“TurboFan” – Experimental new optimizing compiler for Google's V8 JS engine(groups.google.com) |
“TurboFan” – Experimental new optimizing compiler for Google's V8 JS engine(groups.google.com) |
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?
As to who the partners are, you can see commits from Intel adding x87 support and Imagination doing MIPS, for example.
As to why not develop in the open sooner, good question - this is the second time v8 does this, after CrankShaft (third time if you count the initial unveiling of chrome and v8). Maybe it's just how they work.
That's just my theory anyway.
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 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).
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?
In traditional browser engines it's even worse because layout runs on the main thread, which is also shared with your JavaScript, so painting ends up transitively blocked waiting for your scripts to finish. That is not the case in Servo (disclaimer: I work on Servo), but making layout run off the main thread is hard for many reasons—some inherent to the problem, some historical in the design of current engines—so all current browsers run JS and layout together.
To elaborate on how this works in Gecko. The rendering pipeline has several optional stages:
requestAnimationFrame (Scripts) -> Style flush -> Reflow flush -> display list construction -> Layer construction (recycling) -> invalidation -> Paint/Rasterization -> Compositing (on it's own thread).
Gecko tries to only run each stage of the pipeline only if they are needed. Fast operations like a CSS transition on an opacity or transform will only activate the Compositing phase. WebGL only canvas drawing will only activate rAF + Compositing. Meanwhile a JS animation on "top" will run all of these phases.
If you're doing absolute layout, it's easier/better performing to just set top/left to zero, set width and height to their correct values, and then use CSS transforms to position the element where you want it on the page. :)
(Not meant for you, pcwalton, but for others who might not know about a reasonable workaround.)
Could you elaborate on what things you think Servo does better than current rendering engines?
I wrote a [jquery plugin](http://mkoryak.github.io/floatThead/) that 'simulates' `position:sticky` on table headers.
This makes certain things easier in a "sea of nodes" IR. Normally, during optimization you don't have to worry about maintaining a legal schedule of instructions within and between the basic blocks. You just have to respect the control dependencies. However, in order to get executable code you have to impose a schedule on the nodes, whereas with a more conventional CFG IR, you already have a schedule in the form of the basic blocks and the ordering within them.
[1] See Section 2.2-2.4 of Click's paper: http://paperhub.s3.amazonaws.com/24842c95fb1bc5d7c5da2ec735e.... His IR replaces basic blocks with Region nodes. The only instructions that must be linked to Region nodes are ones that inherently have a control dependency. E.g. an "If" node takes a data input and produces two control outputs, which can be consumed by region nodes. "Phi" nodes must also have control inputs, so they can properly associate different control flow paths with the data values that are merged along those paths.