“TurboFan” – Experimental new optimizing compiler for Google's V8 JS engine

Here's something that's been bothering me since Chrome dropped the experimental support for `position: sticky` (and CSS Regions before) and I didn't find the right place to ask (nor is a submission about the JS engine an appropriate venue for this question, I know), so I'm going to hijack this thread:

We know some properties are expensive and when you use them a few times (or with certain values) you get sub-60fps scrolling — but why are they expensive? Are they inherently hard to optimize (e.g. different GPUs across mobile devices), or is it that nobody got to optimize them yet?

> We've reached the point where it's better for our development velocity to work in the open on the bleeding edge branch of V8. That's also better for our collaborators who are working on ports to more platforms.

I wonder who are the collaborators with access to Google's private V8 repo and what platforms they're porting to. If merging TurboFan to the open repo didn't reveal their partners' proprietary plans now, then why not develop in the open sooner?

I hadn't heard of Servo until just now, googled it and it looks really neat.

Could you elaborate on what things you think Servo does better than current rendering engines?

Yes, very curious about this too.

As a first guess, I'm not sure what the v8 strategy is here. The new compiler seems to use the "sea of nodes" approach as opposed to SSA form. A comparison of the two is here

http://static.squarespace.com/static/50030e0ac4aaab8fd03f41b...

The "sea of nodes" approach can give some speedups, but they don't appear huge - 10-20% in that link. Not sure how representative that data is. But it is interesting that modern compilers, like gcc and LLVM, typically use SSA form and not the approach v8 is taking, as further evidence that the "sea of nodes" is not clearly superior.

Perhaps the v8 designers believe there is some special advantage for JS that the new model provides? Otherwise this seems surprising. But hard to guess as to such a thing. If anything, JS has lots of possible surprises everywhere, which makes control flow complex (this can throw, that can cause a deopt or bailout, etc.), and not the best setting for the new approach.

Furthermore, the "sea of nodes" approach tends to take longer to compile, even as it emits somewhat better code. Compilation times are already a big concern in JS engines, more perhaps than any other type of compiler.

Perhaps v8 intends to keep crankshaft, and have turbofan as a third tier (baseline-crankshaft-turbofan)? That would let it only run the slower turbofan when justified. But that seems like a path that is hard to maintain - 2 register allocators, etc., - and turbofan seems like in part a cleanup of the crankshaft codebase (no large code duplications anymore, etc.), not a parallel addition.

Overall the Safari and Firefox strategies make sense to me: Safari pushes the limits by using LLVM as the final compiler backend, and Firefox aside from general improvements has also focused efforts on particular aspects of code or code styles, like float32 and asm.js. Both of those strategies have been proven to be very successful. I don't see, at first glance, what Chrome is planning here. However, the codebase has some intriguing TODOs, so maybe the cool stuff is yet to appear.

The "sea of nodes" approach is just a data structure for representing a program in SSA form. It's orthogonal to anything that has an impact on speed. E.g. GCC uses a tree representation, LLVM uses a CFG, and Hotspot (C2 and Graal) uses a "sea of nodes" representation, but they all represent code in SSA form and that representation is orthogonal to the quality of particular optimizations implemented within the framework.

The speedup reported in that paper is from running constant propagation and dead code elimination at the same time instead of doing them separately, which finds more constants and dead code because the two problems are coupled. The same process can be implemented in a more traditional CFG representation (and generally is--sparse conditional constant propagation).