RFR: 8200729: Conditional compilation of GCs

[Stefan Karlsson]

Hi all,

Please review these patches to allow for conditional compilation of the 
GCs in HotSpot.

Full patch:

http://cr.openjdk.java.net/~stefank/8200729/webrev.04/all/

(See below for a more fine-grained division into smaller patches)

Today Parallel, G1, and CMS, are all guarded by INCLUDE_ALL_GCS. 
INCLUDE_ALL_GCS becomes defined to 1 for our server 
(--with-jvm-variants=server) builds, and is defined to 0 for the minimal 
(--with-jvm-variants=minimal) builds. There are also ways to forcefully 
remove these GCs from the compilation by configuring with, for example, 
--with-jvm-features=all-gcs.

The proposed patch removes INCLUDE_ALL_GCS (and all-gcs) and replaces it 
with INCLUDE_CMSGC, INCLUDE_G1GC, and INCLUDE_PARALLELGC. In addition to 
that, INCLUDE_SERIALGC has been added to guard the Serial GC code.

Future GCs should adopt this scheme before they get incorporated into 
the code base. Note, that there will be some files in gc/shared that are 
going to have to know about all GCs, but the goal is to have very few of 
these INCLUDE_<GC> checks in the non-GC parts of HotSpot.

With this patch, it's also going to be easier to stop compiling CMS when 
the time as come for it to move from deprecated to removed.

Note, that even though this adds great flexibility, and allows for easy 
inclusion / exclusion of GCs, there's no guarantee that a specific 
combination of GCs is going to be maintained in the future. Just like 
not all combinations of the different Runtime features (CDS, NMT, JFR) 
and Compiler features (AOT, COMPILER1, COMPILER2) are guaranteed to 
compile and be supported. I've sanity checked the different GC 
configurations build locally, but I've only fully tested the "server" 
variant, and "minimal" has only been built locally.

There's a more high-level discussion around the flexibility the 
--with-jvm-features flag adds, and if we really should allow it. Since 
this patch only builds upon that existing flexibility, and I don't 
change the two defined jvm variants, I'd appreciate if that discussion 
could be kept out of this review thread. For further discussion around 
this, please see:

http://mail.openjdk.java.net/pipermail/build-dev/2018-April/021663.html

This is the patch queue:

The first patch simply cleans up some INCLUDE_ALL_GCS usages in 
platform-specific files. Some of these changes are already being cleaned 
up by other RFEs:

http://cr.openjdk.java.net/~stefank/8200729/webrev.04/00.removeUnneededIncludeAllGCs/


The second patch pre-cleans some include files:

http://cr.openjdk.java.net/~stefank/8200729/webrev.04/01.fixIncludes/


The following is the main patch, which include all relevant HotSpot 
changes. For a while now, we have been actively working on abstracting 
away GC specific code from non-GC directories, but as can be seen in 
this patch, there are still a few places left. Hopefully, we will get 
rid of most of these in the near future.

http://cr.openjdk.java.net/~stefank/8200729/webrev.04/02.mainPatch/


The last patch adds the make file support to turn on and off the 
different GCs. The content of this patch has evolved from versions 
written by myself, Per, and Magnus Ihse Bursie. Magnus last proposed 
version used the names gc-cms, gc-g1, gc-parallel, and gc-serial, but 
I've changed them to cmsgc, g1gc, parallelgc, and serialgc, so that they 
match the INCLUDE_<GC> defines.

http://cr.openjdk.java.net/~stefank/8200729/webrev.04/03.selectIndivudualGCsMakePatch/

Thanks,
StefanK