Q61310: /Ot May Generate Incorrect Code with Nested Conditionals

Article: Q61310
Product(s): See article
Version(s): 5.10 6.00 | 5.10 6.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | buglist6.00 | mspl13_c
Last Modified: 29-MAY-1990

When using the default optimization of -Ot with nested conditional
expressions, the compiler may generate incorrect code if the
conditionals are the same expression.

The following code sample demonstrates the problem:

#include <stdio.h>

void main (void)
{
   int i;

   for ( i = 0; i < 5; i++)
      if (i < 5)
         printf ("%d\n", i);
}

Output
------

   Actual  Expected
   ------  --------

     1       0
     2       1
     3       2
     4       3
             4

Below are some suggested workarounds:

1. Change "i < 5" in the above for loop, or change "i < 5" in the
   above if statement to "i <= 4".

2. Use -Od to disable optimization when compiling.

3. Use the #pragma optimization ("t", off/on) statement to
   disable/enable the default -Ot optimization before and after the
   function.

4. Use a combination of optimizations (for example, -Ox, -Olt, etc.).

The problem is with the default -Ot optimization, in particular; when
used by itself, the optimizer will generate incorrect code.

In the above code sample, the compiler does not generate the
instruction for the first comparison in the for loop (as shown below).
This causes the instruction pointer to jump to location $FC176 the
first time through the loop.

The end result is the same as if "i" went from 1 to 4 instead of from
0 to 4.
         .
         .
         mov   WORD PTR [bp-2], 0
$F166:
         cmp   WORD PTR [bp-2], 5 ;<- This "cmp" is missing
         jge   $FC167

         push  WORD PTR [bp-2]
         mov   ax, OFFSET DGROUP:$SG170
         push  ax
         call  _printf
         add   sp,4
$FC176:
         inc   WORD PTR [bp-2]
         cmp   WORD PTR [bp-2], 5
         jl    $F166
         .
         .

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

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.