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Comment by nickcw | Hacker News Reader

Chip designers have noticed this similarity in the past have and attempted to push some of the work done by the CPU back into the compiler. Itanium and the Mill CPU both work(ed) like this I believe.

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Mill CPU never existed in silicon, so there are questions to that. But IIRC Ivan said that they didn't do anything that compilers didn't do already for DPSs. He specifically mentioned that they don't want to be like Itanic.

This will never work due to incentive structure, unless the compiler devs are working for the same company that makes the CPU. Otherwise, compiler devs will target a lowest common denominator and call it a day. And even if the compiler devs perfectly support new CPU instructions, compiler users usually want a single binary that can run on as many CPUs as possible, and so will use the lowest common denominator once again. Currently, your CPU will 99% of the time run basic AMD64 instructions, regardless of its capabilities. So Intel and AMD try to make their CPUs really good at running AMD64 code.

> unless the compiler devs are working for the same company that makes the CPU

Every CPU manufacturing company have a compile team.

> This will never work

VLIW processors do work, and for a while now. This type of architecture performs better for data-intensive workloads, so you don't see them in the general-purpose world.

But if you are talking about Mill, yes it will never work.

> But if you are talking about Mill, yes it will never work.

Maybe Mill Computing won't pull it off, but why do you think the approach is bad? It seems sufficiently different from Itanium to not have the exact same problems.

They don't have a "deliver at all costs" philosophy and they are creating a new and different CPU architecture from scratch, a task where even teams oriented at delivering at all costs tend to almost always fail.

The architecture itself looks very good. If it was built in a way similar to RISC-V, it would probably become very influential. (But then, I'm not sure you can create something that innovative by the same procedures RISC-V was created.)

The architecture looks like a dead end compared to other novel architecture like the Reduceron which reached 33% of the performance of a single core 2 duo processor core while running on an FPGA with only 90MHz without using pipelining or out of order execution. The catch is that the execution model of the Reduceron is graph reduction which basically means lambda calculus/lazy functional programming languages. The Reduceron's raw arithmetic performance isn't better, but it doesn't matter because lazy evaluation is memory intensive, the more memory intensive the algorithm, the better the Reduceron performs, which is very unlike classical C machines like x86 or ARM or RISC-V where memory intensitivity must be avoided at all costs for good performance. Where the Reduceron does a single graph reduction per clock cycle, a C machine has to do it with several sequential instructions so a massive clock rate advantage isn't worth much.

Well, first of all, because it shows no results after +10 years. There is definitely no indication that it will work someday.

And above all because there are too many choices that are too specific, outdated and too exotic. (e.g. the split-stream encoding is way too exotic)

I work in a small company that makes processors, and I know from experience that developing a processor is a very complicated subject, you have to go step by step (Mill does not). When you come up with a new design/idea, you try to simulate it, test it and implement it. You don't pile up new ideas without getting feedback on them.

VLIW works _really_ well if you have deterministic memory timings, e.g. DSP/risc style architectures, and you're up for writing a complicated compiler.

And at least the Intel compiler was/is pretty good at producing a single binary that uses whatever features your CPU has available by switching to different code paths depending on the cpuid flags. At least if your CPU is an Intel CPU, it's more conservative with that on AMD CPUs.

> Every CPU manufacturing company have a compile team

Sure, but that doesn't help when all the binaries are compiled for AMD64 anyway.

You can compile your own binaries if max performance is the goal.

Assuming you have access to the source code of course.

Intel and AMD are so good at running AMD64 code that it's often slower to use additional instructions.

It's quite complicated. Sometimes hardware instructions are awkward or redundant enough that compilers ignore them. Or they're broken in hardware and the compiler tries to patch around them. Sometimes there are invariants that are easy for compilers and really annoying for assembly programmers. A given sequence of instructions may execute correctly on multiple (sub)architectures with widely divergent performance behaviour.

The one binary that does minimal to no self modification on boot is popular with users but intrinsically slower. Arm's variable length vector instructions are catering to that.

One binary that is specialised to hardware and expected arguments is the fastest you can do without optimising while the program is starting/running.

However hardware usually changes slower than software. Specifically if a program starts running on a specific x64 chip it's likely to stay on that chip until it finishes running. Linux used to patch itself on boot to pick a faster memcpy (may still do).

It seems obvious that fastest for sufficiently long running processes is to ship a compiler IR, specialise first to the hardware and second to patterns in the data. OpenCL/spir-v have a finaliser premise where you lower to a given GPU just before running on it. Specialising on program data is branded JIT compilation. I don't know of anything doing both yet.

> Otherwise, compiler devs will target a lowest common denominator and call it a day.

Mill's idea for that is that compiler outputs a multi-target binary, which is specialized to the current CPU at install/first-run time, by fixing the size of the belt and other such hardware details.

> "genAsm is a dataflow language, essentially a programmable representation of a single-assignment compiler IR."

https://millcomputing.com/docs/compiler/

You can argue that it's the RISC approach, to some extent.

All of them. And the compiler engineers try to push work into the CPU.

I used to enjoy the Mill CPU videos. Is there some good post-mortem explaining what happened to that project?

https://millcomputing.com/topic/what-is-your-roadmap-for-202...

https://millcomputing.com/topic/on-the-lack-of-progress-repo...

It is reportedly still alive according to their website, but it's not very convincing.

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