EnumData space optimization in j.l.Class (JEP-146)

[Mandy Chung]

On 12/17/12 7:36 AM, Peter Levart wrote:
> Hi David and others,
>
> Here's a patch that eliminates one of two fields in java.lang.Class, 
> related to caching enum constants:
>
> http://dl.dropbox.com/u/101777488/jdk8-tl/JEP-149.enum/webrev.01/index.html 
>
>
> It does it by moving one field to a subclass of HashMap, which is 
> referenced by a remaining field that serves two different 
> purposes/stages of caching.
>

Your observation of merging the enumConstants and enumConstantDirectory 
is a good one.   I see that caching of enumConstantDirectory is 
important as it's used by EnumMap and EnumSet whose performance is 
critical (specified with constant time operations).  I'm unsure about 
Class.getEnumConstants whether it's performance critical and worths the 
complexity of your proposed fix (the enumData field of two types).  If a 
class has cached an enumConstantDirectory, Class.getEnumConstants can 
return a clone of its values().

Anyone knows how Class.getEnumConstants is commonly used and needs to be 
performant?  I suspect it's more typical to obtain the list of enum 
constants statistically (calling Enum.values()) than reflectively.

Mandy

> These are the results of a micro-benchmark that exercises public API 
> that uses the internal j.l.Class API regarding enum constants:
>
> enum MyEnum { ONE, TWO, THREE, FOUR, FIVE, SIX, SEVEN, EIGHT, NINE, TEN }
> EnumSet.noneOf(MyEnum.class): 300_000_000 loops
> MyEnum.valueOf(String): 30_000_000 loops * 10 calls for different names
>
> ** Original JDK8 code
>
> Executing: /home/peter/Apps64/jdk1.8.0-jdk8-tl/bin/java -Xmx4G -cp 
> ../out/production/test test.EnumTest reference
>
>       EnumSet.noneOf(Class): 351610312 340302968 339893333 339774384 
> 339750612 339558414 339547022 339621595
>      MyEnum.valueOf(String): 935153830 897188742 887541353 960839820 
> 886119463 885818334 885827093 885752461
>       EnumSet.noneOf(Class): 339552678 339469528 339513757 339451341 
> 339512154 339511634 339664326 339793144
>
> ** patched java.lang.Class
>
> Executing: /home/peter/Apps64/jdk1.8.0-jdk8-tl/bin/java -Xmx4G -cp 
> ../out/production/test -Xbootclasspath/p:../out/production/jdk 
> test.EnumTest
>
>       EnumSet.noneOf(Class): 351724931 339286591 305082929 305042885 
> 305058303 305044144 305073463 305049604
>      MyEnum.valueOf(String): 955032718 908534137 891406394 891506147 
> 891414312 893652469 891412757 891409294
>       EnumSet.noneOf(Class): 414044087 406904161 406788898 406839824 
> 406765274 406815728 407002576 406779162
>
> The slow-down of about 20% (last line) is presumably a consequence of 
> another in-direction to obtain shared enum constants array when there 
> is already a Map in place. It is still fast though (300M EnumSet 
> instances / 0.4 s).
>
> Here's the source of the micro-benchmark:
>
> https://raw.github.com/plevart/jdk8-tl/JEP-149.enum/test/src/test/EnumTest.java 
>
>
> I don't know what's more important in this occasion. A small space 
> gain (8 or 4 bytes per j.l.Class instance) or a small performance gain 
> (20%).
>
> Regards, Peter
>