RFR: 8292329: Enable CDS shared heap for zero builds [v2]

[Aleksey Shipilev]

On Tue, 23 Aug 2022 16:49:01 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> ZERO uses UseSerialGC by default. When we dump the default CDS archive during the build process, it fails to dump the shared heap (which requires G1GC).
>> 
>> The fix is to force -XX:+UseG1GC when dumping the default CDS archive during the build process.
>> 
>> Speed up:
>> 
>> (Before)
>> $ perf stat -r 40 ./images/jdk/bin/java -version
>> 0.018080 +- 0.000388 seconds time elapsed ( +- 2.15% )
>> 
>> (After)
>> $ perf stat -r 40 ./images/jdk/bin/java -version
>> 0.011986 +- 0.000205 seconds time elapsed ( +- 1.71% )
>
> Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:
> 
>   @shipilev review comments

Zero can default to G1 after #9994. Together with this patch, it should also be able to use the shared heap from CDS, I think. It also tells me that it is good to force G1 at CDS dump time, even without Zero, because the ergonomics might select Serial on some machines. (Like, as I remember from the `compilerDefinitions.cpp` code, might happen on Windows x86_32 that drops to "client emulation" mode.).

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PR: https://git.openjdk.org/jdk/pull/9984