IV61042: INCORRECT OPTIMIZATION OF VARIABLE UPDATES USING INTEGER XOR OPERATOR ("^")

APAR status

• Closed as program error.

Error description

• Error Message: Program operates incorrectly when it contains a
  statement of the form: "x = x ^ 1";
  .
  Stack Trace: N/A
  .
  

Local fix

• -Xjit:disableLocalVP,disableGlobalVP will prevent the error. If
  applied globally this may have a significant cost in performance
  which will vary from application to application, but can be in
  the region of 20%. If this can be targeted at only the methods
  exhibiting the problem the performance impact will be far less,
  and possibly not measurable. For example to apply the work
  around to all the methods of a.b.ClassA
  use:-Xjit:{a/b/ClassA.*}(disableLocalVP,disableGlobalVP)
  

Problem summary

• In certain cases, the JIT optimizer can incorrectly transform an
  integer xor expression by wrongly considering the consequences
  of downstream if statements, when those if statements
  redundantly test the same condition.For example, if the user
  program contained code like the following:void foo(int x) {//
  Here, x could be anythingx = x?1; // Update variable x using
  integer xor operator with constant value 1if (x?1 == 0) {//
  Here, the optimizer can prove that x?1 must be zeroif (x?1 == 0)
  {  // Redundant check...}}}The JIT optimizer could perform an
  incorrect optimization that would cause the code to behave as
  follows:void foo(int x) {x = 0; // Incorrectly optimized valueif
  (0 == 0) {...}}
  

Problem conclusion

• This APAR will be fixed in the following Java Releases:
     5.0  SR16 FP7  (5.0.16.7)
     6    SR16 FP1  (6.0.16.1)
     626  SR8  FP1  (6.1.8.1)
     7    SR7  FP1  (7.0.7.1)
     7.1  SR1  FP1  (7.1.1.1)
  
  The faulty logic was removed from the JIT optimizer.
  

Temporary fix

Comments

APAR Information

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

  IV60353

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

Fix information

• Fixed component name

  JAVA 5 JIT

• Fixed component ID

  620500124

Applicable component levels

• R500 PSY

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