BUG: Bad Result If Call Inline Function Twice in Expression

• Microsoft Visual C++ for Windows, versions 1.5, 1.51, 1.52
• Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0, 4.1, 4.2, 5.0

SYMPTOMS

Invalid results may be generated under these conditions:

• Your code defines an inline function that uses global or static data, and it invokes that function twice within an expression.
• You are using the /Ob1 or /Ob2 or /Ox compiler optimization switch with the 32-bit compiler, or you are using the /Ob1g or /Ob2g or /Ox switch with the 16-bit compiler.
• The global or static variable appears in the return expression for the function.

RESOLUTION

• Do not use the above optimizations.

  -or-

• Assign the return value of one of the function calls to a local variable. Then use the local variable in the expression.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. We are researching this behavior and will post new information here in the Microsoft Knowledge Base as it becomes available.

Sample Code to Reproduce Problem

   /* Compile options needed 32-bit Compiler: /Ob1, or /Ob2, or /Ox
   /* Compile options needed 16-bit Compiler: /Ob1g, or /Ob2g, or /Ox
   */

   #include <stdio.h>

   int  nGlobal = 0;

   inline int test(int nIn) {
      return  nGlobal = nIn;
   }

   void main() {
      // This will incorrectly evaluate to true.
      if  (test(1) == test(2))
          printf("test(1) == test(2)\n");
      else
          printf("test(1) != test(2)\n");

      // This will correctly evaluate to false.
      int n = test(2);

      if  (test(1) == n)
          printf("test(1) == test(2)\n");
      else
          printf("test(1) != test(2)\n");
   }