Native Image Committer and Community Meeting 2023-11-16

[christianwimmer]

List of all past and upcoming meetings: #3933

New and Noteworthy

• Staying up-to-date with JDK 22 promoted builds:
  [GR-49852] [GR-49613] Update JVMCI to 22+22-jvmci-b01. #7748
  [GR-49718] Update JVMCI to 22+20-jvmci-b02. #7721

• Allow to provide options to the native image generator using an environment variable:
  [GR-39407] Add support for NATIVE_IMAGE_OPTIONS environment variable. #7745

• Experimental option handling:
  [GR-47922] Improve list of experimental options. #7720
  [GR-47922] Enforce -H:±UnlockExperimentalVMOptions in CI. #7709

• How to diagnose and debug when a field offset computed at image build time is wrongly used at image run time:
  [GR-48949] For instance objects, move the identity hashcode from the object header into the object. #7835 tries to make the field offsets "more different" by default.
  [GR-49869] Add an option to artificially increase the header size. #7767 to force errors if you suspect that some field offset is wrong.

• Improve analysis time
  [GR-41614] Move resource registration to beforeAnalysis phase #7670 removes the repeated re-scanning of the whole class path during analysis (whenever the analysis has reached an incremental fixed point)
  [GR-48230] Use common pool for class loading, analysis, and compilation. #7661

• Thightening the rules around recurring callbacks:
  [GR-43125] Verify that recurring callbacks don't allocate Java heap memory. #7787

• Renaming of compiler packages
  First rename: [GR-48318] Move compiler packages and module to jdk.compiler.graal namespace. #7621
  Second rename: [GR-49610] Rename jdk.compiler.graal to jdk.graal.compiler. #7650

• Remove unnecessary code now that ParseOnce is always enabled:
  [GR-48579] Remove StaticAnalysisResults. #7766
  [GR-48579] Remove parseOnce checks. #7817

• Compatibility
  [GR-45651] Throw missing registration errors for the JCK. #7827
  [GR-45806] Execute bootstrap methods at run time #7834
  [GR-47937] Make lambda class names format consistent with the JDK name format #7775
  [GR-49691] Maximum size of serial GC memory pools is undefined. #7694

• Monitoring / tools:
  [GR-49904] [GR-50008] Add test to verify JFR mirror events. #7795
  [GR-48279] JFR refactoring and cleanups. #7774
  Use calling convention to derive incoming param register and stack slot locations #7256
  [GR-49305] Allow sampling in method prologue and epilogue. #7755

Deep Dive: Execute bootstrap methods at run time

Bootstrap methods

• Used by ConstantDynamic and InvokeDynamic bytecode
• Allow to create a constant or invoke a method not fixed in bytecode
• Instantiate constant or method the first time it is needed
• ConstantDynamic: the value returned by the bootstrap method is just pushed to the expression stack
• InvokeDynamic: the value returned by the bootstrap method must be a CallSite, which wraps a MethodHandle, which is invoked

Native image used to execute bootstrap methods always at image build time

• That's what we want for "known" bootstrap methods like for lambda functions and string concatenation
• But it is not safe in general because bootstrap methods can be arbitrary user code
  • They can have side effects
  • They can fail with an exception
  • They can return instances of classes that are not marked as --initialize-at-build-time.

#7834 adds support to execute bootstrap methods at run time. This is the default for all bootstrap methods from the JDK that are not known to be safe.

openjdk/jdk#14927 already merged the necessary JVMCI changes into OpenJDK.