IY87090: POBLEMS WITH UNIONS WHEN USING -O OR ANY OTHER HIGHER OPTIMIZATION LEVEL.

April 2009 XL C/C++ Enterprise Edition V8.0 for AIX PTF
May 2007 XL C/C++ Enterprise Edition V8.0 for AIX PTF
December 2006 XL C/C++ Enterprise Edition V8.0 for AIX PTF
May 2008 XL C/C++ Enterprise Edition V8.0 for AIX PTF
August 2008 XL C/C++ Enterprise Edition V8.0 for AIX PTF
February 2007 XL C/C++ Enterprise Edition V8.0 for AIX PTF
November 2007 XL C/C++ Enterprise Edition V8.0 for AIX PTF
August 2006 XL C/C++ Enterprise Edition V8.0 for AIX PTF
April 2010 XL C/C++ Enterprise Edition V8.0 for AIX PTF
June 2011 XL C/C++ Enterprise Edition V8.0 for AIX PTF
July 2009 XL C/C++ Enterprise Edition V8.0 for AIX PTF
October 2009 XL C/C++ Enterprise Edition V8.0 for AIX PTF
July 2007 XL C/C++ Enterprise Edition V8.0 for AIX PTF
August 2007 XL C/C++ Enterprise Edition V8.0 for AIX PTF
February 2008 XL C/C++ Enterprise Edition V8.0 for AIX PTF
November 2008 XL C/C++ Enterprise Edition V8.0 for AIX PTF

APAR status

• Closed as program error.

Error description

• Statements need to be declared twice when using the -O option.
  Otherwise, an incorrect output is obtained.
  
  When compiled with xlC -O test.cpp the actual output is obtained
  only if "buffer[0] = ..." are stated twice.
  
  -------------output-------------
  
  exp: 0 1 a5 a5
  act: be ef a5 a5
  
  -------------end of output----
  
  
  
  ========Test Case===========
  
  extern "C" {
    int printf(const char *,...);
  }
  
  inline void putInt16(char *buffer, int   value) {
      typedef const union {
          int d_variable;
          char d_bytes[1];
      }& T;
  
      buffer[0] = T(value).d_bytes[sizeof value - 2];
      buffer[1] = T(value).d_bytes[sizeof value - 1];
  
      buffer[0] = T(value).d_bytes[sizeof value - 2];
      buffer[1] = T(value).d_bytes[sizeof value - 1];
  
  }
  
  
  int main() {
    const unsigned short unsignedNumber = 1;
  
    for (int di = 0; di < 1; ++di) {
      char buffer[4] = {0xa5,0xa5,0xa5,0xa5};
      putInt16(buffer , unsignedNumber);
      printf("act: %01x %01x %01x %01x\n", buffer[0], buffer[1],
  buffer[2], buffer[3]);
    }
    return 0;
  }
  
  =========End of Test case =========
  

Local fix

• Modify the union and variable T such that T.d_variable holds the
  value of 'value'.
  
  for example:
  
   union{
          int d_variable;
          char d_bytes[1];
      }T;
  
      T.d_variable = value;
  
      buffer[0] = T.d_bytes[sizeof value - 2];
      buffer[1] = T.d_bytes[sizeof value - 1];
  

Problem summary

• Anyone optimizing and using inlining.
  
  A function that is being inlined is taking the address of a
  parameter.  At the call site the parameter is a literal.  The
  generated code is trying to take the address of a constant.
  

Problem conclusion

• We changed the compiler to store the literal into a
  temporary since the parameter is address taken.
  

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:

  LI71776 LI71880 LI71907 LI71933

Fix information

• Fixed component name

  XLC C++ AIX

• Fixed component ID

  5724M1200

Applicable component levels

• R800 PSY U813114

     UP07/09/13 I 1000