RFR: 8255455: Pre-generate ThreadHeapSampler::_log_table [v7]

[Ioi Lam]

On Fri, 30 Oct 2020 15:24:17 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 11 additional commits since the last revision:
> 
>  - Merge branch 'master' into threadHeapSampler_constexpr
>  - Check log_table on startup in debug builds
>  - Add explicit include of logging
>  - Add const, fix copyright
>  - Desugar constexpr into code generator and output
>  - 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
>  - ... and 1 more: https://git.openjdk.java.net/jdk/compare/63c0fc2d...0eebe57c

LGTM

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Marked as reviewed by iklam (Reviewer).

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