RFR: 8255455: Refactor ThreadHeapSampler::_log_table as constexpr [v4]

[Serguei Spitsyn]

On Wed, 28 Oct 2020 14:36:59 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> The static `ThreadHeapSampler::_log_table` is currently initialized on JVM bootstrap to an overhead of ~67k instructions (linux-x64). By turning the initialization into a constexpr, we can precalculate the helper table at compile time, which trades a runtime overhead for a small, 8kb, static footprint increase.
>> 
>> I compared `fast_log2` with the `log2` builtin with a naive benchmarking experiment[1] (not included in this PR) and show that the `fast_log2` is ~2.5x faster than `log2` on my system. And that without the lookup table we'd be much worse. So I think it makes sense to preserve this optimization, but get rid of the startup overhead:
>> 
>> [5.428s][debug][heapsampling] log2, 0.0751173 secs
>> [5.457s][debug][heapsampling] fast_log2, 0.0298244 secs
>> [5.622s][debug][heapsampling] fast_log2_uncached, 0.1645569 secs
>> 
>> I've verified that this refactoring does not affect performance in this naive setup.
>> 
>> [1] https://github.com/openjdk/jdk/compare/master...cl4es:log2_micro?expand=1
>
> 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 six additional commits since the last revision:
> 
>  - Merge branch 'master' into threadHeapSampler_constexpr
>  - Declare and define log_table as a static constexpr inside threadHeapSampler.cpp
>  - Merge branch 'master' into threadHeapSampler_constexpr
>  - Merge branch 'master' into threadHeapSampler_constexpr
>  - Remove _log_table_initialized assert
>  - Refactor ThreadHeapSampler::_log_table as constexpr

Hi Ioi and Claes,

This looks nice.
One nit:
+      _values[i] = (log(1.0 + static_cast<double>(i+0.5) / N)
                     / log(2.0));

Need spaces around '+' sign. Also, it can be just one line.

Thanks,
Serguei

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Marked as reviewed by sspitsyn (Reviewer).

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