Q66105: Code Generation Error with /Gs

Article: Q66105
Product(s): See article
Version(s): 6.00  6.00a | 6.00 6.00a
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | buglist6.00 buglist6.00a | mspl13_c
Last Modified: 22-OCT-1990

When the sample code shown below is compiled with stack checking
disabled (/Gs) and default optimizations, the compiler generates
incorrect code.

The following code/assembly listing shows that the stack frame is
never initialized upon entry to the function. In other words, the base
pointer, BP, is not pushed onto the stack and the stack pointer, SP,
is not moved into BP.

However, the procedure continues to reference BP to access the
parameters passed to the function. Furthermore, at the end of the
assembly procedure, the stack frame is restored; the value of BP is
moved into SP and BP is popped off the stack.

Turning off default optimizations (/Od) or rearranging portions of the
code causes the stack frame to be initialized upon entry to the
function.

Sample Code
-----------

struct foo {
     int i;
};

unsigned u;
char ch;
void func(char);

unsigned test(struct foo *a[], int *x, int y)
{
     struct foo *tmp;

     u=(ch << 8);
     func(ch);

     *x=y;
     tmp=a[y];
}

Partial .COD listing
--------------------

Note: The program is compiled with the following: cl /c /Gs /Fc program.c

_test     PROC NEAR
                                     ; no stack frame set up

     *** 000000     56        push si
;    tmp = -2
;    y = 8
;    x = 6
;    a = 4

     .
     .
     .

;|***      *x=y;
;|***      tmp=a[y];
; Line 17
     *** 000013     8b 5e 08  mov  bx,WORD PTR [bp+8] ;y  *uses bp*
     *** 000016     8b 76 06  mov  si,WORD PTR [bp+6] ;x
     *** 000019     89 1c          mov  WORD PTR [si],bx
;|*** }
; Line 18
     *** 00001b     5e        pop  si
     *** 00001c     8b e5     mov  sp,bp    ; un-does the stack
     *** 00001e     5d        pop  bp       ; frame it never set up
     *** 00001f     c3        ret

_test     ENDP

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

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