RFR: 8255455: Refactor ThreadHeapSampler::_log_table as constexpr [v2]

[Claes Redestad]

On Tue, 27 Oct 2020 23:08:21 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> Claes Redestad has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
>> 
>>  - Merge branch 'master' into threadHeapSampler_constexpr
>>  - Remove _log_table_initialized assert
>>  - Refactor ThreadHeapSampler::_log_table as constexpr
>
> Changes requested by iklam (Reviewer).

The change to declare the log_table as constexpr exposed an issue with the std `log` function not being constexpr, which now cause a build error on Windows and Mac. It works on Linux, but likely not robustly so since I seem to have been stumbling into a non-compliant quirk: https://stackoverflow.com/questions/50477974/constexpr-exp-log-pow

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PR: https://git.openjdk.java.net/jdk/pull/880