[9] RFR(S): 8005873: JRuby test_respond_to.rb asserts with: MT-unsafe modification of inline cache

[Paul Sandoz]

On May 14, 2014, at 10:50 PM, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> wrote:

>>>> Or perhaps use Unsafe.CAS directly within setCachedLambdaForm?
>>> Yes, Unsafe is another option here. But since cache updates should be
>>> rare, Unsafe is an overkill here IMO - locking should be fine.
>> 
>> I don't get it, AtomicReferenceArray.CAS implementation uses Unsafe.CAS
> By overkill I meant code complexity - it increases for no benefit.
> 
> If we use Unsafe directly, we need to duplicate offset calculation logic from AtomicReferenceArray.
> 

While i find that simple [*], i tend to agree that a lock in setCachedLambdaForm is simpler for this case (and cachedLambdaForm could be left as is).

Paul.

[*] There are constants on Unsafe, Unsafe.ARRAY_OBJECT_BASE_OFFSET and Unsafe.ARRAY_OBJECT_INDEX_SCALE, although commonly one would prefer to do address = (i << shift) + base, rather than address = i * scale + base:

  boolean r = Unsafe.compareAndSwapObject(lambdaForms, 
      which * Unsafe.ARRAY_OBJECT_INDEX_SCALE + Unsafe.ARRAY_OBJECT_BASE_OFFSET, 
      null, form);
  return r ? form : lambdaForms[which];  // always returns non-null value