IY51377: Incorrect behaviour of va_start()/va_arg() at -O2

APAR status

• Closed as user error.

Error description

• ABSTRACT:     Incorrect behaviour of va_start()/va_arg() at -O2
  ========
  
  COMPILE COMMAND:   xlC -O t.C
  ===============
  
  OS LEVEL:    AIX 4.3.3, 5.1, and 5.2
  ========
  
  DESCRIPTION:
  ===========
  
  The customer's testcase is generating incorrect output at low
  opt, but correct output when no optimization is used or when IPA
  is used.
  
  The testcase uses va_start()/va_arg() to handle varying number
  of arguments to a function.  The function, c, is defined as
  c(char *a1, char *a2, ...). The body of c uses va_arg to print
  the addresses of the 2nd and 3rd arguments.
  
  
  TESTCASE:
  ========
  ******************** t.C *******************
  
  #include <iostream>
  #include <stdarg.h>
  
  class t1
  {
  public:
      void b(char *a1)
      {
          c(a1, "2", "3");
      }
  protected:
      void c(char *a1, char *a2, ...)
      {
          va_list ml;
          va_start(ml, a1);
          char *dummy;
          dummy = a2;
          fprintf(stdout, "Adress a1 = %d/Value a1 = %s\n", a1,
  a1);
          fprintf(stdout, "Adress a2 = %d/Value a2 = %s\n", a2,
  a2);
          dummy = va_arg(ml, char*);
          fprintf(stdout, "Adress arg2 = %d/Value arg2 = %s\n",
  dummy, dummy);
          dummy = va_arg(ml, char*);
          fprintf(stdout, "Adress arg3 = %d/Value arg3 = %s\n",
  dummy, dummy);
          va_end(ml);
      }
  };
  
  int main()
  {
      t1 mt1;
      mt1.b("1");
      return 0;
  }
  
  ******************** t.C *******************
  
  ACTUAL OUTPUT:
  =============
  $ xlC -O t.C
  $ a.out
  Adress a1 = 268444288/Value a1 = 1
  Adress a2 = 268444290/Value a2 = 2
  Adress arg2 = -559038737/Value arg2 =
  Adress arg3 = 268444292/Value arg3 = 3
  
  
  EXPECTED OUTPUT:
  ===============
  The expected output is that the values of a2 and arg2 should be
  the same. This correct output is seen when the testcase is
  compiled without optimization or with -O4 or -O5 (i.e. IPA
  turned on).
  
  $ xlC t.C
  $ a.out
  Adress a1 = 268444560/Value a1 = 1
  Adress a2 = 268444562/Value a2 = 2
  Adress arg2 = 268444562/Value arg2 = 2
  Adress arg3 = 268444564/Value arg3 = 3
  $
  

Local fix

Problem summary

Problem conclusion

Temporary fix

Comments

• This problem is not a compiler defect, but is working as
  designed, according to the C++ Standard.
  
  The way that va_start is being used is incorrect. va_start is
  being called with the second parameter being the first parameter
  from the function definition, instead of the rightmost parameter
  (before the ', ...').  According to the C and C++ Standards, the
  second parameter of va_start() is the rightmost parameter in the
  parameter list of the function, right before the ", ...".  In
  the testcase, va_start is called with the leftmost parameter
  instead of the rightmost one.
  
  Here is subclause 18.7 paragraph 3 from the C++ Standard, which
  addresses this:
  
  "...  The parameter parmN is the identifier of the rightmost
  parameter in the variable parameter list of the function
  definition (the one just before the ...).  If the parameter
  parmN is declared with a function, array, or reference type, or
  with a  type that is not compatible with the type that results
  when passing an argument for which there is no parameter, the
  behaviour is undefined."
  

APAR Information

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

Fix information

Applicable component levels