Q40113

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Q40113: Incorrect Code Generation with /J

Article: Q40113
Version(s): 5.00 5.10 | 5.10
Operating System: MS-DOS    | OS/2
Flags: ENDUSER | buglist5.00 buglist5.10
Last Modified: 5-JAN-1989

If the following program is compiled with the /J compiler option
(which changes the default for char type from signed to unsigned) in
the Microsoft C Optimizing Compiler Versions 5.00 and 5.10, it will
not calculate the value of the long integer variable "result"
correctly:

#include <stdio.h>
void main(void)
{
unsigned long result = 'b';

result = result - 'a' + 1  ;
printf("result = %lx", result) ;
}

Compiled without /J, the program works correctly. Replacing "-a+1"
with "-96", or using a char type variable to store the value "a" also
prevents the problem.

Microsoft has confirmed this to be a problem in Versions 5.00 and
5.10. We are researching this problem and will post new information as
it becomes available.

The following is the assembly-code listing generated with /J :

;|*** unsigned long result = 'b';
        mov     WORD PTR [bp-4],98      ;result
        mov     WORD PTR [bp-2],0
;|***
;|*** result = result - 'a' + 1  ;
        add     WORD PTR [bp-4],-96     ;This sets the carry flag
                                        ;incorrectly.
        adc     WORD PTR [bp-2],0

The following is the code generated without /J :

;|*** unsigned long result = 'b';
        mov     WORD PTR [bp-4],98      ;result
        mov     WORD PTR [bp-2],0
;|***
;|*** result = result - 'a' + 1  ;
        sub     WORD PTR [bp-4],96      ;result
        sbb     WORD PTR [bp-2],0