HPCC-32790 Avoid allocating hash mappings inside a critical section

[ghalliday]

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[github-actions]

Jira Issue: https://hpccsystems.atlassian.net//browse/HPCC-32790

Jirabot Action Result:
Workflow Transition To: Merge Pending
Updated PR

[ghalliday]

Pushed for review of the approach. This has not been performance tested yet - I will add information once I have done that.

[richardkchapman]

I can't see any flaws. You are second-guessing the optimizer a bit - which isn't always a good idea.

[mckellyln]

Clever and approved.
I was thinking of something much more basic like a preallocated list.

[ghalliday]

Although the change looks good I am not seeing any improvement in performance with it in place when combined with HPCC-32791. On its own it shows an impovement for the inplace, but slow down for the default compression.

I am tempted to leave until I understand why performance reduces - likely the overhead of the thread variable.

[ghalliday]

Note, even changing so that it is a simple pointer (with no thread cleanup) - even though would leak - does not improve performance significantly.