RFR: 8277385: Zero: Enable CompactStrings support

[Aleksey Shipilev]

On Thu, 18 Nov 2021 15:14:55 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> This enables `CompactStrings` for Zero. When we were doing original Compact Strings in JDK 9, we disabled the support on non-primary platforms, hoping relevant maintainers would follow up with platform-specific work. Here is me following up, as Zero maintainer :) 
> 
> There is little to do on Zero side, as it is pure interpreter without String intrinsics. Other platforms had old-shaped String intrinsics, so for them enabling the feature would mean implementing Compact-String-shaped intrinsics too. But this is irrelevant for Zero. There is still benefit of doing less work with smaller Strings.
> 
> There are no regressions on the benchmarks I tried, and some benchmarks improve significantly. Notably, specjvm:{compiler,sunflow,derby,xmlvalidation} improve about 5%, specjvm:{serial,xmltransform} improve about 20% on x86_64.
> 
> Additional testing:
>  - [x] Linux x86_64 Zero benchmarks
>  - [x] Linux x86_64 Zero `tier1`

Thanks! Extended Zero testing seems fine.

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PR: https://git.openjdk.java.net/jdk/pull/6459