RFR: 8255455: Refactor ThreadHeapSampler::_log_table as constexpr

[Lin Zang]

On Tue, 27 Oct 2020 14:00:34 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

Dear @cl4es, 
I am not a reviewer, just have 1 comment that maybe you need to update the Year info in the headers of touched files. 

Thanks.
Lin

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