesn't require determinism to work, but it means that your performance is especially sensitive to nondeterminism because the compiler cannot account for loads and stores that occur online. The SDM (see vol 1, section 9.5.1) describes this idea pretty well.