LI81019: TERNARY OPERATOR REMOVING VARIABLE ASSIGNMENT

APAR status

• Closed as program error.

Error description

• Compile-time evaluation of a ternary operator is removing valid
  code resulting in incorrect output.  The evaluation changes:
  
  rc = (((x=1), 0) ? 1 : 0);
  
  into:
  
  rc = 0;
  
  The 'x=1' assignment statement disappears.
  
  
     === TEST CASE ===
  
  #include <stdio.h>
  int rc, x=-5;
  int main()
  {
     printf("x should equal 1\n");
     rc = (((x=1), 0) ? 1 : 0);
     printf("rc=%d x=%d\n", rc, x);
     return 0;
  }
  
  $ xlc test.c
  $ ./a.out
  x should equal 1
  rc=0 x=-5
  $
  

Local fix

• To stop the compile-time ternary operator evaluation add
  the benign built-in __builtin_constant_p():
  
  rc = (((x=1), __builtin_constant_p(0), 0) ? 1 : 0);
  

Problem summary

• PROBLEM DESCRIPTION:
  The compiler may fold conditional expressions (?, &&, ||) and
  incorrectly discard potential side-effects of evaluating the
  conditional expression.
  
  USERS AFFECTED:
  C++ conditional expressions with side-effects.
  

Problem conclusion

• The fix disables folding of conditional constant expressions
  with side-effects.
  

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:

  IJ21746

Fix information

• Fixed component name

  XL C/C++ LINUX

• Fixed component ID

  5725C7310

Applicable component levels