RFR(XS): JDK-8010941: MinJumpTableSize is set to 18, investigate if that's still optimal

[Christian Thalinger]

On Sep 11, 2013, at 12:53 AM, Niclas Adlertz <niclas.adlertz at oracle.com> wrote:

> On 2013-09-10 20:44, Christian Thalinger wrote:
>> Are Zero and Shark builds broken after this change?
>> 
>> -- Chris
>> 
>> On Sep 10, 2013, at 1:49 AM, Niclas Adlertz <niclas.adlertz at oracle.com> wrote:
>> 
>>> On 2013-09-09 11:33, Aleksey Shipilev wrote:
>>>> On 09/09/2013 01:16 PM, Niclas Adlertz wrote:
>>>>> On 2013-09-06 14:02, Aleksey Shipilev wrote:
>>>>>> On 09/06/2013 03:54 PM, Niclas Adlertz wrote:
>>>>>>>> It is customary for us (perf guys) to create multiple
>>>>>>>> @GMB methods with different problems sizes, and run them in single JMH
>>>>>>>> session.
>>>>>>> That would have been smarter, yes. I'll do that next time. Thanks.
>>>>>>> And thank you for helping me with JMH.
>>>>>> Sure, any time.
>>>>>> 
>>>>>>>> Yes, "10" seems the turn-point for X64; for SPARC, I'd set it to "5".
>>>>>>> Ok, I'll have one value for each platform instead.
>>>>>>> 
>>>>>>> WEBREV: http://cr.openjdk.java.net/~adlertz/JDK-8010941/webrev02/
>>>>>> Good!
>>>>>> 
>>>>>> I worry about the 32-bit x86 though. It seems the register pressure with
>>>>>> jump tables is higher? If so, the turn-point for x86 can be even larger.
>>>>>> Can you double-check the 32-bit x86 VM?
>>>>>> 
>>>>>> -Aleksey.
>>>>>> 
>>>>> Hi Aleksey.
>>>>> 
>>>>> I don't think the register pressure is higher using a jump table since
>>>>> it's just a jump to an address with an offset.
>>>>> In fact, tests seem to show the opposite; with a lookup table
>>>>> (implemented as a binary search) fewer registers seem to have a negative
>>>>> impact.
>>>>> When we have 6 cases, it's actually faster to use a jump table on 32-bit
>>>>> x86 instead of a lookup table (compared to 10 on x64).
>>>> 
>>>> Thumbs up. Let's go with your change then.
>>>> 
>>>> -Aleksey.
>>>> 
>>> 
>>> Thank you Aleksey.
>>> 
>> 
> 
> Hi Chris,
> 
> This is a C2 specific flag so only C2 specific files are altered.

Right.  Sorry, I missed that.  Looks good.

-- Chris

> 
> Kind Regards,
> Niclas Adlertz