FIX: Global Code Optimization Generates Incorrect Code

The information in this article applies to:

• The Microsoft C/C++ Compiler (CL.EXE), included with:

      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft Visual C++ for Windows, versions 1.0 and 1.5
  

SYMPTOMS

The code below illustrates a bug in the C/C++ compiler that occurs when the /Ox, /Ol, /Oe, or /Og compiler switch is used. As a result of the optimization, the code below is stuck executing an infinite loop because of source code optimization. By using the /Fc compiler option, the code within the if statement is optimized out of the source code, as shown in the assembly output.

RESOLUTION

There are four workarounds to this problem:

• Use the fast compiler. You can force the compiler to use this option by adding the /f option.

  -or-

• Compile without the /Ox, /Ol, /Oe, or /Og options.

  -or-

• Disable optimization for the function generating the incorrect code. This is done by using the optimize pragma as follows:

        #pragma optimize("",off)
  

        int test_func(void)
        {
         /* .... */
        }
  

        #pragma optimize("",on)
  

  -or-

• In the case of the code shown below in the "MORE INFORMATION section, repeat the line:

        state=0;
  

  to cause the compiler to generate the correct assembly code.

STATUS

Microsoft has confirmed this to be a bug in the products listed above. This problem was corrected in C/C++ compiler version 9.0, included with Visual C++ 32-bit Edition, version 2.0.

MORE INFORMATION

The following sample demonstrates this problem.

Sample Code

/* Compile options needed: /c /Fc /Ox (or /Ol, /Oe, or /Og)
*/
      #include <stdio.h>

      static int fault, state;

      int test_func (void)
      {
         if (state == 2)
         {
            state=0;         // No code generated for this block if
      optimized
         /* state=0; */      // works if this line's comment is removed
            if (fault == 0)  //
               return(1);      //
            else             //
               return(-1);     //
         }
         else
            return(0);
      }

      void main(void)
      {
         state = 2;

         while (test_func() == 1);
         printf("done\n");
      }