m68k problem and libstdc++

[Dale Johannesen]

On Thursday, January 3, 2002, at 04:55 PM, Craig Rodrigues wrote:
> An assembler error is resulting:
> Error: invalid instruction for this architecture; needs 68020 or 
> higher -- statement `cas.l %d0,%d1,(%a0)' ignored
>
> I suspect it is because of the code in
> libstdc++-v3/config/cpu/m68k/atomicity.h:
>
> static inline _Atomic_word
> __attribute__ ((__unused__))
> __exchange_and_add (volatile _Atomic_word *__mem, int __val)
> {
>   register _Atomic_word __result = *__mem;
>   register _Atomic_word __temp;
>   __asm__ __volatile__ ("1: move%.l %0,%1;"
>                         "   add%.l %2,%1;"
>                         "   cas%.l %0,%1,%3;"
>                         "   jbne 1b"
>                         : "=d" (__result), "=&d" (__temp)
>                         : "d" (__val), "m" (*__mem), "0" (__result)
>                         : "memory");
>   return __result;
> }
>
> I'm not a 68000 asm expert.  Does anyone have an idea as
> to what the fix could be?

CAS is an indivisible instruction intended to allow synchronization
of multiple processors (the sequence above is straight out of one of
Motorola's examples).  I don't think there's any way to get that
functionality on pre-68020 chips, so you'll have to conditionalize
it away somehow.  If supporting such chips is no longer required,
and I would guess it's not, the easiest fix will be to pass a flag
to the assembler telling it the target is 68020 or higher.
(I know the architecture fairly well, at least I used to, but I'm
not expert on multiprocessor synchronization.)