Q65743: Invalid Offset Generated for jmp After In-Line Assembly

Article: Q65743
Product(s): See article
Version(s): 6.00    | 6.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | buglist6.00 fixlist6.00a | mspl13_c
Last Modified: 24-SEP-1990

The Microsoft C version 6.00 compiler can generate incorrect offsets
for jmp instructions following the termination of in-line assembly
blocks in certain situations.

The sample code below will generate the incorrect offset for the
return statement following the termination of its in-line assembly
block. The only known workaround is to compile with the /qc option,
thus invoking the quick compiler.

If the optimize pragma is removed from this code, it will generate the
following error with any command-line optimization switch.

   fatal error C1001: Internal Compiler Error
   (compiler file '@(#)main.c:1.176', line 807)
   Contact Microsoft Product Support Services

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

#pragma optimize("",off)

#include<stdio.h>

long double asin(long double f_in)
{
    _asm
    {
        fld      f_in
        fld      st
        fmul     st,st
        fld1
        fcom     st(1)
        fstsw    ax
        sahf
        jc       toobig
        fsubr
        fsqrt
/*      fxch     st(1)  uncomment for acos */
        fpatan
        jmp      done
toobig:
        fstp     st
        fstp     st
        fabs
        fchs
        fsqrt
done:
        fstp     f_in
     }
     return f_in;
}

main()
{
    char s[200];
    long double f_in,f_out;
    f_in=0.0;
    while(gets(s)!=NULL)
    {
        if (sscanf(s,"%Lf",&f_in)==1)
        {
           f_out=asin(f_in);
           printf("in %Lf asin %Lf\n",f_in,f_out);
        }
    }
    exit(0);
}

The following is incorrect code generated as viewed from within
CodeView's mixed source/assembly mode (compiled with cl /Od /Zi
file.c):

29:         fstp     f_in
0047:004C 9B             WAIT
0047:004D DB7E04         FSTP      TByte Ptr [BP+04]
30:          }
31:          return f_in;
0047:0050 9B             WAIT
0047:0051 DB6E04         FLD       TByte Ptr [BP+04]
0047:0054 9B             WAIT
0047:0055 D9C0           FLD       ST(0)    ; different from below
0047:0057 9B             WAIT
0047:0058 DDD9           FSTP      ST(1)    ; perhaps repetitive
0047:005A 90             NOP
0047:005B 9B             WAIT
0047:005C E9FAFF         JMP       0059 ;notice incorrect offset here
32:     }
0047:005F 5E             POP       SI
0047:0060 5F             POP       DI
0047:0061 8BE5           MOV       SP,BP
0047:0063 5D             POP       BP
0047:0064 C3             RET
33:

The following is correct code (compiled with cl /qc /Od /Zi file.c):

0047:004C 9B             WAIT
0047:004D DB7E04         FSTP      TByte Ptr [BP+04]
30:          }
31:          return f_in;
0047:0050 9B             WAIT
0047:0051 DB6E04         FLD       TByte Ptr [BP+04]
0047:0054 90             NOP
0047:0055 9B             WAIT
0047:0056 E90000         JMP       0059              ; offset correct here
32:     }
0047:0059 5F             POP       DI
0047:005A 5E             POP       SI
0047:005B 8BE5           MOV       SP,BP
0047:005D 5D             POP       BP
0047:005E C3             RET
33:

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

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