IZ78390: INCORRECT RESULT WITH -O2

September 2010 PTF for XL C for AIX, V11.1
September 2010 PTF for XL C/C++ for AIX, V11.1
January 2011 PTF for XL C/C++ for AIX, V11.1
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
July 2010 PTF for XL C/C++ for AIX, V11.1
July 2010 PTF for XL C for AIX, V11.1
January 2011 PTF for XL C for AIX, V11.1
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

• The following test case produces incorrect result if compiled
  with -O2.
  
  
  ==========
  TEST CASE:
  ==========
  
  #include <stdio.h>
  int main()
       {
       double d = 1.0;
       short i;
       struct DBLM { double d;};
       union {
          struct DBLM a[2];
          struct { struct DBLM low, by; } s;
          } du;
       for(i=0;i<2;i++)
          du.a[i].d = d;
       if(du.s.by.d == 0)
          printf("ERROR: Should not get here! %g\n",du.s.by.d);
       return 0;
       }
  
  
  
  BAD:
  
  $xlc_r ./t.c -O2
  $./a.out
  ERROR: Should not get here! 0
  
  GOOD:
  xlc_r ./t.c
  $./a.out
  (prints out nothing)
  

Local fix

• Use XL C/C++ for AIX release prior to V10.1.
  

Problem summary

• PROBLEM DESCRIPTION: Incorrect result with -O2.  The compiler
  does not always alias array and nested struct members of an
  Union together.  This causes a correctness problem at
  Optimization.
  
  USERS AFFECTED: C only.  All platforms.  Nested array and
  struct members within a union.
  

Problem conclusion

• The compiler now properly recognizing that an element of an
  array member and a member of a sub-struct member of a union
  overlap the same memory location. The problem is fixed.
  

Temporary fix

Comments

APAR Information

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

  IZ74157

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

Fix information

• Fixed component name

  XL C FOR AIX

• Fixed component ID

  5724X1200

Applicable component levels