The way Java does it is their JIT written in Java is optionally AOT compiled to native code, so it doesn't have a startup time or warmup time problem and it can be PGOd.
We do know that the returns are worth it, because people are able to develop new optimisations in the Java version that people just won't attempt in the C++ version because the code is so much harder to work with (but possibly just due to the age of the C++ version) and its achieving 13% speedup over the C++ version in practice at places like Twitter, which is worth millions of dollars.
This is fascinating. Is the claim here that the C++ version could have never been as fast as the current Java one or just that people have been optimizing the Java one and getting improvements past the C++ one?
Also, it sounds like the old C++ JIT has to be maintained and shipped in the event that the Java-based JIT isn't available at startup, right? If so that makes sense as a stop-gap and it would be more of a tiered JIT, not a port. Tiered JITs are definitely proven, successful technology.
> Is the claim here that the C++ version could have never been as fast as the current Java one
The claim is that the work to add new optimisations to the old C++ code is so difficult that people aren't prepared to do it. The Java code is easier to write and debug, enough so that people are managing to add new optimisations that they haven't added to the C++ code.
> the old C++ JIT has to be maintained and shipped in the event that the Java-based JIT isn't available at startup
Well only until the new Java JIT is mature. The AOT build of the Java JIT can be shipped in the binaries so it's always there.
> If so that makes sense as a stop-gap and it would be more of a tiered JIT, not a port. Tiered JITs are definitely proven, successful technology.
It's used as a new top-tier yes, replacing the C++ top tier. The C++ JIT is likely to go I think, in the medium-to-long term.
We do know that the returns are worth it, because people are able to develop new optimisations in the Java version that people just won't attempt in the C++ version because the code is so much harder to work with (but possibly just due to the age of the C++ version) and its achieving 13% speedup over the C++ version in practice at places like Twitter, which is worth millions of dollars.