RFR: 8185925 & 8153682 (footprint reduction of java.lang.StackFrameInfo)

[Brent Christian]

Hi, Remi

Thanks for the idea.  From my reading of the jol ouput:

http://cr.openjdk.java.net/~bchristi/8185925/StackFrameInfo.jol.rmDeclClass.txt 


I believe retainClassRef is already being stored in a single byte (i.e. 
SIZE of 1):

  OFFSET  SIZE     TYPE DESCRIPTION
...
      10     1  boolean StackFrameInfo.retainClassRef

Thanks,
-Brent

On 11/6/17 11:45 PM, Remi Forax wrote:
> Hi Brent,
> if you declare retainClassRef as a byte, you can even compress the data structure a little bit more, no ?
> a boolean (as a field) uses 4 bytes while a byte uses welll 1 byte.
> Given that bci is a short, the VM will put both bci and retainClassRef on the same 32 bits slot.
> 
> cheers,
> Rémi
> 
> ----- Mail original -----
>> De: "Brent Christian" <brent.christian at oracle.com>
>> À: "core-libs-dev" <core-libs-dev at openjdk.java.net>, "hotspot-dev" <hotspot-dev at openjdk.java.net>
>> Envoyé: Mardi 7 Novembre 2017 01:23:16
>> Objet: RFR: 8185925 & 8153682 (footprint reduction of java.lang.StackFrameInfo)
> 
>> Hi,
>>
>> There are a couple opportunities to reduce the memory footprint of
>> java.lang.StackFrameInfo (the internal implementation of
>> java.lang.StackWalker.StackFrame):
>>
>> 8153682[1] : StackFrameInfo.declaringClass could be removed
>> 8185925[2] : StackFrameInfo::walker field can be replaced with bitmap to
>> save footprint
>>
>> I had a look using jol[3].  Removing only 'walker' helps only under 32-
>> and 64-bit, but not with compressed oops.  Removing both 'walker' and
>> 'declaringClass' brings a benefit to compressed oops as well (though not
>> for 16-byte aligned).
>>
>> The size change, in bytes, for each execution mode is as follows:
>>
>>                               32-bit: 32->24
>>                               64-bit: 56->40
>>                   64/compressed oops: 32->24
>> 64/compressed oops, 16-byte aligned: 32->32
>>
>> (For reference, the jol reports for the baseline and specimen are at [4]
>> and [5], respectively.)
>>
>> Please review my code change for this.  The webrev is here:
>> http://cr.openjdk.java.net/~bchristi/8185925/webrev.03/
>>
>> An automated test run is in progress.
>>
>> Thanks!
>> -Brent
>>
>> --
>> 1. https://bugs.openjdk.java.net/browse/JDK-8153682
>> 2. https://bugs.openjdk.java.net/browse/JDK-8185925
>> 3. http://openjdk.java.net/projects/code-tools/jol/
>> 4.
>> http://cr.openjdk.java.net/~bchristi/8185925/StackFrameInfo.jol.baseline.txt
>> 5.
>> http://cr.openjdk.java.net/~bchristi/8185925/StackFrameInfo.jol.rmDeclClass.txt