Well it's hard to summarize what I said in the Coffee Compiler club chat in a HN comment, but there were a number of things that went wrong there. I half agree with Cliff and half agree with the V8 blogpost. TurboFan evolved into a very complicated compiler that made a number of things harder on itself that it should have been.

The sea of nodes is just extending SSA renaming on values to both control and effects. Effect dependencies are equivalent SSA renaming of the state of the world, allowing relaxed ordering of effectful operations and more general transforms. That means that GVN and load elimination are the same thing when effect dependencies are explicitly part of the graph.

Making control and effect dependencies explicit is great!

What makes the sea of nodes complicated is relaxing linear control and effects to allow more reorderings. Many optimizations require a more general algorithm (which is sometimes inefficient, but mostly not) and other optimizations can be almost impossible. E.g. reasoning about what happens between two instructions is impossible--there is no such thing, except after scheduling. For most optimizations, the chain of dependencies is enough. Not all. Loop transforms become more complicated, making regions of code that are uninterruptible (e.g. fully initializing an object before it can be see by the GC) is tough, and a few other things.

Overall I would say that TurboFan's main problem was that did not relax effect edges and it tried to introduce speculation too late and tried to that in the sea of nodes representation. It would have been a better design to do some optimizations on a CFG representation prior to the heavy lifting in optimizations that work on the sea of nodes.

One of TurboFan's good architectural decisions was to separate operators from the node representation, so that reasoning could be somewhat independent of how nodes represent dataflow and effects, but it looks like that got junked in favor of the class-based organization (https://github.com/v8/v8/blob/main/src/maglev/maglev-ir.h) which is pure 90s tech lifted straight from C1 and Crankshaft. When I see an IR that's 11K lines in a header, I find it astonishing. Pity, that 11K knot isn't just self-contained, it will replicate itself over and over and over in the compiler and make a big mess in the end.

I think the main part of the V8 blogpost I agree with is that the sea of nodes is difficult to debug, especially for big graphs. I don't see any way around that except a whole crapton of testing, better tools, graph verifiers, etc. There's a learning curve to any compiler, and complex compilers have complex failure modes. Still, I think some people on the V8 team just always hated the sea of nodes and blamed all of their problems on it. It didn't help that all of the senior people who developed expertise with the IR moved on.

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Leszek 4 hours ago | parent [-]

(disclaimer: current V8 team member replying to senior ex-V8 team member)

I'd say there were a lot more things problematic in practice with SoN than not relaxing effect edges enough - I'd argue that the bigger problem was that a single effect chain was not enough to represent the flexibility that SoN promised, while keeping the costs, and getting that flexibility would mean effectively extending the effect chain to one effect colour per object per field. Maybe this was a JS specific idiosyncrasy, but my experience was that the effect chain became almost homomorphic to the control chain (and when it wasn't, we had bugs), and then you may as well merge the two into a CFG - if you have to skip over links in your effect chain to skip over certain kinds of effect then you can equally well skip over zero effect nodes too. With SoN, we got all the costs and (almost none) of the benefits, hoisting really isn't so difficult that you have to design your whole IR around it.

As for IR design and TFs good architectural decision, idk, I don't think it's all that different from what we ended up with in maglev. All those classes are just convenience views onto a consistent node layout (with e.g. the same trick as TF of putting inputs behind the node), and so far we haven't had issues with it - time will tell I suppose.

Overall, this narrative that TF, with it's SoN and other serial decisions, was super clever and built by very smart senior engineers that just all moved on and left behind just us dummies that don't get it -- I've honestly never argued against it. Hell, I can even agree with it, same as I totally believe Cliff when he says that he could easily solve every problem we struggled with (likely by doing it in the scheduler). Tony Stark built one in a cave with a bunch of scraps, but unfortunately I'm not Tony Stark, and we've ended up choosing human comprehension (instead of superhuman) as a design constraint so that us dummies can still work on it after all the senior engineers got promoted away or bored. I think this is a good decision and I stand by it.

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titzer 3 hours ago | parent [-]

Well obviously I don't think that you're a bunch of dummies so please don't throw out strawmen like that.

I don't know if Cliff is on the same page w.r.t. how much speculation is necessary (and when) to make JS go fast. In particular, inserting speculative guards has the nice property of improving downstream information for dominated control flow paths. Dominated control flow paths are few and far between when the control chain is very relaxed (in fact, that's the point of a relaxed dependency representation--the graph doesn't have induced dominance information, just dependencies). So relaxing ordering actually works against making use of speculation decisions, because speculation decisions have all these downstream (forward) benefits. It actually makes a lot of sense to work out what speculations are going to be done and propagate their effects on a fully-scheduled CFG[1].

The case is not the same in Java. In C2, afaict, all speculation happens at graph build time, which is driven by abstract interpretation of the bytecode, which follows the control flow order. That means it can make use of dominance information.

AFAIK Graal does scheduling multiple times, whenever it needs to know explicit control information. This allows its speculations to propagate forward to dominated paths.

> Maybe this was a JS specific idiosyncrasy, but my experience was that the effect chain became almost homomorphic to the control chain (and when it wasn't, we had bugs), and then you may as well merge the two into a CFG - if you have to skip over links in your effect chain to skip over certain kinds of effect then you can equally well skip over zero effect nodes too. With SoN, we got all the costs and (almost none) of the benefits, hoisting really isn't so difficult that you have to design your whole IR around it.

I think this is the core of the problem. Control and effects are different things and it wasn't until long after TurboFan that I understood this well enough to even articulate it to myself, let alone to others. The only nodes that really need control inputs are those that have write effects or could diverge (not terminate). Reads of mutable state don't need control, they just need to have an effect input to order them w.r.t. writes. Writes don't need to depend on reads, like they did in TF IR; they are anti-dependencies that can be treated differently. From conversations with Cliff, I think C2 does treat antidependencies differently. Truth be told, writes don't even need to have control dependencies if the scheduler replicates code to make sure that different versions of the world are not simultaneously live (i.e. they are affine resources). The control dependencies in TF graphs were basically a CFG embedded in the graph and making writes depend on control more or less achieved that affine-ness.

While JS has far too many things that can change the world, it's clearly not the case that all the effect chains were linear, because load elimination would never happen, and no code motion would ever be possible. While it's hard to know how you think of "multiple effect chains"--it doesn't have to be represented by a multiple of edges in the graph. You can still have a single effect edge per node in most cases.

In the end it seemed like some people were really intent on only thinking about optimization from a CFG perspective and chaining a CFG through all effectful things, completely defeating the purpose of a dependency graph. People think about IRs in different ways, sure, but the dependency graph view and mindset is inherent in the sea of nodes representation. "Walking the code forward" is a common mental mode for CFG optimizations but is just alien to sea of nodes.

[1] To cut to the chase, and hindsight is 20/20, I think the best design for JS optimization is to use a CFG for a lot of frontend and middle optimizations, before lowering, to use it to insert speculations, and then to thread only a minimal effect / control chain to make a sea of nodes graph, run all the optimizations again, and reschedule it to a CFG. TBH I am not sure whether lowering should happen on a CFG or SoN--it does matter, but I don't think there's a definitive answer. But definitely run GVN on SoN after lowering--there's a bazillion common subexpressions to find then. It'd be great if optimizations can be written to be independent of whether a node is hooked up in a CFG or a SoN, to allow reuse between the two different representations. Or if the representation was sufficiently parametric that nodes could be hooked up in either configuration.

	▲	Leszek 3 hours ago \| parent [-]
		The dummies phrasing is my own, and I stand by it - I simply find the CFG way of thinking much easier to reason about than the SoN way, and I find myself falling back into it no matter how hard I try to follow what I totally (abstractly) appreciate is a more mathematically beautiful dependency/anti-dependency graph - I think that's a limitation of my ability to maintain that concept in my mind and that other people can do it better than me. I find it far easier to reason on CFG terms most of the time, and to raise the CFG temporarily into some dependency representation for doing eliminations/hoisting/LICM, than I do maintaining that dependency representation across all phases and accurately reasoning in each of those about what concrete dependency I forgot about when writing the reduction (concrete recent example, I tried to make reading from holes segfault by unmapping the hole, to force potential holey field access to always first compare against the sentinel, but I was foiled by TF since the load had no dependency on the hole compare branch and could be hosted above it). Implying that this was simply "some people" insisting on only thinking one way without trying to think the SoN way, or that it was some sort of CFG prejudice, is the actual strawman here.