RFR: 8187033: [PPC] Imporve performance of ObjectStreamClass.getClassDataLayout()

[Peter Levart]

Hi Ogata,

On 09/04/2017 10:23 AM, Kazunori Ogata wrote:
> Hi Peter,
>
> Thank you for your comment.  I thought the compiler must insert memory
> fence at the end of object initializer, but I agree relying on it is not
> correct w.r.t. JMM.
>
> Then I'll take 1) 1) Put VarHandle.fullFence() between initialization of
> ClassDataSlot[] and writing the reference to non-volatile dataLayout.
> (Webrev: http://cr.openjdk.java.net/~horii/8187033/webrev.01-fence/), as
> it achieved the best performance.
>
>
> Regards,
> Ogata
>

If playing with mutable plain fields in multiple threads, it is 
mandatory to read the plain field just once in program. Your implementation:

1196     ClassDataSlot[] getClassDataLayout() throws InvalidClassException {
1197         // REMIND: synchronize instead of relying on volatile?
1198         if (dataLayout == null) {
1199             ClassDataSlot[] slots = getClassDataLayout0();
1200             VarHandle.fullFence();
1201             dataLayout = slots;
1202         }
1203         return dataLayout;
1204     }

reads dataLayout field twice (line 1198 and 1203). Those two reads may 
reorder and 1st return non-null value, while the 2nd return previous 
value - null. You should use a local variable to store the 1st read and 
return the local variable at the end. Like:

      ClassDataSlot[] getClassDataLayout() throws InvalidClassException {
          ClassDataSlot[] slots = dataLayout;
          if (slots == null) {
              ClassDataSlot[] slots = getClassDataLayout0();
              VarHandle.fullFence();
              dataLayout = slots;
          }
          return slots;
      }


Regards, Peter