gcc volatile behaviour

["Robert J. Brown" <rj at eli dot elilabs dot com>]

>>>>> "Zoltan" == Zoltan Kocsi <zoltan@bendor.com.au> writes:

    Zoltan> I have some problem with gcc/egcs crosscompiling to m68k.
    Zoltan> The problem probably would be present on any target,
    Zoltan> actually.

    Zoltan> When the value of an assignment expression where the
    Zoltan> left-hand side is volatile is used, gcc re-fetches the
    Zoltan> left-hand object. That is:

    Zoltan>   volatile int a, b; a = b = 0;

    Zoltan> compiles to

    Zoltan>         clr.l b move.l b,%d0 move.l %d0,a
 
    Zoltan> Apart from the fact that the involvement of d0 is not
    Zoltan> really a necessity (with -O3), I do not understand why
    Zoltan> does it fetch 'b' ?

Actually, when a and b are volatile, a = b = 0; is not exactly the
same thing as a = 0; b = 0; because assignment is left-associative,
and the value of the expression assigned to a in a=b=0; is the value
of the expression b=0; which  is exactly 0 only when b is
non-volatile.  When b is volatile, the value of b can only be
determined by fetching b.  So the compiler did the right thing!

    Zoltan> Similarly, when one does stringcopy:

    Zoltan>    strcpy( char *a char *b ) { while( *a++ = *b++ ); }
   
    Zoltan> compiles the loop to the inefficient but correct

    Zoltan> .L7: move.b (%a0)+,%d0 
                 move.b %d0,(%a1)+ 
                 jbne .L7

    Zoltan> Changing the declaration of 'a' to volatile char *a, the
    Zoltan> result is this:

    Zoltan> .L7: move.b (%a1)+,(%a0) 
                 move.b (%a0)+,%d0 
                 jbne .L7
 
    Zoltan> This means that the copy is not terminated when the '\0'
    Zoltan> is *written* to the area pointed by 'a' but when a zero is
    Zoltan> *read back* from the given area. In case of HW buffers
    Zoltan> this may not be what the programmer had in mind, for
    Zoltan> buffers sometimes are write only, for example.

From the man page:

DESCRIPTION
       The  strcpy() function copies the string pointed to be src
       (including the terminating `\0' character)  to  the  array
       pointed  to by dest.  The strings may not overlap, and the
       destination string dest must be large  enough  to  receive
       the copy.

IMO since the man page defines the terminator wrt the source string,
then the above implementation of strcpy is flawed!  Volatile should
have nothing to do with it this time.

    Zoltan> In every case, when the left hand operand of an assignment
    Zoltan> expression is volatile and the value of the expression is
    Zoltan> ised, gcc and egcs read back the value from the volatile
    Zoltan> object instead of using the value written to the object.

    Zoltan> Is there any particular reason why gcc and egcs do this ?

    Zoltan> Thanks,

    Zoltan> Zoltan

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