[gpderetta]

> and it's a different idea than decoding multiple instructions in a cycle and sending them to one of many execution units (which is usually just called "dispatch", though "out of order execution

Being able to execute multiple instructions is more properly superscalar execution, right? In-order designs are also capable of doing it and the separate execution unit do not even need to run in lockstep (consider the original P5 U and V pipes).

[wtallis]

Right; it's easy to forget that superscalar CPU cores don't actually have to be in-order, but most of them are out-of-order because that's usually necessary to make good use of a wide superscalar core.

(What's the best-performing in-order general purpose CPU core? POWER6 was notably in-order and ran at quite high clock speeds for the time. Intel's first-gen Atom cores were in-order and around the same time as POWER6 but at half the clock speed. SPARC T3 was ran at an even lower clock speed.)

[gpderetta]

POWER6 might indeed have been the last In-Order speed demon.

[sillywalk]

The IBM Z10 came out a year later. It was co-designed with POWER6 as part of IBM's eClipz project, and shared a number of features / design choices, including in-order execution.

[ajross]

In-order parallel designs are "VLIW". The jargon indeed gets thick. :)

But as to OO: the whole idea of issuing sequential instructions in parallel means that the hardware needs to track dependencies between them so they can't race ahead of their inputs. And if you're going to do that anyway, allowing them to retire out of order is a big performance/transistor-count win as it allows the pipeline lengths to be different.

[gpderetta]

VLIW is again a different thing. It uses a single instruction that encodes multiple independent operations to simplify decoding and tracking, usually with exposed pipelines.

But you can have, for example, a classic in-order RISC design that allows for parallel execution. OoO renaming is not necessary for dependency tracking (in fact even scalar in order CPUs need dependency tracking to solve RAW and other hazards), it is "only" needed for executing around stalled instructions (while an in order design will stall the whole pipeline).

Again P5 (i.e the original Pentium) was a very traditional in order design, yet could execute up to two instructions per cycle.

[ajross]

> VLIW is again a different thing.

No it isn't. I'm being very deliberate here with refusing pedantry. In practice, "multiple dispatch" means "OO" in the same way that "VLIW" means "parallel in order dispatch". Yes, you can imagine hypothetical CPUs that mix the distinction, but they'd be so weird that they'd never be built. Discussing the jargon without context only confuses things.

> you can have, for example, a classic in-order RISC design that allows for parallel execution.

Only by inventing VLIW, though, otherwise there's no way to tell the CPU how to order what it does. Which is my point; the ideas are joined at the hip. Note that the Pentium had two defined pipes with specific rules about how the pairing was encoded in the instruction stream. It was, in practice, a VLIW architecture (just one with a variable length encoding and where most of the available instruction bundles only filled one slot)! Pedantry hurts in this world, it doesn't help.

[wtallis]

> Note that the Pentium had two defined pipes with specific rules about how the pairing was encoded in the instruction stream. It was, in practice, a VLIW architecture (just one with a variable length encoding and where most of the available instruction bundles only filled one slot)!

This is ridiculous. There are no nop-filled slots in the instruction stream, and you can't even be sure which instructions will issue together unless you trace backwards far enough to find a sequence of instructions that can only be executed on port 0 and thus provide a known synchronization point. The P5 only has one small thing in common with VLIW, and there's already a well-accepted name for that feature, and it isn't VLIW.

[ajross]

Meh. P5's decode algorithm looks very much like VLIW to me, and emphatically not like the 21264/P6 style of dispatch that came to dominate later. I find that notable, and in particular I find senseless adherence to jargon definitions[1] hurts and doesn't help in this sphere. Arguing about how to label technology instead of explaining what it does is a bad smell.

[1] That never really worked anyway. ia64, Transmeta's devices and Xtensa HiFi are all "VLIW" by your definition yet work nothing like each other.

[gpderetta]

I'm sorry, but if P5 was VLIW then the word has lost all meanings. They couldn't possibly be more different.

[ajross]

The point was exactly that "VLIW" as a term has basically no meaning. What it "means" in practice is parallel in-order dispatch (and nothing about the instruction format), which is what I said upthread.

[atq2119]

VLIW means Very Large Instruction Word. It is a property of the instruction set, not of the processor that implements it.

You could have a VLIW ISA that is implemented by a processor that "unrolls" each instruction word and mostly executes the constituent instructions serially.

[tliltocatl]

Also you can have out-of-order VLIW. Later Itaniums were like that, because turns VLIW doesn't help much with random memory access latency.