3 ms·

> we're also going to see someone give VLIW another shot Dunno why people would keep thinking this is a good idea, but if someone wants to try eating it with a

> we're also going to see someone give VLIW another shot Dunno why people would keep thinking this is a good idea, but if someone wants to try eating it with a new VLIW design yet again, I’m always down for some popcorn. Dynamically finding ILP at instruction dispatch time is always going to be smarter than a compiler trying to guess at compile time. IMO, it’s worth the area. Even if you assume the compiler gets it right and finds tons of ILP, it’ll get it wrong for the next hardware rev when your core inevitably changes and you have to break apart your VLIW instructions and get ILP back dynamically anyway. To say nothing of shared pipelines in an SMT processor, where VLIW basically runs counter to SMT and the only way to know which units are going to be available on a specific cycle is during runtime. > and they're going to succeed this time. I don’t see any reason why VLIW would succeed more in future attempts than in past attempts.

cakoose 6y ago
Even if compiler-only ILP is doomed to fail, maybe VLIW optimists are just hoping something useful might come out of everything we've learned? For example, compilers already model the CPU's pipeline and ILP capacity to inform instruction selection. VLIW designs are an attempt to make that cooperation more explicit, similar in spirit to branch/prefetch hints. Maybe there's still a way to do that well? And re: microarchitectural changes in the next hardware rev: https://news.ycombinator.com/item?id=25237056 https://news.ycombinator.com/item?id=25237056 These ideas might ultimately fail, but they seem worth looking in to, at least.
- djcapelis 6y ago
  I think you are correct that there is a section of the field enchanted by the idea of VLIW and desperately hoping to justify any possible reason why building these chips was less of a bad notion than they’ve turned out to be each and every time. I don’t think we’re really learning much by pretending building a new one is a good idea. Or that the last efforts had much value. No one is saying don’t make progress on compilers. Look around you and you’ll see the things you just referenced already exist in practice in production today. (Among others: compiler/architecture co-design is a well established and fertile area of work. Compiling to intermediate formats which specialize for a device later is in use to some degree in App Store, which keeps programs in an intermediate format and thins binaries down to specific slices on demand for iOS devices, etc.) None of these technologies justify building a new VLIW chip or have frankly much to do with those efforts. The best thing we could learn from the huge amount of effort that’s gone into VLIW architectures is actually learn the lesson that in front of us, demonstrated repeatedly, that unless something substantial changes, VLIW does not produce a better architecture for processor cores and the assumption that it would was wrong.

cakoose 6y ago

Even if compiler-only ILP is doomed to fail, maybe VLIW optimists are just hoping something useful might come out of everything we've learned? For example, compilers already model the CPU's pipeline and ILP capacity to inform instruction selection. VLIW designs are an attempt to make that cooperation more explicit, similar in spirit to branch/prefetch hints. Maybe there's still a way to do that well? And re: microarchitectural changes in the next hardware rev: https://news.ycombinator.com/item?id=25237056 https://news.ycombinator.com/item?id=25237056 These ideas might ultimately fail, but they seem worth looking in to, at least.

djcapelis 6y ago
I think you are correct that there is a section of the field enchanted by the idea of VLIW and desperately hoping to justify any possible reason why building these chips was less of a bad notion than they’ve turned out to be each and every time. I don’t think we’re really learning much by pretending building a new one is a good idea. Or that the last efforts had much value. No one is saying don’t make progress on compilers. Look around you and you’ll see the things you just referenced already exist in practice in production today. (Among others: compiler/architecture co-design is a well established and fertile area of work. Compiling to intermediate formats which specialize for a device later is in use to some degree in App Store, which keeps programs in an intermediate format and thins binaries down to specific slices on demand for iOS devices, etc.) None of these technologies justify building a new VLIW chip or have frankly much to do with those efforts. The best thing we could learn from the huge amount of effort that’s gone into VLIW architectures is actually learn the lesson that in front of us, demonstrated repeatedly, that unless something substantial changes, VLIW does not produce a better architecture for processor cores and the assumption that it would was wrong.