Request for review: 8003848: Make ConstMethod::generic_signature_index optional and move Method::_max_stack to ConstMethod

[Coleen Phillimore]

Jiangli,

This code looks fine, although I don't like that max_stack is being 
moved without moving max_locals.   They're usually used together in the 
same place.   If you moved max_locals and size_of_parameters also with 
this change into constMethod, I think it would be better.   Also, we 
want to move these fields anyway for more sharing with CDS.

Thanks,
Coleen

On 11/26/2012 9:08 PM, serguei.spitsyn at oracle.com wrote:
> Jiangli,
>
> It looks good.
>
> Thanks,
> Serguei
>
> On 11/21/12 11:53 AM, Jiangli Zhou wrote:
>> Hi,
>>
>> Please review the following memory reduction change for Method and 
>> ConstMethod:
>>
>>   http://cr.openjdk.java.net/~jiangli/8003848/webrev/
>>
>> Majority of java methods do not have generic signature. For those 
>> methods, ConstMethod::generic_signature_index are always 0. 
>> ConstMethod::generic_signature_index can be optional and only 
>> allocated for methods with generic signature.
>>
>> The Method class has 2-byte unused due to padding for alignment. 
>> Method::_max_stack can be moved to ConstMethod after changing 
>> ConstMethod::generic_signature_index to be optionally allocated. This 
>> removes an u2 field in Method, and can avoid the 2-byte unused space 
>> waste due to padding.
>>
>> Together, above would save 4-bytes for each java method without 
>> generic signature.
>>
>> Tested with jprt, runthese and vm.quick.testlist. No measurable 
>> performance difference with the change for specjvm98 and specjbb2005.
>>
>> ============================================================================== 
>>
>> logs.method_baseline.jbb:
>>   Benchmark           Samples        Mean     Stdev Geomean Weight
>>   specjbb2005               3    56252.53    284.79
>> ============================================================================== 
>>
>> logs.method.jbb.1:
>>   Benchmark           Samples        Mean     Stdev   %Diff P 
>> Significant
>>   specjbb2005               3    56350.15    626.79    0.17 
>> 0.823            *
>> ============================================================================== 
>>
>>
>> ============================================================================== 
>>
>> logs.method_baseline.spec:
>>   Benchmark           Samples        Mean     Stdev Geomean Weight
>>   specjvm98                 8      543.39     11.93
>> ============================================================================== 
>>
>> logs.method.spec.1:
>>   Benchmark           Samples        Mean     Stdev   %Diff P 
>> Significant
>>   specjvm98                 8      544.21      9.56    0.15 
>> 0.881            *
>> ============================================================================== 
>>
>>
>>
>> Thanks,
>> Jiangli
>