IJ55085: JAVA JIT: GC ASSERT DUE TO AN INVALID OBJECT REFERENCE

APAR status

• Closed as program error.

Error description

• Error Message: The GC asserts with the message "Object neither
  in heap nor stack-allocated". Please note that this assert
  message is by no means unique to the specific issue described by
  this APAR. Therefore, an instance of this assert message is not
  definitive proof that this APAR issue had occurred.
  .
  Stack Trace: N/A
  .
  This problem can occur on any J9 platform as the issue is found
  in common code used on all J9 platforms. The problem will most
  likely have no negative consequences, but in rare cases a GC
  assert will occur. There is a very small possibility that a
  segmentation fault can occur, but there have been no reports of
  this actually occurring.
  

Local fix

• The issue can be avoided by disabling the Virtual Guard Head
  Merger JIT optimization using the following java command line
  option:
  -Xjit:disableVirtualGuardHeadMerger
  This option should have a minor performance impact. We estimate
  that the effects would be no more than 1-2% on the high end for
  typical scenarios.
  

Problem summary

• The JIT was allowing an object field load to occur outside an
  object type guard. This allowed for the possibility of loading
  from an incorrect offset in cases where the guard would have
  prevented the execution of inlined code. Since the results of
  the load would only be used inside the guarded section, there
  would be no possibility of this load causing incorrect behavior
  of the java method, but the existence of this incorrect value in
  a register might be noticed by the GC resulting in an assert.
  There is also the possibility that the invalid load could reach
  past the end of an object, and if that object was located at the
  very end of the java heap, the load could result in a
  segmentation fault rather than a GC assert.
  

Problem conclusion

• The JIT was modified to prevent loading values outside an object
  type guard.
  .
  This APAR will be fixed in the following Releases:
  .
  IBM Semeru Runtimes
     21              21.0.8.0
     17              17.0.16.0
     11              11.0.28.0
  IBM SDK, Java Technology Edition
     8    SR8 FP50  (8.0.8.50)
  .
  Downloads and supplementary documentation can be found at the
  following locations:
  - For non z/OS operating systems:
    - IBM Semeru Runtimes, Version 11 and later
      https://www.ibm.com/semeru-runtimes/downloads/
    - IBM SDK, Java Technology Edition, Version 8
      https://www.ibm.com/support/pages/java-sdk-downloads/
  - For the z/OS operating system:
    - Java SDK Products on z/OS
      https://www.ibm.com/support/pages/java-sdk-products-zos
  

Temporary fix

Comments

APAR Information

• APAR is sysrouted FROM one or more of the following:

• APAR is sysrouted TO one or more of the following:

Fix information

• Fixed component name

  JIT

• Fixed component ID

  620700124

Applicable component levels