IZ91166: INCORRECT OUT WITH -O2 -Q64

May 2011 PTF for XL C for AIX, V11.1
May 2011 PTF for XL C/C++ for AIX, V11.1
August 2011 PTF for XL C/C++ for AIX, V11.1
November 2011 PTF for XL C/C++ for AIX, V11.1
January 2012 PTF for XL C for AIX, V11.1
January 2012 PTF for XL C/C++ for AIX, V11.1
April 2012 PTF for XL C for AIX, V11.1
April 2012 PTF for XL C/C++ for AIX, V11.1
July 2012 PTF for XL C/C++ for AIX, V11.1
October 2012 PTF for XL C for AIX, V11.1
October 2012 PTF for XL C/C++ for AIX, V11.1
February 2013 PTF for XL C for AIX, V11.1
February 2013 PTF for XL C/C++ for AIX, V11.1
XL C/C++ for AIX Fix Pack 14 (May 2013 PTF) for 11.1
XL C for AIX Fix Pack 14 (May 2013 PTF) for 11.1
XL C/C++ for AIX Fix Pack 15 (August 2013 PTF) for 11.1
XL C for AIX Fix Pack 15 (August 2013 PTF) for 11.1
XL C/C++ for AIX Fix Pack 16 (November 2013 PTF) for 11.1
XL C for AIX Fix Pack 17 (February 2014 PTF) for 11.1
XL C/C++ for AIX Fix Pack 17 (February 2014 PTF) for 11.1
XL C/C++ for AIX Fix Pack 18 (May 2014 PTF) for 11.1
XL C/C++ for AIX Fix Pack 19 (August 2014 PTF) for 11.1
XL C/C++ for AIX Fix Pack 20 (November 2014 PTF) for 11.1
XL C for AIX Fix Pack 21 (April 2015 PTF) for 11.1
XL C/C++ for AIX Fix Pack 21 (April 2015 PTF) for 11.1
XL C/C++ for AIX Fix Pack 22 (September 2015 PTF) for 11.1
XL C for AIX Fix Pack 23 (March 2016 PTF) for 11.1
XL C/C++ for AIX Fix Pack 23 (March 2016 PTF) for 11.1
XL C for AIX Fix Pack 24 (September 2016 PTF) for 11.1
XL C/C++ for AIX Fix Pack 24 (September 2016 PTF) for 11.1
August 2011 PTF for XL C for AIX, V11.1
November 2011 PTF for XL C for AIX, V11.1
July 2012 PTF for XL C for AIX, V11.1
XL C for AIX Fix Pack 16 (November 2013 PTF) for 11.1
XL C for AIX Fix Pack 18 (May 2014 PTF) for 11.1
XL C for AIX Fix Pack 19 (August 2014 PTF) for 11.1
XL C for AIX Fix Pack 20 (November 2014 PTF) for 11.1
XL C for AIX Fix Pack 22 (September 2015 PTF) for 11.1

APAR status

• Closed as program error.

Error description

• Incorrect output is generated when a test case is compiled with
  -O2 -q64.
  
  ===== Test Case
  test.c
  
  int foo() { return 0; }
  
  int bar(int *p)
  {
      int rc;
  
      rc = foo();
      if (rc < 0)
         return rc;
  
      p[0] = 0;
      p[1] = 0x8000;
      return 0;
  }
  
  #include <stdio.h>
  
  int main()
  {
     int rc, x[2];
  
     rc = bar(x);
     printf("rc %d\n", rc);
     return 0;
  }
  
  ====== Observed Output
  # xlc_r -q64 -O0 test.c && a.out
  rc 0
  # xlc_r -q64 -O2 test.c && a.out
  rc 32768                             <== Incorrect
  # xlc_r -q64 -O3 test.c && a.out
  rc 0
  # xlc_r -q64 -O4 test.c && a.out
  rc 0
  # xlc_r -q64 -O5 test.c && a.out
  rc 0
  

Local fix

• The test case passes when compiled at all optimization levels,
  except for -O2. In addition, the test case also passed when
  compiled with -q32 or -qarch=pwr5 or lower.
  

Problem summary

• PROBLEM DESCRIPTION:
  An internal function returns incorrect instructions, which
  makes other optimization discard it, hence leading to wrong
  code.
  
  USERS AFFECTED:
  Users of code (1) containing functions whose return can be
  optimized and (2) contains optimization opportunities and (3)
  is compiled at O2 q64.
  

Problem conclusion

• This problem was here from the beginning of the optimization
  routine, it was just exposed recently by the extra step which
  triggered dead code. The fix is straightforward.
  

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:

  LI76111

Fix information

• Fixed component name

  XL C FOR AIX

• Fixed component ID

  5724X1200

Applicable component levels