What's the difference between effect and control edges of V8's TurboFan?

Question

I've read many blog posts, articles, presentation and videos, even inspected V8's source code, both the bytecode generator, the sea-of-nodes graph generator and the optimization phases, and still couldn't find an answer.

V8's optimizing compiler, TurboFan, uses an IR of type "sea of nodes". All of the academic articles I found about it says that it's basically a CFG combined with a data-flow graph, and as such has two type of edges to connect nodes: data edges and control edges. Basically, if you take only the data edges you form a data-flow graph while if you choose the control edges you get a control flow graph.

However, TurboFan has one more edge type: "effect edges" (and effect phis). I suppose that this is what this slide means when it says that this is not "sea" of nodes but "soup" of nodes, because I couldn't find this term anywhere else. From what I understood, effect edges help the compiler keep the structure of statements/expressions that if reordered will have a visible side-effect. The example everyone uses is o.f = o.f + 1: the load has to come before the store (or we'll read the new value), and the addition has to come before the store, too (or otherwise we'll store the old value and uselessly increment the result).

But I cannot understand: isn't this the goal of control edges? From searching through the code I can see that almost every node has an effect edge and a control edge. Their uses isn't obvious though. From what I understand, in sea of nodes you use control edges to constrain evaluation order. So why do we need both effect and control edges? Probably I'm missing something fundamental, but I can't find it.

TL;DR: What's the use of effect edges and EffectPhi nodes, and how they're different from control edges.

Answer 1

The idea of a sea-of-nodes compiler is that IR nodes have maximum freedom to move around. That's why in a snippet like o.f = o.f + 1, the sequence load-add-store is not considered "control flow". Only conditions and branches are. So if we slightly extend the example:

if (condition) {
  o.f = o.f + 1;
} else {
  // something else
}

then, as you describe in the question, effect dependencies ensure that load-add-store are scheduled in this order, and control dependencies ensure that all three operations are only performed if condition is true (technically, if the true-branch of the if-condition is taken). Note that this is important even for the load; for instance, o might be an invalid object if condition is false, and attempting to load its f property might cause a segfault in that case.

Answer 2

Control edges are edges between basic blocks caused by branch statements. The effect edge is an edge that indicates that the instructions inside the basic block must be in a certain order.