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

[Claes Redestad]

On Tue, 27 Oct 2020 23:08:13 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
>
> src/hotspot/share/runtime/threadHeapSampler.cpp line 48:
> 
>> 46: };
>> 47: 
>> 48: const FastLogTable<ThreadHeapSampler::FastLogCount> ThreadHeapSampler::_log_table;
> 
> To guarantee that `_log_table` is evaluated at C++ compile time, it's best to change the code to
> 
> constexpr FastLogTable<ThreadHeapSampler::FastLogCount> _log_table;
> 
> There are 2 reasons:
> 
> 1. C++ guarantees compile-time evaluation only if the expression is used in a "constexpr context". You can read more from [here](https://isocpp.org/blog/2013/01/when-does-a-constexpr-function-get-evaluated-at-compile-time-stackoverflow).
> 2. In the future, if the `FastLogTable` constructor is modified in a way that cannot be evaluated at compile time, (e.g., someone removes `constexpr` from the constructor's declaration by mistake, the C++ compiler will catch that and tell you that `_log_table` cannot be `constexpr`.
> 
> Unfortunately,  you cannot use `constexpr` in forward declarations, so you should either move the definition of `FastLogTable` to the hpp file, or remove the declaration of `_log_table` from the hpp file.

Ok, makes sense.

I went with removing the `_log_table` declaration from the hpp file. I think things came out a bit cleaner as a result, too.

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