VarHandle Performance

[liqunl]

VarHandle has been added as a feature of Java9, to replace Unsafe.

In openj9, VarHandle is implemented using MethodHandle. In the JIT, we recognize and transform a VarHandle call into an invokeGeneric on a MethodHandle object which is an element of Varhandle.handleTable. The invokeGeneric becomes a invokeExact on a MethodHandle after conversion in the trees.

In simplified trees, the call is transformed from

call VarHandle.set
   aload VH

to

acall MethodHandle.asType
   aloadi VarHandle.handleTable[i]
      aload VH
   aload <call site MethodType>
call MethodHandle.invokeExact
   ==>acall MethodHandle.asType

As stated in http://openjdk.java.net/jeps/193, users of VarHandle is encouraged to store VarHandle objects into static final fields to allow constant folding to fold away some checks.

For predictable behaviour and performance at runtime VarHandle instances should be held in static final fields

Currently, our goal is to optimize VarHandle calls on VarHandle instances that are held in static final fields. The following are the main things we need,

• Guarded static final field folding Guarded static final field folding #3018
• Inline the MethodHandle call chain in targeted inlining
• Enhanced MethodHandle inlining opt strategy.
• Optimization to Unsafe calls made by VarHandle access methods, depends on Codegen Support for atomic method symbols eclipse-omr/omr#2958

Other optimizations include

• Try to replace acall MethodHandle.asType with conversion trees for invoke generic.
  asType will create a new MethodHandle for invoke generic, stopping the JIT from targeted inlining.
• More optimized java code in VarHandle subclasses.

Changes we have made
#671
#663
#264
#235
#210
#2121
eclipse-omr/omr#1796

[liqunl]

FYI @andrewcraik

[pshipton]

According to @andrewcraik the work targeted to 0.11.0 is completed. Moving to 0.12.0 for any remaining work.

[pshipton]

The expected performance improvements are delivered in the 0.12 milestone. Re-targeting to 0.14 (the next quarterly delivery) for the remaining work. If this happens to complete before the 0.13 (Java 12) milestone the target milestone will be adjusted.