IJ35502: RVO OVERWRITING PARAMETER TO FUNCTION

APAR status

• Closed as program error.

Error description

• Return value optimization (RVO) is overwriting a function input
  parameter object reference, resulting in an incorrect addition
  calculation.
  
  The function calling the function using RVO (i.e. make() calling
  combine() ), should not be using a class return value that
  shares a memory location with one of the parameters.
  
  
  === TEST CASE ===
  
  #include <stdio.h>
  
  struct Numbers
  {
      Numbers() : a(0) , b(0) { }
      Numbers(int av, int bv) : a(av), b(bv) { }
  
      int a;
      int b;
  };
  
  Numbers combine(const Numbers& a, const Numbers& b)
  {
      Numbers x;          //  (1)
      x.a = a.a + b.a;
      x.b = a.b + b.b;
      return x;
  }
  
  Numbers make()
  {
      Numbers a(1, 1);    //  (2)
      Numbers b(2, 2);
  
      a = combine(a, b);
      return a;
  }
  
  int main()
  {
      Numbers a = make();
      printf("%d %d\n", a.a, a.b);
  }
  
  
  
  $ xlC test.cpp
  $ ./a.out
  2 2
  $
  
  Expected output is:
  3 3
  

Local fix

• Add a user provided copy assignment operator:
  
  
  Numbers &operator=(const Numbers &other) { a = other.a; b =
  other.b; return *this; }
  

Problem summary

• PROBLEM DESCRIPTION:
  Temporary elision is too aggressive and elides a necessary
  temporary, causing invalid runtime behaviour.
  
  USERS AFFECTED:
  Multiple levels of NRVO.
  

Problem conclusion

• The temp elision has been corrected.
  

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:

Fix information

• Fixed component name

  XL C/C++ FOR AI

• Fixed component ID

  5725C7200

Applicable component levels