Commits on Jan 4, 2022

1. [jnienv-gen] Add possible C#9 function pointer backend …

   Context: 926e4bc
   Context: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-9.0/function-pointers
   Context? dotnet#666
   
   Commit 926e4bc allowed `jnienv-gen` to emit multiple different
   JNIEnv invocation strategies at the same time, allowing
   `tests/invocation-overhead` to try them "all at once" for side-by-
   side comparisons.
   
   Add support for JNIEnv invocation strategy which relies on
   C#9 Function Pointers, a'la:
   
   	partial struct JNIEnv {
   	    public  delegate* unmanaged <IntPtr /* env */, jobject> ExceptionOccurred;
   	}
   	partial class JniEnvironment {
   	    partial class Exceptions {
   	        public static unsafe JniObjectReference ExceptionOccurred ()
   	        {
   	            IntPtr __env = JniEnvironment.EnvironmentPointer;
   	            var tmp = (*((JNIEnv**)__env))->ExceptionOccurred (__env);
   	            return new JniObjectReference (tmp, JniObjectReferenceType.Local);
   	        }
   	    }
   	}
   
   This *should* allow for performance better than "JIPinvokeTiming",
   as it avoids P/Invoke overheads to a set of `java_interop_*` C
   functions (926e4bc), while *also* avoiding the overheads involved
   with using `Marshal.GetDelegateForFunctionPointer()` as used by
   "JIIntPtrs".
   
   …but it doesn't:
   
   	$ JI_JVM_PATH=$HOME/android-toolchain/jdk-11/lib/jli/libjli.dylib dotnet tests/invocation-overhead/bin/Debug/net6.0/invocation-overhead.dll
   	# SafeTiming timing: 00:00:04.2123508
   	#	Average Invocation: 0.00042123508ms
   	# XAIntPtrTiming timing: 00:00:02.1625501
   	#	Average Invocation: 0.00021625500999999998ms
   	# JIIntPtrTiming timing: 00:00:02.3620239
   	#	Average Invocation: 0.00023620239ms
   	# JIPinvokeTiming timing: 00:00:01.8993587
   	#	Average Invocation: 0.00018993587ms
   	# JIFunctionPointersTiming timing: 00:00:02.0278083
   	#	Average Invocation: 0.00020278083ms
   
   (Compare and contrast with 926e4bc, circa 2015!)
   
   Of particular note is that the Average Invocation time for
   JIFunctionPointersTiming takes 7% longer than JIPinvokeTiming.
   
   We may not continue investigation of C#9 Function Pointers
   for `JNIEnv` binding purposes, but we can preserve this code for
   future investigation.

   

   jonpryor committed Jan 4, 2022
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