On Tue, 27 Oct 2020 14:56:33 GMT, Claes Redestad <redes...@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 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). 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. ------------- PR: https://git.openjdk.java.net/jdk/pull/880
