[9] RFR (X): 8163880: Constant pool caching of fields inhibited/delayed unnecessarily

[Zoltán Majó]

Hi Vladimir,


thank you for looking at this issue once more.

On 08/25/2016 09:05 PM, Vladimir Kozlov wrote:
> Okay I think I understand what you are trying to say but it was not my 
> point.

Oh, I see.  Then I misunderstood you.

> I was questioning why put_code is set when bytecode could be get 
> instruction. Reading comments and code around I think I understand now.
> We set both get_code and put_code marking that the field is resolved. 
> But in some cases we delay marking to force field's reresolution.

Yes, that is also my understanding.

>
> In next code you can use is_static:
>
>     bool uninitialized_static = ((bytecode == Bytecodes::_getstatic || 
> bytecode == Bytecodes::_putstatic) &&
>                                  !klass->is_initialized());

OK, I've updated the code.

>
> Code could be rearranged to be more compact (similar to original code):
>
>     Bytecodes::Code put_code = (Bytecodes::Code)0;
>     Bytecodes::Code get_code = (Bytecodes::Code)0;
>     if (!uninitialized_static) {
>       get_code = ((is_static) ? Bytecodes::_getstatic : 
> Bytecodes::_getfield);
>       if ((is_put && !has_initialized_final_update) || 
> !info.access_flags().is_final()) {
>         put_code = ((is_static) ? Bytecodes::_putstatic : 
> Bytecodes::_putfield);
>       }
>     }

I agree and have updated the code accordingly.

Here is the updated webrev:
http://cr.openjdk.java.net/~zmajo/8163880/webrev.02/

JPRT testing passes (testset hotspot).

Thank you!

Best regards,


Zoltan

>
> Thanks,
> Vladimir
>
> On 8/23/16 5:31 AM, Zoltán Majó wrote:
>> Hi Vladimir,
>>
>>
>> thank you for the feedback! Please see details below.
>>
>> On 08/22/2016 08:20 PM, Vladimir Kozlov wrote:
>>> Typo (double 'not'):
>>> +   // and the required IllegalAccessError would not not be thrown.
>>
>> OK, I've corrected that.
>>
>>>
>>> I think has_initialized_final_update should be part of assert to see
>>> it in hs_err:
>>>
>>> +   if (has_initialized_final_update) {
>>> +     assert(info.access_flags().is_final(), "Fields with initialized
>>> final updates must be final");
>>> +   }
>>
>> Yes, I've updated that as well.
>>
>>>
>>> Why is_put check is not done in outer check? It is missing for
>>> is_final():
>>>
>>> +   if (!uninitialized_static) {
>>> +     if ((is_put && !has_initialized_final_update) ||
>>> !info.access_flags().is_final()) {
>>
>> I think it's an optimization to do it this way: For non-final-fields, we
>> want to set up CP caching faster (i.e., we want to reduce the number of
>> times we call into the runtime to
>> LinkResolver::resolve_field_access()).We had that optimization in place
>> before JDK-8160527, but not afterwards, which caused a performance
>> regression.  Please see some details below.
>>
>> The CP cache does caching for get and put bytecodes individually.
>> Normally, the caching for put (in 'put_code') should be set up by put
>> instructions, the caching for get (in 'get_code') should be set up by
>> get instruction.  The optimization is to let get instructions set up
>> caching for put bytecodes in some select cases.  Here is the way that
>> (not) workedbefore/after JDK-8160527.
>>
>>
>> 1) The table below describes the way resolution worked before
>> JDK-8160527.  The table shows if put_code was set (S) or not set (NS)
>> after InterpreterRuntime::resolve_get_put().
>>
>>                | Instruction triggering resolution
>>                |===================================
>> Field type     | get          | put
>> ===================================================
>> final          | NS           |S
>> non-final      | S            | S
>>
>> http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/29ed49c42486#l1.38
>>
>>
>> 2) Unfortunately, JDK-8160527 changed the table to:
>>
>>                | Instruction triggering resolution
>>                |===================================
>> Field type     | get          | put
>> ===================================================
>> final          | NS           |*NS*
>> non-final      | *NS*         | S
>>
>> http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/29ed49c42486#l1.30
>>
>> So the VM does not set up caching in *2additional cases*, which causes a
>> performance regression.
>>
>>
>> 3) The current webrev proposes the following:
>>
>>                | Instruction triggering resolution
>>                |===================================
>> Field type     | get          | put
>> ===================================================
>> final          | NS           |*S (unless has_initialized_final_update
>> is true for the field)*
>> non-final      | S            | S
>>
>> That table looks the same as what we had before JDK-8160527, except for
>> fields with initialized final updates. For those fields we need to defer
>> setting up CP caching so that the appropriate exception can be thrown at
>> the right place (by LinkResolver::resolve_field_access()).
>>
>>
>> 4) If we would do what you suggested (I hope I understood your
>> suggestion correctly)thenwe'd move the is_put condition to the outer 
>> check:
>>
>> +   if (!uninitialized_static && is_put) {
>> +     if (!has_initialized_final_update ||
>> !info.access_flags().is_final()) {
>>
>> Then the table would look:
>>
>>                | Instruction triggering resolution
>>                |===================================
>> Field type     | get          | put
>> ===================================================
>> final          | NS           |S(unless has_initialized_final_update is
>> true for the field)
>> non-final      | *NS*         | S
>>
>> So we would have *still one case* where CP caching is not set up and
>> that would most likely result in some performance regression (relative
>> to pre-JDK-8160527).
>>
>>
>> I hope I have not overlooked anythingabove.
>>
>> Here is the updated webrev:
>> http://cr.openjdk.java.net/~zmajo/8163880/webrev.01/
>>
>> JPRT testing is running.
>>
>> Thank you!
>>
>> Best regards,
>>
>>
>> Zoltan
>>
>>
>>>
>>> Thanks,
>>> Vladimir
>>>
>>> On 8/22/16 4:20 AM, Zoltán Majó wrote:
>>>> Hi,
>>>>
>>>>
>>>> please review the fix for JDK-8163880.
>>>>
>>>> https://bugs.openjdk.java.net/browse/JDK-8163880
>>>>
>>>> Problem & solution: The performance regression reported by the bug is
>>>> caused by unnecessary delay (or even inhibition) of constant pool
>>>> caching for fields targeted by put instructions. For a more
>>>> detailed description of the problem and possible solutions, please
>>>> see the following comment in the bug report [1].
>>>>
>>>> Webrev:
>>>> http://cr.openjdk.java.net/~zmajo/8163880/webrev.00/
>>>>
>>>> Testing:
>>>> - performance evaluation with the affected benchmark shows that fix
>>>> above solves the performance regression;
>>>> - all hotspot test with -Xmixed and -Xcomp (running).
>>>>
>>>> Thank you!
>>>>
>>>> Best regards,
>>>>
>>>>
>>>> Zoltan
>>>>
>>>> [1]
>>>> https://bugs.openjdk.java.net/browse/JDK-8163880?focusedCommentId=13990030&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13990030 
>>>>
>>>>
>>>>
>>