IV82902: JAVA JIT - INCORRECT COMPARISION RESULTS ON X86 CPUS

APAR status

• Closed as program error.

Error description

• Error Message: Incorrect behavior in ICU4J.
  .
  Stack Trace: N/A
  .
  The problem can only occur on X86 (Linux/Windows on AMD or Intel
  CPUs) and we have only seen the problem on Java8. The failing
  test-case did not hit the problem in Java6 or Java7, but we
  believe that the problem is possible but much more difficult to
  encounter then it is in Java8.
  The result of the issue could in theory surface in many ways,
  but incorrect behavior is the most likely result and is how it
  was seen in the ICU4J scenario where the problem was first
  discovered.
  

Local fix

• The problem can be avoided with almost no performance impact by
  setting/exporting the following environment variable:
  TR_disableNoTestEFlags=1
  

Problem summary

• The JIT may fail to generate a "test/cmp" instruction when a
  previous instruction sets the right flag register bits to direct
  a "jmp" instruction, but the previous instruction was working
  against a register width that the "jmp" was not meant to react
  against. i.e. A "shr" instruction working against rdi where the
  comparison should have been against a byte (dil). Using the
  flags set by the "shr" might cause the "jmp" instruction to
  behave in a way that the Java code was not intending. A
  "test/cmp" instruction against "dil" is requited in such cases.
  

Problem conclusion

• The JIT was modified such that the register width is considered
  when determining if a "test/cmp" instruction is needed when
  previous instructions have already set the conditions flags that
  the "test/cmp" instruction would set.
  .
  This APAR will be fixed in the following Java Releases:
     8    SR3       (8.0.3.0)
     6 R1 SR8 FP25  (6.1.8.25)
     6    SR16 FP25 (6.0.16.25)
     7    SR9 FP40  (7.0.9.40)
     7 R1 SR3 FP40  (7.1.3.40)
  .
  Contact your IBM Product's Service Team for these Service
  Refreshes and Fix Packs.
  For those running stand-alone, information about the available
  Service Refreshes and Fix Packs can be found at:
             https://www.ibm.com/developerworks/java/jdk/
  

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

• R130 PSY

     UP

• R260 PSY

     UP

• R600 PSY

     UP