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.

Visual Studio supports 64bit integer types even in 32 bit mode. In fact, the JDK requires a compiler with 64bit integer support - it's used in jlong typedef, for example.

I checked the typedef for LARGE_INTEGER in Windows SDK, and it looks exactly like the one on the MSDN page - no ifdefs to check if we are compiling in 64bit mode.

> Though I wonder if given endian-ness the layout of the structs simply coincides with that of a native 64-bit variable?

That's it exactly. 

By the way, here's a nice article from Raymond Chen about this:
https://devblogs.microsoft.com/oldnewthing/20040825-00/?p=38053

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PR Comment: https://git.openjdk.org/jdk/pull/16336#issuecomment-1780486149