8377909: Replace SummaryDiff's array implementation with a hashtable

[jdksjolen]

Hi,

This PR replaces the array based approach to the SummaryDiff to a hashtable based one. This allows us to save space, as now we only store the diffs for the MemTags that actually changed. This will become more valuable as the number of MemTags in use increases.

As we anticipate most operations to only affect a small number of MemTags I utilize the stack for a small starting array of size 4. This helps us avoid unnecessary mallocs. In order to avoid unnecessary copying, I also changed a few functions from returning SummaryDiff to taking in one as a reference.

Old: sizeof(SummaryDiff) == 464
New: sizeof(SummaryDiff) == 112

Testing: Ran NMT Gtests, currently running tier1-tier3

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Progress

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Issue

• JDK-8377909: Replace SummaryDiff's array implementation with a hashtable (Enhancement - P4)

Reviewing

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$ git fetch https://git.openjdk.org/jdk.git pull/29717/head:pull/29717
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Update a local copy of the PR:
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$ git pull https://git.openjdk.org/jdk.git pull/29717/head

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$ git pr checkout 29717

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https://git.openjdk.org/jdk/pull/29717.diff

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