Q49631

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Q49631: /Zr Switch Causes Incorrect Code Generation

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Article: Q49631
Product(s): See article
Version(s): 2.00 2.01
Operating System(s): MS-DOS
Keyword(s): ENDUSER | S_C buglist2.00 buglist2.01 S_QuickASM | mspl13_c
Last Modified: 6-NOV-1989

When the /Zr switch is used to generate pointer checking instructions
for Quick C, the generated code can cause problems with long
arithmetic.

The following example shows that the code generated with the /Zr
switch can cause incorrect results:

#include <stdio.h>
#include <malloc.h>

void main (void)
{
  long *lp;
  lp = (long *)malloc(sizeof(long));
  *lp = 65530L;
  *lp += 65530L;
  if( *lp != (65530L + 65530L) )
  {
        printf("*lp not added correctly\n");
        exit(1);
  }
  else
  {
        printf("*lp added correctly\n");
        exit(0);
  }
}

When compiled without the /Zr switch, the code generated for the line
"*lp += 65530;" is as follows:

    MOV     BX, WORD PTR [lp]
    ADD     WORD PTR [BX], -06  ; 65530
    ADC     WORD PTR [BX+02],+00

When compiled with the /Zr switch, the code generated is as follows:

    MOV     BX, WORD PTR [lp]

    AND     BX, BX              ; Code inserted by /Zr
    JNZ     _main+3c (005C)     ;   "      "     "   "
    CALL    0010                ;   "      "     "   "

    ADD     WORD PTR [BX], -06  ; 65530

    AND     BX, BX              ; Code inserted by /Zr
    JNZ     _main+46 (0066)     ;   "      "     "   "
    CALL    0010                ;   "      "     "   "

    ADC     WORD PTR [BX+02],+00

The problem with the inserted code is that the AND instruction sets
the carry flag to 0 (zero). Therefore, prior to the ADC instruction,
which adds in the initial state of the carry flag, the carry flag is
set back to zero producing the incorrect results.

Microsoft has confirmed this to be a problem with with QuickC Version
2.00 and 2.10. We are researching this problem and will post new
information as it becomes available.

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