RFR 8173227: [JVMCI] HotSpotJVMCIMetaAccessContext.fromClass is inefficient

[Doug Simon]

+        while (javaType == null) {
+            WeakReference<ResolvedJavaType> type = resolvedJavaType.get(javaClass);
+            javaType = type.get();
+            if (javaType == null) {
+                resolvedJavaType.remove(javaClass);
+            }

Maybe add a small comment as to why the loop is necessary.

Otherwise, looks good.

> On 24 Jan 2017, at 09:50, Tom Rodriguez <tom.rodriguez at oracle.com> wrote:
> 
> http://cr.openjdk.java.net/~never/8173227/webrev
> https://bugs.openjdk.java.net/browse/JDK-8173227
> 
> HotSpotJVMCIMetaAccessContext.createClass is a synchronized wrapper around a WeakHashMap with WeakReference values. During bytecode parsing this can be a heavy contention point between between multiple threads. Additionally the use of WeakReferences for the keys and values is expensive. A ClassValue<WeakReference<ResolvedJavaType>> would avoid the contention around the synchronized method and would eliminate one of the two required WeakReferences.  Tested with Graal.
> 
> tom