FIX: Loop Optimization Causes Infinite Do-While Loop

MS-DOS                 | WINDOWS   | WINDOWS NT

The information in this article applies to:

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

      - Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax
      - Microsoft C for OS/2, versions 6.0 and 6.0a
      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft Visual C++ for Windows, versions 1.0 and 1.5
      - Microsoft Visual C++ 32-bit Edition, version 1.0
  

SYMPTOMS

The use of loop optimization (/Ol, or /Ox for the C/C++ compiler 8.0 for Windows NT) in a do-while loop that terminates after a single iteration may cause an infinite loop. The code below can be used to demonstrate this behavior. An infinite loop is generated when the expression (i <= e) from the program below is true during the first loop iteration.

CAUSE

Examining the assembly/source code file generated by using the /Fc compiler option reveals that the comparison operation differs with the optimized and non-optimized versions. The optimized version will only reenter the loop if the two values are not equal, whereas the non-optimized version correctly checks if i is less than or equal to e.

Optimized version:

;|***   while (i<=e);

    *** 000059 ff 4e f4           dec WORD PTR [bp-12]
    *** 00005c 75 f1              jne $D536

;|***   while (i<=e);

  L00537:
    *** 000054 8b 46 f6           mov ax,WORD PTR -10[bp]
    *** 000057 39 46 fc           cmp WORD PTR -4[bp],ax
    *** 00005a 7f 03 e9 e7 ff     jle L00536

RESOLUTION

There are two workarounds to this problem:

1. Use the fast compiler option /f.

   -or-

2. Disable optimization during the function where the infinite loop occurs by using the optimize pragma:

         #pragma optimize("",off)
   

         void bad_loop_function(void)
         {
         /* ... */
         }
   

         #pragma optimize("",on)
   

STATUS

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

MORE INFORMATION

The following sample code can be used to demonstrate the problem.

Sample Code

/* Compile options needed: /Ol
*/

#include <stdio.h>

void main(void)

   int a, b, c;
   int e, i;

   scanf("%d%d%d", &a, &b, &c);
   printf("%d%d%d", a, b, c);

   i=a-c;
   e=b-c;
   do
   {
      printf("x");
      i++;
   }
   while (i<=e);
   printf("\n");