RFR: 8253037: G1: Improve check for string dedup

[Thomas Schatzl]

On Sun, 1 Nov 2020 07:04:29 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this change to G1 evacuation's checking for string
> deduplication.  The old code would go out of line to the G1StringDedup
> support code for every object when deduplication is enabled, only to usually
> discover the object isn't a string.  Instead we now have a simpl inline
> test for the combination of dedup enabled and object is a string, and only
> call out to the dedup support code when that's true.  This eliminates some
> work for every non-string (non-array) object when dedup is enabled.
> 
> The performance impact seems to be pretty small and hard to measure, since
> enabling deduplication has other costs that seem to overwhelm the cost
> here.  I'm hoping to improve that with JDK-8254598.
> 
> Testing:
> tier1 on Oracle supported platforms.
> Performance testing with deduplication enabled.

(Not necessarily requesting changes, but feedback to my comments)

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Changes requested by tschatzl (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/981