IV26973: BAD LOOP VERSIONER TEST LEADS TO POOR PERFORMANCE, OR TO A CRASH IN JITTED CODE

APAR status

• Closed as program error.

Error description

• Error Message: This could be a performance problem, or it could
  appear as a crash with vmState=0x00000000.
  .
  Stack Trace:
  .
  

Local fix

• -Xjit:disableLoopVersioner
  

Problem summary

• The jit optimization "loop versioner" attempts to make a fast
  version of a loop that omits certain checks in the loop by
  performing equivalent checks before the loop starts.  If the
  loop contains an array access using an indexing expression that
  changes every loop iteration, the optimization can erroneously
  try to pull that indexing expression out of the loop, resulting
  in a nonsense check before the loop.  This corrupt check can
  cause control to flow to the slow version of the loop, thereby
  harming performance; or else the corruption could be so severe
  that the generated code crashes.
  

Problem conclusion

• This defect will be fixed in:
  6.0.1 SR3
  7.0.0 SR2
  6.0.0 SR11
  5.0.0 SR14
  .
  The loop versioner optimization was fixed so it doesn't try to
  emit checks based on the array indexing expression under these
  circumstances.
  

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:

  IV26974

Fix information

• Fixed component name

  JIT

• Fixed component ID

  620700124

Applicable component levels

• R260 PSY

     UP

• R600 PSY

     UP