IJ02084: MISSING POST-INCREMENT FOR GLOBAL VARIABLE IN CONDITIONAL WITH UNUSED OBJECT

XL C/C++ for AIX Fix Pack 20 (March 2018 Update) for 12.1
XL C/C++ for AIX Fix Pack 21 (January 2019 Update) for 12.1

APAR status

• Closed as program error.

Error description

• A post-increment of a global variable in an IF conditional is
  not incremented when an unused object with a defined
  destructor is present in the same function:
  
          int glob_var = 0;     // GLOBAL VALUE
          ...
          {
              UnusedObj a;
              if (glob_var++) { return; }
  
  If the unused object does not have a defined destructor,
  the post-increment succeeds.
  
  
  
     === TEST CASE ===
  
  #include <cstdio>
  #include <cassert>
  
  int glob_var = 0;
  
  class UnusedObj {
    public:
      ~UnusedObj() { } // If commented out, problem does not occur
  };
  
  void initialize()
  {
      UnusedObj a;
      if (glob_var++) { return; }  // Post-increment
  }
  
  void shutdown()
  {
      UnusedObj a;
      assert(glob_var > 0);
  
      printf("%d\n",glob_var);
      if (--glob_var != 0) { return; }
  }
  
  int main(int argc, char *argv[])
  {
      initialize(); initialize();
      shutdown(); shutdown();
  }
  
  
  $ xlC apar.cpp
  
  OUTPUT:
  $ ./a.out
  1
  Assertion failed: glob_var > 0, file  apar.cpp, line 20
  IOT/Abort trap (core dumped)
  $
  
  EXPECTED OUTPUT:
  $ ./a.out
  2
  1
  $
  

Local fix

• Remove destructor for unused object.
  

Problem summary

• PROBLEM DESCRIPTION:
  Insertion of post-increment code into block may be inserted
  incorrectly.
  
  USERS AFFECTED:
  Post increment in the condition of an if-statement, if last
  statement in the conditional block is a return statement and
  implicit object destruction is pending.
  

Problem conclusion

• Compiler may generate incorrect code in some cases with
  post-increment in the condition of an if-statement, if last
  statement in the conditional block is a return statement and
  implicit object destruction is pending.
  

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

• RC10 PSY

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