IV60353: 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 anything
      x = x^1; // Update variable x using integer xor operator
  with constant value 1
      if (x^1 == 0) {
        // Here, the optimizer can prove that x^1 must be zero
        if (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 value
      if (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:

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

  IV61042

Fix information

• Fixed component name

  JIT

• Fixed component ID

  620700124

Applicable component levels

• R600 PSY

     UP

• R260 PSY

     UP