RFR: 8250563: Add KVHashtable::add_if_absent [v3]

[Calvin Cheung]

On Tue, 8 Sep 2020 15:27:49 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> Please review this XS change. I added a new **KVHashtable::add_if_absent** function (modeled after
>> ResourceHashtable::put_if_absent from JDK-8244733).
>> - I used "add" instead of "put" to be consistent with the naming convention in utility/hashtable.hpp
>> - I also fixed a type in the comments in resourceHashtable.hpp
>> 
>> Running mach5 tiers1/2.
>
> Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:
> 
>   (a) remove const& from add_if_absent; (b) use EQUALS template param instead of ==

Marked as reviewed by ccheung (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/66