[9] RFR(XS) 8059780: SPECjvm2008-MPEG performance regressions on x64 platforms

[Dean Long]

I see now.  Thanks Vladimir and John.

dl

On 11/3/2014 10:10 AM, Vladimir Kozlov wrote:
> On 11/3/14 9:40 AM, Dean Long wrote:
>> By the way, do we know why the lcm change caused  a regression?  Is 
>> reordering the worklist
>> using pop() a design feature?
>
> No, putting last element in a place of removed one is not used here by 
> "generate optimal code" design but because it is fastest way to remove 
> element from array.
> If we only used selection code at lines 534-548 it would not matter 
> which is the order.
> The problem in special nodes (lines 462-469) which depends on the 
> order and this, I think caused, the problem.
>
> Unfortunately this code in lcm does not produce as optimal code as we 
> would like (see examples in RFEs).
> We were unlucky that reordering of nodes due to Atom changes caused 
> this regression.
>
> Thanks,
> Vladimir
>
>>
>> dl
>>
>> On 10/31/2014 5:58 PM, Vladimir Kozlov wrote:
>>> I forgot to say that lcm.cpp changes were small part of 8052081 
>>> changes. The rest of changes will stay.
>>>
>>> Thanks,
>>> Vladimir
>>>
>>> On 10/31/14 5:55 PM, Vladimir Kozlov wrote:
>>>> http://cr.openjdk.java.net/~kvn/8059780/webrev/
>>>> https://bugs.openjdk.java.net/browse/JDK-8059780
>>>>
>>>> We found performance regression (3-5%) on x64 after changes 8052081 
>>>> [1]
>>>> were pushed. It is caused by changes in lcm.cpp. Those changes were 
>>>> done
>>>> because Atom is in-order cpu and some benchmarks on Atom showed 
>>>> benefit
>>>> from these changes.
>>>>
>>>> But Atom is not our core platform and the regression is big.
>>>>
>>>> Backout changes made in lcm.cpp in jdk9 and 8u40 (8052081 was 
>>>> backported
>>>> to 8u40).
>>>>
>>>> We have RFEs to improve block's instructions order which may help both
>>>> platforms:
>>>>
>>>> https://bugs.openjdk.java.net/browse/JDK-6958833
>>>> https://bugs.openjdk.java.net/browse/JDK-7101232
>>>>
>>>> Tested with SPECjvm2008-mpegaudio.
>>>>
>>>> Thanks,
>>>> Vladimir
>>>>
>>>> [1] https://bugs.openjdk.java.net/browse/JDK-8052081
>>