[jdk19] RFR: 8289302: Restore CriticalJNINatives

[Martin Doerr]

On Wed, 29 Jun 2022 12:10:35 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> This change restores the code in the JVM that implemented the internal JavaCritical or CriticalJNINatives functionality.  Customers are using this feature and there's no replacement yet for it.
> This change is a backout and merge of jDK-8258192: Obsolete the CriticalJNINatives flag.
> This change also adds the deprecation message that was missed, which I checked is okay with the customer.
> 
> Tested with tiers1-4.

`UseSystemMemoryBarrier` reduces the overhead of Java thread state transitions significantly (when returning from native and when calling JNI functions).
But there's one additional thing which should be checked regarding handling of primitive arrays. Critical JNI functions provide direct access to the arrays in the Java heap while the regular JNI functions need to use Get/Set<PrimitiveType>ArrayRegion (which is fast for small arrays or when copying is required anyway) or Get<PrimitiveType>ArrayElements&Release<PrimitiveType>ArrayElements (slower) or GetPrimitiveArrayCritical&ReleasePrimitiveArrayCritical (which can avoid copying costs, but require expensive interaction with GC). It's probably worth checking the JNI code for good selection of array access functions.

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PR: https://git.openjdk.org/jdk19/pull/90