Q52198: Optimization Can Cause Integer Division to Always Return 1

Article: Q52198
Product(s): See article
Version(s): 2.00 2.01
Operating System(s): MS-DOS
Keyword(s): ENDUSER | s_quickasm buglist2.00 buglist2.01 | mspl13_c
Last Modified: 17-JAN-1990

When optimizing code under QuickC (using either the Options menu from
within the environment, or the -Ot or -Ox switches from the command
line), integer division can generate incorrect code. This occurs only
when all of the following conditions are satisfied:

1. The -Ot or -Ox optimization level is chosen.

2. The dividend and the divisor are both variables.

3. The divisor is the product of a multiply (or shift) operation that
   immediately precedes the division.

To work around this problem, compile modules that meet all the above
criteria with either -Od or -Ol.

When this problem occurs, the divisor is being divided into itself.
Therefore, if the returned value for the division is not exclusively
1, you are not experiencing this problem.

The sample program below demonstrates the problem. Microsoft has
confirmed this to be a problem in QuickC Versions 2.00 and 2.01. We
are researching this problem and will post new information here as it
becomes available.

The following program demonstrates this problem when compiled as
specified (compile the module with either -Od or -Ol to generate code
that runs correctly):

/*
    DIVISION.C
    Compile: qcl -Ot (or -Ox) division.c
*/

#include <stdio.h>

void main ( void )

{
    int a, b, c, d ;

    a = 100 ;
    b = 5 ;
    c = b*5 ;
    d = a/c ;

/*
    When compiling with -Ot or -Ox, the failed error message will
    result AND the value of d will be set to one.
*/

  if (d == 4)
    printf("Success, d is %d, should be 4!\n", d) ;
  else
    printf("Failed, d is %d, should be 4!\n", d) ;

}

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