Q66216: Loop Optimizations May Generate Bad Code on Nested Loops

Article: Q66216
Product(s): See article
Version(s): 6.00   | 6.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | buglist6.00 buglist6.00a | mspl13_c
Last Modified: 24-OCT-1990

The code below illustrates a problem in using loop optimizations with
nested loops.

With loop optimizations disabled, the compiler calculates the value of
x for each iteration of the loop and produces the correct result.

When loop optimizations are turned on, the compiler initializes the
value of x and subtracts it by 10 on each loop iteration. The problem
occurs here with the miscalculation of x when variable i increments.

Workaround
----------

The only workaround is to compile without loop optimizations, which
can be done in the following ways:

1. Do not use the /Ol option.

2. Insert the #pragma optimize statement in the code to turn off loop
   optimization for the particular function.

Microsoft has confirmed this to be a problem in C version 6.00 and
6.00a. We are researching this problem and will post new information
here as it becomes available.

Sample Code
-----------

#include<stdio.h>

void main(void)
{
     int i, j, x;

     for (i=0; i<2; i++)
     {
          for (j=0; j<4; j++)
          {
               x=(i*4+3-j)*10;
               printf("%d\n", x);
          }
     }
}

The following table demonstrates the output when the code above is
compiled with and without loop optimizations:

   Correct Output Without /Ol     Incorrect Output with /Ol
   --------------------------     -------------------------

      30                             30
      20                             20
      10                             10
       0                              0
      70                             -10
      60                             -20
      50                             -30
      40                             -40

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