The guts are substantially different and seemingly more sophisticated. Notable things:

1) The compiler under compiler/dex seems to do a pretty simple set of optimizations and generate either code directly (compiler/dex/quick) or using LLVM through an intermediate code called GBC (compiler/dex/portable).

2) There's a new file format, OAT. Contrary to some reports, it doesn't appear to be a new bytecode, but rather a new file format for DEX bytecode. My guess it exists to support features like caching generated native code or compiling applications to images that don't have to be JIT-ed. The compiler has support for writing out ELF files, and LLVM can load ELF files dynamically with MCLinker.

3) There seems to be the start of a more sophisticated compiler under compiler/sea_ir. My guess is that it's intended to enable more high-level optimizations in the future. It seems to be a sea-of-nodes IR like the one used in Hotspot's C2 compiler, but doesn't seem to be either finished or plugged in. The change log suggests that it's still getting off the ground.

4) Judging by the commit logs, there seems to be a parallel-marking garbage collector, though otherwise it doesn't seem to be anything fancy.

So I guess no moving GC still, because of LLVM. That's unfortunate.

Does anyone know whether Android/Dalvik has made decisions that lock them into non-moving/conservative GC or whether it's still a possibility in the future?

Google made changes to how the JNI returns references to Java objects in Ice Cream Sandwich (4.0) so that native code can work with a moving GC.

http://android-developers.blogspot.com/2011/11/jni-local-ref...

Doesn't answer your question, just some supporting evidence.

Hopefully that means that Google throws some developers at giving LLVM better GC support, so the rest of us benefit.

This is very similar to C#'s NGEN compiler, which compiles C# apps to native code. It's used in Windows Phone to provide pre-compiled versions of applications when people download them.

I thought Dalvik was quiet because the principal author (Dan Bornstein) left Google a while ago.

Is ART from the recent FlexyCore acquisition, or something else that has been going on for a while?

The commit history suggests that it started more than 2 years ago

While Google's acquisition of Flexycore was only made public recently, the process must have started years ago. As Ars Technica noted,

http://arstechnica.com/gadgets/2013/10/google-buys-flexycore...

> FlexyCore doesn't seem to have done anything on the Internet for the last two years. The company actively promoted its product, releasing 11 videos in a one-year span, and then just stopped. The lack of recent activity makes it unclear if Flexycore has continued development outside the public eye.

One unfortunate thing about Bornstein leaving is he seemed to be the only one working on fixing the 65536 method limit bug/feature for Dalvik executables.

Tell me about it. All we get is simply spinning methods into separate jars every six months or so. Not a solution, IMO.

Comment from the source[1]:

  We have already created that oat file above with
  CreateOatFile. Originally this was just our own proprietary 
  file but now it is contained within an ELF dynamic object
  (aka an .so file)
 

Seems like oat file might be enclosed in an ELF container and might support linking of shared libraries.

[1]https://android.googlesource.com/platform/art/+/kitkat-relea... [2]http://www.retrodev.com/android/dexformat.html