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Maybe it was inaccurate but I got what you were trying to say -- that "incoherent execution" is difficult on SIMD machines. > You may be interested to consider

by jhgb 4y ago

Maybe it was inaccurate but I got what you were trying to say -- that "incoherent execution" is difficult on SIMD machines. > You may be interested to consider how incredibly complex the modern x86 architecture is to implement because it supports sequential program execution as a core invariant principle. As a result, modern computers (which strive to be faster than a PDP-11) have to do a massive amount of work to parallelize that sequential code because parallel execution is the only frontier of fast computation that remains. I'm aware what recent CPUs do with ISA instructions. That flies very badly in the face of the minimum complexity principle as well. The amount of physical resources dedicated these days to making your legacy code run just slightly faster is exceptionally wasteful. > but in modern times calling jump "low level" (or, for that matter, calling branching in general low level) is a very "Do you think that's air your breathing?" kind of position Well, it's low-level for the kind of minimum complexity system that I'd consider ideal. Not necessarily for the abominations forced on us as an accident of history.

shadowgovt 4y ago
You might be interested in a fascinating game called TIS-100 by Zachtronics. The game is a lot of things, but a core premise is that it imagines a computer from a time approximately parallel to the PDP-11 that took the form of small compute components that were connected to each other instead of a monolithic central processor. It's a fun game, and it sort of raises the question of which is the "abomination[s] forced on us as an accident of history." Because when you look around at most of the biological world, you see heavily distributed systems with some centralization, but computers (man-made things that they are) are heavily centralized, clock-locked, deterministic... And energy-intensive. And slow. History is arbitrary but not random, and it's fun to think about how things might have been different if the first machines started embarrassingly parallel with follow-up work to consolidate the data instead of embarrassingly centralized with us now in the era of how to make the monoliths fast. It's interesting to think about what's "ideal" about a machine that supports arbitrary jumps (and the global address space that demands, and the sequential execution necessary to prevent decoherence, and the memory protection demanded because some addresses are not executable code and should never be executed, etc., etc.).
- jhgb 4y ago
  > and it sort of raises the question of which is the "abomination[s] forced on us as an accident of history." What I meant by that is the legacy of AMD64 having thousands of instructions, many of them with arbitrary opcode encoding, half-assed SIMD ISA instead of a proper vector ISA, and the ability to emulate an 8086, all purely for reasons of backwards compatibility. If you started designing a computing ecosystem completely from scratch, surely you wouldn't end up with an AMD64-based IBM PC descendant as your best idea you could come up with?
  - saagarjha 4y ago
    This would be kind of a sad direction to go in if we didn’t have any constraints, since we already have new architectures going in this direction anyways. When you take the crust of the ISA off though all processors look similar and that’s where the interesting bits lie.

by jhgb 4y ago

shadowgovt 4y ago

You might be interested in a fascinating game called TIS-100 by Zachtronics. The game is a lot of things, but a core premise is that it imagines a computer from a time approximately parallel to the PDP-11 that took the form of small compute components that were connected to each other instead of a monolithic central processor. It's a fun game, and it sort of raises the question of which is the "abomination[s] forced on us as an accident of history." Because when you look around at most of the biological world, you see heavily distributed systems with some centralization, but computers (man-made things that they are) are heavily centralized, clock-locked, deterministic... And energy-intensive. And slow. History is arbitrary but not random, and it's fun to think about how things might have been different if the first machines started embarrassingly parallel with follow-up work to consolidate the data instead of embarrassingly centralized with us now in the era of how to make the monoliths fast. It's interesting to think about what's "ideal" about a machine that supports arbitrary jumps (and the global address space that demands, and the sequential execution necessary to prevent decoherence, and the memory protection demanded because some addresses are not executable code and should never be executed, etc., etc.).

jhgb 4y ago
> and it sort of raises the question of which is the "abomination[s] forced on us as an accident of history." What I meant by that is the legacy of AMD64 having thousands of instructions, many of them with arbitrary opcode encoding, half-assed SIMD ISA instead of a proper vector ISA, and the ability to emulate an 8086, all purely for reasons of backwards compatibility. If you started designing a computing ecosystem completely from scratch, surely you wouldn't end up with an AMD64-based IBM PC descendant as your best idea you could come up with?
- saagarjha 4y ago
  This would be kind of a sad direction to go in if we didn’t have any constraints, since we already have new architectures going in this direction anyways. When you take the crust of the ISA off though all processors look similar and that’s where the interesting bits lie.