RFR: 8318709: Improve System.nanoTime performance on Windows

[David Holmes]

On Tue, 24 Oct 2023 08:23:01 GMT, Daniel Jeliński <djelinski at openjdk.org> wrote:

> - use LARGE_INTEGER.QuadPart instead of assembling the jlong from high/low parts
> - precalculate counts_per_nano to avoid costly floating-point division in counter to nanosecond conversion
> 
> Benchmark before:
> SystemTime.nanoTime                avgt   15  19,366 � 0,383  ns/op
> 
> After:
> SystemTime.nanoTime                avgt   15  15,812 � 0,385  ns/op
> 
> Tier1-2 clean.

src/hotspot/os/windows/os_windows.cpp line 804:

> 802: static jlong initial_performance_count;
> 803: static jlong performance_frequency;
> 804: static double counts_per_nano; // NANOSECS_PER_SEC / performance_frequency

Doesn't that calculate nanos_per_count?

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PR Review Comment: https://git.openjdk.org/jdk/pull/16336#discussion_r1372673336