5 ms·

VLIW was really marooned in time: driven by overconfidence in the compiler (which had shown that you could actually expose pipeline hazards), and underestimates

VLIW was really marooned in time: driven by overconfidence in the compiler (which had shown that you could actually expose pipeline hazards), and underestimates of the coming abundance of transistors (which make superscalar OoO really take off, along with giant onchip caches). well, and multicore to sop up even more available transistors.

panick21_ 4y ago
The problem is that VLIW has already proven not to work in the 90s with lots of companies investing money in it but not making it a product. EPIC was basically VLIW++ with lots of added stuff that should overcome the issues with it, but not doing that successfully. I don't think they underestimated the amount of transistors, they just thought that EPIC would be a better way to use them. OoO had already proven itself in the 90s as well, so its not like this was unknown when they designed EPIC.
- kragen 4y ago
  otoh, for the previous 20 years, things like the 432 and lispms and burroughs large systems had been losing, in favor of architectures that pushed all the hard work onto compilers so it makes sense that in 01995 you'd look at ooo and vliw and extrapolate that vliw/epic was going to beat the crap out of ooo
  - panick21_ 4y ago
    Granted it makes some amount of sense. But the issue is that with EPIC you still can address every part of the processor unless you want to continue to grow the instruction. So you end up having to do OoO anyway but you just made it much more complex and hard to reason about. I'm not a chip designer but is what I understood to be one of the issues. Also if this compiler stuff wasn't jet written, unlike with RISC where people at Standford showed successful compilation for RISC already before people even developed any high performance RISC chips. I don't want to claim I'm smarter then those people, clear all the people working on these VLIW processor were a lot smarter then me. But then again many smart people worked on Alpha and they didn't go the VLIW route.

panick21_ 4y ago

The problem is that VLIW has already proven not to work in the 90s with lots of companies investing money in it but not making it a product. EPIC was basically VLIW++ with lots of added stuff that should overcome the issues with it, but not doing that successfully. I don't think they underestimated the amount of transistors, they just thought that EPIC would be a better way to use them. OoO had already proven itself in the 90s as well, so its not like this was unknown when they designed EPIC.

kragen 4y ago
otoh, for the previous 20 years, things like the 432 and lispms and burroughs large systems had been losing, in favor of architectures that pushed all the hard work onto compilers so it makes sense that in 01995 you'd look at ooo and vliw and extrapolate that vliw/epic was going to beat the crap out of ooo
- panick21_ 4y ago
  Granted it makes some amount of sense. But the issue is that with EPIC you still can address every part of the processor unless you want to continue to grow the instruction. So you end up having to do OoO anyway but you just made it much more complex and hard to reason about. I'm not a chip designer but is what I understood to be one of the issues. Also if this compiler stuff wasn't jet written, unlike with RISC where people at Standford showed successful compilation for RISC already before people even developed any high performance RISC chips. I don't want to claim I'm smarter then those people, clear all the people working on these VLIW processor were a lot smarter then me. But then again many smart people worked on Alpha and they didn't go the VLIW route.