Ivan Godard

Positively antique! We’ve hoped for someone to join that wants to do it, but so far… You interested?

An update: Conforming is no built into branch ops, so the only use of a separate op is to recover live values from the mix of live and dead, as you suggest. Consequently the former “conform” op is no spelled “rescue”.

Rescue is cheap in execution time but expensive in code space when a value is only used far from where it is defined. A significant part of the specialize heuristics are devoted to finding a balance between use of rescue and of spill-fill.

We looked into auto-kill, but so far it seems to not fit the hardware timing; figuring out how things get reordered in the specializer is faster than trying to do it at runtime.

Rescue is only useful when the currently live values fit on the belt but some are out of reach. If the live-value count exceeds the belt size then there’s nothing for it but spill and refill when belt pressure is lower.

Don’t know anything about “A20” either, nor why you ask. I did mention “L4”, see https://en.wikipedia.org/wiki/L4_microkernel_family

We are doing a micro-kernel, which is necessarily from scratch although we may borrow approaches from L4.

Much less burdensome on a Mill, but still will be a long time and a lot of work, yes.

However, we need to start the OS work so as to have a test and verification code suite for the system aspects of the chip. Even an Arduino needs interrupt handling, atomics, … Standard benchmarks don’t exercise those parts of the design.

Various announcements coming soon (for some value of soon).

Ivan

What’s a daxpy?
Daxpy is a BLAS function, the most commonly used: “double precision A times X + Y”; see https://en.wikipedia.org/wiki/Basic_Linear_Algebra_Subprograms#Level_1. It’s why hardware has FMA (a.k.a MAC) ops.

And when you write “cycle,” do you mean a clock cycle or a Mill instruction?
Absent stalls, Mill issues all instructions of one bundle (formerly described as “all operations of one instruction”, except people got confused by the terminology) in one clock cycle. So cycle and bundle (instruction) are equivalent.

We expect that the next talk will be less about how the architecture works and more about how well it works – measured performance numbers, guesstimates for area and power, that kind of thing.

As a slight variation on the multiple values in one go theme, would a “touch” like operation that specifies multiple belt positions (maybe just in the last N positions) that are needed soon again help in any way?

That’s what “rescue” does.

Also, never having worked with an architecture that exposed a scratchpad to me, I am wondering how its typically used. If it would be for constants that are needed a few times during execution of code, I imagine normal memory addressing and the cache system would work just fine. Is there a typical cutoff point where the scratchpad starts to benefit and at what sizes?

The compiler doesn’t load anything twice from memory/cache that it knows is unchanged; memory bandwidth is limited and cache is a power hog. Generally literal constants and some LEAs are simply redone if needed a second time, rather than being run off to scratch. We should probably revisit this for quad FP literals, assuming that a spill/fill is cheaper than a 16-byte literal, but we don’t have much real experience with quad yet. Some programs have memory arrays of constants that they load with explicit addressing; LLVM is a little erratic in how it treats those loads (to constexpr or not), but we just take what LLVM gives us.

• This reply was modified 5 years, 8 months ago by  Ivan Godard.

Rescue does in fact have twice the range of other references. phases are an abstraction, and there are actually no intra-cycle data movements. Consequently, at the end of the cycle (when rescue takes place) there are the belt’s worth of operand that started the cycle plus however many were dropped during the cycle physically available, which is up to twice belt size. Congratulations; I don’t think anyone before has spotted that to ask about it.

I’m not sure I’ve understood your suggestions; if my response here seems inapt to you then please post again.

First, as to “sector”. This seems to be a notion where there are multiple distinct address spaces, with one of them being designated as “current” some how, so one can address within a sector using fewer bits than would be required to address across sectors. If that’s right, then in effect we already have such sectoring: “sectors” are created and destroyed automatically by the call/return mechanism, so each called function has its own private sector. This is a little different from your sectors though, because there is no inter-sector addressing; a function is stuck with its sector and cannot switch to another.

Then, as to the “chain of waterfalls” idea, where each space “falls off the end” into the next space. This won’t really help, because eventually everything ever produced at the top will eventually fall over every waterfall – which means that the path to DRAM (the last) would have to have the same bandwidth as the rate of result production at the top, in the core; the whole machine would run at memory speed, which in core terms is sloooow. The chain is also pointless, because most operands are finished with before they fall off, so there’s no need to preserve them over the subsequent falls.

So there needs to be some way to designate which operands are to be discarded and which passed on to the next level. The Mill belt by default discards everything, with the program explicitly saving any still alive. It could be done the other way, saving everything except what is explicitly discarded, but then one would still need to say where things are to be saved, and yes, an auto-save to a belt-like next level is possible. However, that level in turn would have to be kept congruent over control flow, which costs bits too. Which is the better trade-off depends on the likelihood of control flow over the life of an operand on the second level and the size of that level; looking at the code we are trying to execute, we decided that using spatial addressing on the second (scratchpad) level worked best.

Then, you suggest a “save many” and “restore many” so as to reduce the save/restore entropy, analogous to the register bulk save/restore of legacy ISAs. A “many” operation would be easier on a microcoded machine (Mill doesn’t use microcode) but could certainly be implemented on a non-micro physical architecture. However, to be useful the lifetimes of the operands in the “many” would need to be similar, so they could be saved and restored as a group. While the start of lives in a group would be similar (everything needing saving that was produced since the last save), the end of lives turn out to be very different, almost random, obviating a “restore many”. This suggests that a “save many” and “restore one” design would be possible. It’s not clear to me whether the entropy saving of a “save many” operation would justify its hardware complication; needs thinking about.

Then about sector+offset addressing. The value of this depends on whether multiple references are contiguous enough to pay the cost of switching sectors (and keeping the “current sector” congruent over control flow). Looking into that and doing actual measurement would be a good intern project, but by eyeball based on the code that we have running now I’m inclined to think that the reference patterns are too random to offer enough clustering value. It would also complicate the software, which is why sectoring has dropped out of use in legacy architectures.