[Exp] Experimenting with "value-based" classes and oop testing
david.simms at oracle.com
Fri Jan 19 08:25:21 UTC 2018
On 18/01/18 17:09, Remi Forax wrote:
> ----- Mail original -----
>> De: "David Simms" <david.simms at oracle.com>
>> À: "valhalla-dev" <valhalla-dev at openjdk.java.net>
>> Envoyé: Mercredi 17 Janvier 2018 13:26:33
>> Objet: [Exp] Experimenting with "value-based" classes and oop testing
>> So to lay some ground-work for testing value oops versus reference oops,
>> I've created the following patch "Value based classes" (VBC) for the
>> experimental branch (exp):
>> * Added "INCLUDE_VBC" feature macro to clearly identify "Value-based
>> class" feature
>> * klass.hpp:572 Klass may be marked as "value-based" via a new access flag
>> o systemDictionary.cpp:2053 uses the flag "ValueBasedClasses" to
>> mark the current JDK value-based classes, or the user may
>> specify their own
>> * Prototype of efficient "oopDesc::klass_is_value_based()" test has
>> been implemented by encoding "odd/even" klass ptr into the
>> "oop._metadata" (i.e. oop klass ptr)
>> o klass.cpp:174 affectively double the klass ptr alignment (from 8
>> to 16 bytes), and ensures value classes are aligned "odd" (or to
>> 8 bytes), and regular classes to 16 bytes
>> o Wastes a little memory, but given that instanceKlass weigh
>> around 460 bytes, an extra 8 bytes seemed like a small price...
>> o ...testing for value classes simply involves test the effective
>> least significant bit in the oop metadata field, should work for
>> both compressed (lsbit) and uncompress klass ptr (is 8 byte
>> + klass.inline.hpp:74 should be relatively simple to implement
>> in both JIT and template assembler
>> One can use this as basis for building experiments with alternative
>> behaviour for L-World proposal, e.g. monitor operations involving values.
>> Comments ?
> clever trick,
> in the test, because you do not use LocalDateTime, @run can be simplified to
> @run testng/othervm -XX:ValueBasedClasses=java/time/LocalTime,ValueBased ValueBased
Yeah, this test was resurrected from a previous experiment and then
gutted. I should add further features using oop->klass_is_value_based()
so the test actually does something useful.
> my other question is what is the purpose to have a value based class with mutable fields ?
Hehe, yeah ValueBased test itself doesn't follow the rules, will adjust.
Also thinking of adding some form of auto-value classification to class
file parser, identify value type candidates in existing benchmarks, so
we can see L-World costs
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