Request for review 7177409: Perf regression in JVM_GetClassDeclaredFields after generic signature changes

[Jiangli Zhou]

Hi David,

Thanks for the review! Please see comments below.

On 06/19/2012 06:41 PM, David Holmes wrote:
> HI Jiangli,
>
> On 20/06/2012 3:32 AM, Jiangli Zhou wrote:
>> Please review the following webrev that address the performance
>> regression in JVM_GetClassDeclaredFields with very large class.
>>
>> http://cr.openjdk.java.net/~jiangli/7177409/webrev.00/
>>
>> In fieldDescriptor::generic_signature(), it now returns NULL immediately
>> if the access_flags indicates the field has no generic signature. This
>> avoids iterating though the fields unnecessarily. Mikael Gerdin has
>> confirmed the change resolves the performance issue for
>> JVM_GetClassDeclaredFields (thanks, Mikael!).
>
> Your change fixes the problem but I can't help but feel that there is 
> an API problem here, and a usage problem in that this fragment in 
> reflection.cpp:
>
>   if (java_lang_reflect_Field::has_signature_field() &&
>       fd->generic_signature() != NULL) {
>     Symbol*  gs = fd->generic_signature();
>
> should really be:
>
>   if (java_lang_reflect_Field::has_signature_field() &&
>       fd->has_generic_signature()) {
>     Symbol*  gs = fd->generic_signature();
>
> If the query is basically constant time the performance would not be 
> impacted. In fact the current code seems to be relying on the C 
> compiler to reuse the result of generic_signature() so that it doesn't 
> actually call it twice. If it is calling it twice then an API change 
> (for methods too) would yield an even better performance improvement.

Yes, most compiler probably would not generate code to reload the 
fd->generic_signature(). But adding an has_generic_signature() API and 
use here seems good to me too. I can make that along with the bug fix.

Thanks,

Jiangli

>
> Maybe an RFE ...
>
> David