IJ20662: INCORRECT RESULTS FOR EXPRESSIONS WITH SHIFTS

APAR status

• Closed as program error.

Error description

• Error Message: A method using a combination of shift and
  arithmetic 'or' expressions started producing incorrect results
  after having been called many times. The incorrect behaviour
  continued until the JVM was restarted.
  .
  Stack Trace: N/A
  .
  

Local fix

• The problem can be avoided by setting the following operating
  system environment variable before starting the JVM:
  TR_DisableROLSimplification=1
  

Problem summary

• There is a commonly used idiom in Java to rotate the bits in a
  numeric value. It typically looks something like the following
  for the case of rotating a 32 bit value by 8 bits:
  x = x << 24 | x >>> 8;
  Since this is often a performance critical calculation that can
  be represented simply in the instructions sets of many
  processors, the JIT recognises the pattern and optimises it for
  optimal encoding. Unfortunately, the optimisation failed to
  correctly test the bit length of the variable being rotated, and
  as a result would incorrectly transform expressions that would
  be a valid rotation of a 32 bit variable when the variable was
  in fact 64 bits.
  

Problem conclusion

• The optimisation has been fixed to avoid the ambiguity between
  different sizes of the target variable.
  For more details, see
  https://github.com/eclipse/omr/pull/4471
  .
  This APAR will be fixed in the following Java Releases:
     8    SR6 FP5   (8.0.6.5)
  .
  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