LI77453: INCORRECT LOOP COUNTER WITH -QIPA

XL C/C++ for Linux Fix Pack 9 (August 2013 Update) for 11.1
XL C/C++ for Linux Fix Pack 8 (June 2013 Update) for 11.1

APAR status

• Closed as program error.

Error description

• The counter in the for loop goes past its boundaries when
  compiling with -qipa and causes an incorrect output at runtime.
  
  The issue goes away if you remove -qipa from the compile line.
  
  
  ===== COMPILE COMMAND:
  xlC_r -o tc test.c -O -q64 -qipa
  
  ===== TESTCASE:
  $ cat test.c
  #include <stdio.h>
  
  int main()
  {
          unsigned int ctr=6;
          unsigned int stop =  (unsigned int)-1;
          unsigned int start =  5;
  
          printf("start=%u  ; stop=%u\n",start,stop);
  
          for(ctr = start ; ctr != stop ; --ctr)
          {
                  printf("ctr=%u\n",ctr);
  
                  if (ctr>start)
                  {
                          printf("Shouldn't get here\n");
                          exit(1);
                  }
          }
  }
  $
  
  ===== ACTUAL OUTPUT:
  $ ./tc
  start=5  ; stop=4294967295
  ctr=5
  ctr=4
  ctr=3
  ctr=2
  ctr=1
  ctr=0
  ctr=4294967295
  Shouldn't get here
  $
  
  ===== EXPECTED OUTPUT:
  $ ./tc
  start=5  ; stop=4294967295
  ctr=5
  ctr=4
  ctr=3
  ctr=2
  ctr=1
  ctr=0
  $
  

Local fix

• Compile with -qstrict_induction. Although, you may need to
  monitor the runtime performance when using this option.
  

Problem summary

• USERS AFFECTED:
  Users using -qhot=level=0 or above may be affected by this
  issue.
  
  PROBLEM DESCRIPTION:
  When -qhot=level=0 or higher is used, users may encounter
  incorrect output due to an internal induction variable
  overflow.
  

Problem conclusion

• The fix consists of detecting a loop normalization opportunity
  where the induction variable overflows; so that users do not
  have to face a performance impact of using -qstrict_induction.
  

Temporary fix

Comments

APAR Information

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

  IV33243

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

Fix information

• Fixed component name

  XL C/C++ FOR LI

• Fixed component ID

  5724X1400

Applicable component levels

• RB10 PSN IV33243

     UP06/09/13