ID Number: Q62305
6.00 | 6.00
MS-DOS | OS/2
buglist6.00 fixlist6.00a
Summary:
In the case below, the Microsoft C 6.0 compiler will generate
incorrect code for enregistering the arguments of a function declared
with the _fastcall attribute.
Microsoft has confirmed this to be a problem in C version 6.0. This
problem was corrected in C version 6.0a.
The following code provides an example of this behavior:
Sample Code
-----------
/* Compile options needed: none
*/
#include <stddef.h>
void init_windows( void );
void _fastcall wxxopen( short up_r,
short up_c,
short lo_r,
short lo_c,
char *title,
short ctrl,
short dth,
short wth,
short wc,
short fch );
void init_windows()
{
wxxopen( 00, 00, 02, 79, NULL, 10, 0, 0, 0, 32 );
wxxopen( 04, 01, 11, 78, NULL, 10, 0, 0, 0, 32 );
wxxopen( 22, 00, 24, 79, NULL, 10, 0, 0, 0, 32 );
}
More Information:
The incorrectly generated code is for the third call to wxxopen().
Rather than 0 (zero) being placed into the DX register for the second
argument, a copy of the AX register, which contains 22, is moved into
DX. This can be clearly seen in the following .COD extract from the
above source:
;|*** wxxopen( 22, 00, 24, 79, NULL, 10, 0, 0, 0, 32 );
; Line 22
*** 000041 b8 4f 00 mov ax,79
*** 000044 50 push ax
*** 000045 2b c0 sub ax,ax
*** 000047 50 push ax
*** 000048 b8 0a 00 mov ax,10
*** 00004b 50 push ax
*** 00004c 2b c0 sub ax,ax
*** 00004e 50 push ax
*** 00004f 50 push ax
*** 000050 50 push ax
*** 000051 56 push si
*** 000052 b8 16 00 mov ax,22
*** 000055 8b d0 mov dx,ax
*** 000057 bb 18 00 mov bx,24
*** 00005a e8 00 00 call @wxxopen
The conditions for the error to occur appear to be very narrowly
defined. The argument being placed into DX must be a constant 0
(zero). The compiler must be in such a state that it considers the AX
register to contain 0 (zero) from a previous operation. This is
apparently the state after the SUB AX, AX instruction above.
Regrettably, the AX register has since been used to hold the first
enregistered argument.
It is difficult to convince the compiler to reach this state. The
sequence of three calls to wxxopen() with the specified arguments has
been found to reliably produce the error on the third call.
Additional reference words: fastcall 6.00 6.00a