Final RFR: 8005232 (JEP-149) Class Instance size reduction (original) (raw)

Aleksey Shipilev aleksey.shipilev at oracle.com
Wed Jan 9 12:19:13 UTC 2013

Good stuff.

On 01/07/2013 02:46 AM, David Holmes wrote:

http://cr.openjdk.java.net/~dholmes/8005232/webrev/

Sorry for obnoxious first-time reviewer questions: a) I think the CAS loop in newReflectionData() is misleading in its reuse of the parameters. Can we instead inline it? Or, can we read the fields for reflectionDataandreflectionData and reflectionDataandclassRedefinedCount, with no parameters passed? b) Had we considered filling up the complete ReflectionData eagerly on first access? This will make ReflectionData completely final. I would guess this goes against the per-use laziness? c) What's the merit of using Unsafe instead of field updaters here? (Avoiding the dependency on j.u.c?) d) I think the code can be further simplified if we stick with reflectionData() returning non-null object all the time, and check for useCaches instead in the usages, at the expense of creating the methods which actually get the reflection data. e) Should useCaches be final? That will allow aggressive optimizations for (c).

This is a saving of 7 reference fields ie. 28 bytes, resulting in a new Class instance size of 80 bytes. This saves a further 4 bytes due to the fields being 8-byte aligned without any need for padding. So overall we save 32 bytes per class instance.

Shameless promotion follows. This tool [1] should help to estimate the class layout in the face of object alignment, compressed references, paddings, alignment and whatnot.

-Aleksey.

[1] https://github.com/shipilev/java-object-layout/

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