IJ17054: JAVA JIT ON POWER: CRASH IN FREEBESTREGISTER()

APAR status

• Closed as program error.

Error description

• Error Message: SIGSEGV is received when JIT compiling a method
  which makes use of a java switch statement where subtracting
  case<OSB>i<CSB> and case<OSB>i-1<CSB> would cause a 32bit
  integer overflow.
  .
  Stack Trace:
  TR_PPCMachine::freeBestRegister(TR_Instruction*,TR_Register*,TR_
  PPCRealRegister*,bool)
  TR_PPCMachine::assignOneRegister(TR_Instruction*,TR_Register*,bo
  ol)
  TR_PPCRegisterDependencyGroup::assignRegisters(TR_Instruction*,T
  R_RegisterKinds,unsigned int,TR_CodeGenerator*)
  TR_PPCDepLabelInstruction::assignRegisters(TR_RegisterKinds)
  TR_PPCCodeGenerator::doRegisterAssignment(TR_RegisterKinds)
  TR_CodeGenerator::generateCode()
  TR_Compilation::generateCode()
  TR_Compilation::compile()
  .
  The problem can only occur when JIT compiling Java code that
  contains a switch statement that would cause a 32bit integer
  overflow like the following example:
  switch( type ){
     case -2112854795:
        // code
        break;
     case 523817410:
        // code
        break;
     }
  The overflow occurs when the JIT compiler attempts to calculate
  the stride between two cases:
  stride = 523817410 - (-2112854795)
  The result overflows a 32bit integer resulting in an unexpected
  negative value which eventually results in a crash later in the
  compilation.
  The problem can only occur on POWER JVMs (AIX, LinuxPPC,
  LinuxPCCle) and can not occur on other platforms like x86 or
  zSeries.
  

Local fix

• The problem can be avoided by disabling the JIT optimization
  called GRA. This might have a moderate effect on performance by
  causing the JIT to generate less optimal code, but when applied
  only to one method (as seen below) the effect should be
  unnoticeable. Use the following java command line option:
  -Xjit:{package/class.method*}(disableGRA)
  Where <package/class.method> is taken from the javacore file
  generated when the problem occurred.
  

Problem summary

• The JIT failed to check for a possible overflow condition which
  would result in a miscalculation of the number of registers that
  are required to generate code for handling a switch state.
  

Problem conclusion

• The JIT was modified so that it would detect a overflow
  condition and properly calculate the number of registers
  required to handle the switch statement.
  .
  This APAR will be fixed in the following Java Releases:
     8    SR5 FP40  (8.0.5.40)
     7    SR10 FP50 (7.0.10.50)
     7 R1 SR4 FP50  (7.1.4.50)
  .
  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