[9] RFR(L) 8013267 : move MemberNameTable from native code to Java heap, use to intern MemberNames

Peter Levart peter.levart at gmail.com
Mon Nov 3 20:09:29 UTC 2014

Hi David,

I was thinking about the fact that java.lang.invoke code is leaking into 
java.lang.Class. Perhaps, If you don't mind rewriting the code, a better 
code structure would be, if j.l.Class changes only consisted of adding a 

+ // A reference to canonicalizing cache of java.lang.invoke.MemberName(s)
+ // for members declared by class represented by this Class object
+ private transient volatile Object memberNameData;

...and nothing else. All the logic could live in MemberName itself 
(together with Unsafe machinery for accessing/cas-ing Class.memberNameData).

Now to an idea about implementation. Since VM code is not doing any 
binary-search and only linearly scans the array when it has to update 
MemberNames, the code could be changed to scan a linked-list of 
MemberName(s) instead. You could add a field to MemberName:

class MemberName {
     // next MemberName in chain of interned MemberNames for particular 
declaring class
     private MemberName next;

Have a volatile field in MemberNameData (or ClassData - whatever you 
call it):

class MemberNameData {
     // a chain of interned MemberName(s) for particular declaring class
     // accessed by VM when it has to modify them in-place
     private volatile MemberName memberNames;

     MemberName add(Class<?> klass, int index, MemberName mn, int 
redefined_count) {
         mn.next = memberNames;
memberNames = mn;
         if (jla.getClassRedefinedCount(klass) == redefined_count) { // 
no changes to class
             ... code to update array of sorted MemberName(s) with new 'mn'
             return mn;
         // lost race, undo insertion
memberNames = mn.next;
         return null;

This way all the worries about ordering of writes into array and/or size 
are gone. The array is still used to quickly search for an element, but 
VM only scans the linked-list.

What do you think of this?

Regards, Peter

On 11/03/2014 05:36 PM, David Chase wrote:
>>> My main concern is that the compiler is inhibited from any peculiar code motion; I assume that taking a safe point has a bit of barrier built into it anyway, especially given that the worry case is safepoint + JVMTI.
>>> Given the worry, what’s the best way to spell “barrier” here?
>>> I could synchronize on classData (it would be a recursive lock in the current version of the code)
>>>    synchronized (this) { size++; }
>>> or I could synchronize on elementData (no longer used for a lock elsewhere, so always uncontended)
>>>    synchronized (elementData) { size++; }
>>> or is there some Unsafe thing that would be better?
>> You're worried that writes moving array elements up for one slot would bubble up before write of size = size+1, right? If that happens, VM could skip an existing (last) element and not update it.
> exactly, with the restriction that it would be compiler-induced bubbling, not architectural.
> Which is both better, and worse — I don’t have to worry about crazy hardware, but the rules
> of java/jvm "memory model" are not as thoroughly defined as those for java itself.
> I added a method to Atomic (.storeFence() ).  New webrev to come after I rebuild and retest.
> Thanks much,
> David
>> It seems that Unsafe.storeFence() between size++ and moving of elements could do, as the javadoc for it says:
>>     /**
>>      * Ensures lack of reordering of stores before the fence
>>      * with loads or stores after the fence.
>>      * @since 1.8
>>      */
>>     public native void storeFence();

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