Q43061: Optimization Problem with SHIFTing and ANDing

Article: Q43061
Product(s): See article
Version(s): 5.10   | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | buglist5.10 | mspl13_c
Last Modified: 17-MAY-1989

When compiled with CL, the default optimization setting (/Ot) will
cause the following source code to produce erroneous results. The
problem only appears if the value being shifted is a variable and the
masking value is a constant. The number of bits to shift can be a
constant or a variable. Other combinations of variables and constants
are evaluated correctly by the compiler.

Compiling with /Od solves the problem. Using temporary variables also
produces the correct results.

Microsoft has confirmed this to be a problem with Version 5.10. We are
researching this problem and will post new information as it becomes
available.

Both of the expressions should evaluate to 0x0000, but when optimized,
they evaluate to 0x0100.

/*  Example of optimizing problem with SHIFTing and ANDing.
 */
#include <stdio.h>

#define SHIFT 0x0002
#define MASK  0x00FF

void main( void )
{
    unsigned short usResult;
    unsigned short usStart;
    unsigned short usShift;

    /*  With a variable, a constant, and another constant.  */
    usStart = 0x0040;

    /***  PROBLEM WITH FOLLOWING EXPRESSION!  ***/
    usResult = (usStart << SHIFT) & MASK;
    printf( "Result #1: %4.4X\n", usResult );

    /*  With a variable, another variable, and a constant.  */
    usStart = 0x0040;
    usShift = SHIFT;

    /***  PROBLEM WITH FOLLOWING EXPRESSION!  ***/
    usResult = (usStart << usShift) & MASK;
    printf( "Result #2: %4.4X\n", usResult );
}

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