Epsilon + Graal

[Roman Kennke]

> I am tinkering with Epsilon support for Graal (targeting AOT binaries with no GC). This patch
> applies to jdk/jdk:
>   http://cr.openjdk.java.net/~shade/epsilon/graal-support.patch
> 
> ...and passes this test suite:
> 
> $ CONF=linux-x86_64-server-fastdebug make images run-test TEST=compiler/aot
> TEST_VM_OPTS="-XX:+UnlockExperimentalVMOptions -XX:+UseEpsilonGC"
> 
> ...and does this:
> 
> $ build/linux-x86_64-server-release/images/jdk/bin/jaotc -J-XX:+UseEpsilonGC --info HelloWorld.class
> --output hello-epsilon.so
> $ build/linux-x86_64-server-release/images/jdk/bin/java -XX:+UnlockExperimentalVMOptions
> -XX:+UseEpsilonGC -Xlog:gc -XX:+UseAOT -XX:AOTLibrary=./hello-epsilon.so  HelloWorld
> HelloWorld
> [0.004s][info][gc] Resizeable heap; starting at 2009M, max: 30718M, step: 128M
> [0.004s][info][gc] Using TLAB allocation; max: 4096K
> [0.004s][info][gc] Elastic TLABs enabled; elasticity: 1.10x
> [0.004s][info][gc] Elastic TLABs decay enabled; decay time: 1000ms
> [0.004s][info][gc] Using Epsilon
> [0.038s][info][gc] Heap: 30718M reserved, 2009M (6.54%) committed, 698K (0.00%) used
> 
> real	0m0.048s
> user	0m0.064s
> sys	0m0.014s

Cool!


> I am confused what to do next. Some process questions:
> 
>  a) Where do I propose the patch? As GitHub PR to oracle/graal, is that right?

Well, the Hotspot part to one of the hotspot mailing list (probably this
here?). For the Graal part, yes, as PR vs oracle/graal.

>  b) The change requires adjustments in JVMCI, how is that handled? I assume JVMCI and Graal changes
> are done independently? In that case, there is a bit of circularity here: I cannot put JVMCI change
> in without breaking runs with Epsilon for a while, and cannot put Epsilon changes in before JVMCI is
> updated?

Dunno about that.

>  c) Pretty sure current patch fails some write barrier verification, because verification assumes
> either G1 or CardTable-based BarrierSet. Do we expect to clean up verification before the Epsilon
> patch, or can it be done within the patch?

I can have a look at that because I'm currently knee-deep in that code
anyway.

>  d) How do we run Graal tests (especially given the need for JVMCI adjustments)? Are they run
> automatically on PR proposal?

Yes, when you file a PR, *some* CI stuff is run automatically.

other than that, you should at least run 'mx unittest' from your
graal/compiler project locally.

Roman