review (S) for 6978249: spill between cpu and fpu registers when those moves are fast

[Tom Rodriguez]

It's very program and processor dependent.  If the moves are fast enough then it never appears to hurt and it can bring a small benefit for real programs.  Tight kernels with more live values can benefit more.  We're still getting a handle on the exact performance characteristics and hopefully will turn it on by default in some configs once we're more confident.  Anyway, your mileage may vary.

tom

On Aug 20, 2010, at 11:13 AM, Hiroshi Yamauchi wrote:

> Hi Tom,
> 
> I'm just curious - how much win is this spilling technique?
> 
> Hiroshi
> 
> On Thu, Aug 19, 2010 at 2:05 PM, Tom Rodriguez <tom.rodriguez at oracle.com> wrote:
>> 
>> On Aug 19, 2010, at 1:49 PM, David Dabbs wrote:
>> 
>>> 
>>> Tom Rodriguez wrote:
>>> 
>>> --8< snip
>>> [in addition to the register spilling changes]
>>> I also moved the logic for PrintFlagsFinal since the initialization of
>>> several subsystems may change some flag values which will be missed by
>>> the current location.  Tested with scimark, ctw and the nsk tests on
>>> 32 and 64 bit.
>>> 
>>> 
>>> Does the "bundling" of the PrintFlagsFinal tweaks with the spilling mods
>>> mean the former won't make it into JDK6 until HS19 lands there?
>> 
>> Yes.  They could always be backported separately under a new bug id.  At the time it didn't seem worth a new bug id.
>> 
>>> 
>>> 
>>> Thank you,
>>> 
>>> David
>>> 
>>> 
>>> p.s. Would this be the right list to post questions regarding HS flags
>>> appropriate for
>>> maximizing HotSpot performance on Nehalem CPUs?
>> 
>> Yes it would be fine to ask about that here.  Mostly we autodetect and use the appropriate settings so there's not a lot which is or should be tuned.
>> 
>> tom
>> 
>>> 
>>> 
>>> 
>>> 
>> 
>>