RFR: 8318709: Improve System.nanoTime performance on Windows

[Daniel Jeliński]

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.

LARGE_INTEGER does not have the same alignment issue because unions are aligned to the alignment required by the largest type, and LONGLONG (aka int64) QuadPart requires 8-byte alignment on 64bit machines.

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PR Comment: https://git.openjdk.org/jdk/pull/16336#issuecomment-1780510386