Perf regression accessing fields in JDK21

[Radim Vansa]

Hello,

I've stumbled upon a performance regression when accessing fields in a 
class with many fields, in interpreted mode. I believe this is caused by 
introduction of FieldStream [1]; another performance regression caused 
by this change was already found upon heap dump [2].

Field access causes iteration through all fields in 
InstanceKlass::find_local_field(...) [3]. After [1] this results in 
decoding many variable-length integers (through 
FieldInfoReader.read_field_info invoked by the next() method) rather 
than simple indexed access, which turns out to be costly. Moreover, when 
we call fd->reinitialize(...) the indexed access [4] results in another 
iteration through the fields [5] rather than O(1) access.

I am attaching a reproducer; this creates a class with 21,000 fields, 
compiles it and executes this (all that the class does is to initialize 
all its fields). On JDK 17 running this reports 581 ms; on JDK 21 the 
test takes 8017 ms on my laptop.

I was able to avoid the second iteration by passing FieldInfo to 
reinitialize() and the execution went down to 5712 ms, but I don't see a 
simple solution that would make the first iteration more efficient. I 
was thinking that the name and signature indices might not be 
variable-size encoded (this would require 4 rather than 2 bytes in 
classes with a short constant pool) but then in reinitialize() we would 
need to load the other flags anyway (hence iterating through the stream 
with variable lengths). The stream could be also reordered (?) into 
several sequences of fixed length records, but this is rather 
complicated and does not really guarantee to fix the regression.

Thank you for your thoughts on this matter.

Radim Vansa


[1] https://bugs.openjdk.org/browse/JDK-8292818

[2] https://bugs.openjdk.org/browse/JDK-8317692

[3] 
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/oops/instanceKlass.cpp#L1783

[4] 
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/runtime/fieldDescriptor.cpp#L97

[5] 
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/oops/instanceKlass.cpp#L1774



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