RFR(s): 8244733: Add ResourceHashtable::xxx_if_absent (was: ResourceHashtable::compute_if_absent)

[Thomas Stüfe]

Hi,

This is a continuation of the review thread:
https://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2020-May/039444.html

I changed the title of the bug somewhat hence the new thread.

JBS: https://bugs.openjdk.java.net/browse/JDK-8244733
Webrev:
http://cr.openjdk.java.net/~stuefe/webrevs/8244733--add-resourcehashtable--compute_if_absent/webrev.01/webrev/

This now adds three new functions to ResourceHashTable:

new_if_absent(), compute_if_absent(), put_if_absent()

which work very similar to their equivalents in j.u.Map.

The advantage they offer is that for a typical "if map does not contain X,
add X+value" pattern we only do the lookup once.

--

So I extended the scope of the item. After playing around with various call
sites I realized that David's proposal of put_if_absent() would be useful,
as well as a variant which just adds a default constructed value. All three
are tested in gtest and used in real call sites.

Ran tests tonight in our CI, all went through.

Thanks, Thomas