Request for review 7178145: Change constMethodOop::_exception_table to optionally inlined u2 table

[Jiangli Zhou]

Hi Keith,

Thanks for the review and comments!

On 06/19/2012 04:01 PM, keith mcguigan wrote:
>
> Hi Jiangli,
>
> In methodOop.hpp, the class ExceptionTable holds a raw pointer into 
> the heap (the pointer to the embedded table).  This is used in 
> methodOop::fast_exception_handler_bci_for() in a loop when searching 
> for the bci.  If class loading is initiated , a Full GC could occur 
> which could invalidate the pointer in ExceptionTable on the next pass 
> through the loop.  Need some sort of handle or recalculation in that 
> loop. Perhaps a No_Safepoint_Verifier to catch any other places like 
> this?

Wow, so the embedded table could move along with the methodOop when 
fast_exception_handler_bci_for() is looping through it if there is a GC 
happening at the same time due to class loading. Thanks for pointing 
that out! The original code was ok as it was using a typeArrayHandle to 
the separate table, which is GC 'safe'.

It seems No_Safepoint_Verifier asserts if it passes a safe point, which 
might not be what fast_exception_handler_bci_for() should do. I'll take 
your first suggestion and initialize the table inside the loop. :)

>
> Also in ClassVerifier::verify_exception_handler_targets().
>
> I didn't look to see if this was possible in generateOopMaps code too, 
> but you might want to take a look at that too -- it's definitely 
> looping with that raw pointer (I didn't follow through to see if it 
> could safepoint).

Will fix those.

>
> Other than that one issue, I think this looks really good.

Thanks a lot!

Jiangli
>
> -- 
> - Keith
>
> On 6/19/2012 5:24 PM, Jiangli Zhou wrote:
>> Hi,
>>
>> Please review the following webrev for method exception handler table
>> memory reduction:
>>
>> http://javaweb.sfbay.sun.com/~jianzhou/webrev.exceptiontable/
>>
>> Per VM spec the method exception handler table is tuples of u2's, but in
>> hotspot the data are stored as ints in constMethodOop::_exception_table
>> (a typeArray). The change reduces the exception handler table memory
>> usage by:
>>
>>   * Changing the exception handler table from int array to u2 array.
>>   * Making the exception handler table as conditionally allocated
>>     inlined table in constMethod, and eliminate the _exception_table
>>     field. The inlined table is only allocated when the exception table
>>     attribute exists for the method.
>>   * Removing the overhead of the extra typeArray, the data is directly
>>     stored in the inlined table when exception handler exists for a 
>> method.
>>
>> Tested with runthese, jprt and vm.quick.testlist. No noticeable
>> performance degradation from specjvm98 and specjbb2005.
>>
>> ============================================================================== 
>>
>> logs.exceptiontable_baseline.1:
>> Benchmark Samples Mean Stdev Geomean Weight
>> specjbb2005 8 56372.58 763.16
>> specjvm98 8 534.50 19.50
>> ============================================================================== 
>>
>> logs.exceptiontable.1:
>> Benchmark Samples Mean Stdev %Diff P Significant
>> specjbb2005 8 56734.05 880.76 0.64 0.395 *
>> specjvm98 8 531.05 21.00 -0.65 0.738 *
>> ============================================================================== 
>>
>>
>> ============================================================================== 
>>
>> logs.exceptiontable_baseline.2:
>> Benchmark Samples Mean Stdev Geomean Weight
>> specjvm98 8 527.63 17.10
>> ============================================================================== 
>>
>> logs.exceptiontable.2:
>> Benchmark Samples Mean Stdev %Diff P Significant
>> specjvm98 8 536.79 20.54 1.74 0.349 *
>> ============================================================================== 
>>
>>
>> Thanks,
>>
>> Jiangli
>>