[9, 8u40] RFR (M): 8057020: LambdaForm caches should support eviction

[Paul Sandoz]

On Dec 6, 2014, at 1:30 PM, Peter Levart <peter.levart at gmail.com> wrote:
> Now what scares me (might be that I don't have an intimacy with LambdaForm class like you do). There is a situation where you publish LambdaForm instances via data race.
> 
> One form of LambdaForm.transformCache is an array of Transform objects (the other two forms are not problematic). Transform class has all fields final except the 'referent' field of SoftReference, which holds a LambdaForm instance. In the following line:
> 
> 377                 ta[idx] = key;
> 
> 
> ...you publish Transform object to an element of array with relaxed write, and in the following lines:
> 
> 271         } else {
> 272             Transform[] ta = (Transform[])c;
> 273             for (int i = 0; i < ta.length; i++) {
> 274                 Transform t = ta[i];
> 275                 if (t == null)  break;
> 276                 if (t.equals(key)) { k = t; break; }
> 277             }
> 278         }
> 279         assert(k == null || key.equals(k));
> 280         return (k != null) ? k.get() : null;
> 
> 
> ...you obtain the element of the array with no synchronization and a relaxed read and might return a non-null referent (the LambdaForm) which is then returned as an interned instance.
> 

Transform still has final fields, thus when constructing a Transform using "key.withResult(form))" i don't think hotspot will reorder the store of the referent, or dependent stores, so that they occur after publication of the key element into the Transform[] array.

So i think things are also are ok between putInCache and getInCache.

> So can LambdaForm instances be published via data races without fear that they would appear half-initialized?
> 

AFAICT yes. LambdaForm has final/stable fields and i presume it does not leak in the constructor (even when compiling on construction). See also MethodHandle.updateForm for rare cases where the lambda form of a MethodHandle is updated.

Paul.
 

> That's what I didn't know when I used a lazySet coupled with volatile get to access array elements in my version:
> 
> http://cr.openjdk.java.net/~plevart/misc/LambdaFormEditor.WeakCache/webrev.01/
> 
> 
> Regards, Peter

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