RFR: 8150044: Generate classlists at build-time

[Claes Redestad]

Hi,

please review this change to generate classlists at build-time

bug: https://bugs.openjdk.java.net/browse/JDK-8150044

webrevs:
top: http://cr.openjdk.java.net/~redestad/8150044/top.01/
jdk: http://cr.openjdk.java.net/~redestad/8150044/jdk.01/
hotspot: http://cr.openjdk.java.net/~redestad/8150044/hotspot.01/

The implementation generates an interim image consisting of a minimal
set of modules, then use this to run a small generator program to
load common utilities and facilities and dump the result of this to a
classlist that is then bundled with the final images.

The smaller number of classes on the default classlist (~1100 instead
of ~2500) requires some adjustment to the metaspace defaults.

This achieves the following:

- Removes a manual, error-prone process to update the versioned
   classlists
- Ensures the classlists shipped with the JDK/JRE is up to date
   with recent JDK changes, e.g., when moving classes from sun.* to
   jdk.internal.*
- Automatically picks up and incorporates the output of jlink plugins
   such as GenerateJLIClassesPlugin into the classlist
- Supports cross-compilation build targets, although it runs using a
   build JDK that can run on the host platform to generate such
   classlists (this isn't ideal, but no worse than the current
   situation, where the versioned classlist for the host platform is
   simply copied to the cross-compiled target)

There are a few concerns/drawbacks:

- It does add complexity to the build, and concern has been voiced that
   this would adversely affect build times. However, I'm happy to say
   that on my machine build times are roughly the same:

Before:
real    2m37.303s
user    35m33.576s
sys    3m46.476s

After:
real    2m36.168s
user    35m31.232s
sys    3m52.268s

(real time varies ± 5s from build to build)

- Startup on the specific applications we've used to generate the
   classlists for previously suffer small regressions. These are
   specifically rather dated AWT and Swing-based applications. OTOH,
   startup characteristics generally improve on other applications
   (minimal VM, jetty, etc...)

Testing: JPRT -testset hotspot

Thanks!

/Claes