[OpenBIOS] [RFC 3/3] SPARC32/64: Mimic Sun's OBP behaviour if a divide by zero occurs.

[Mark Cave-Ayland]

On 11/01/13 18:22, Segher Boessenkool wrote:

> So, how to implement this. Firstly, we should have the same behaviour
> for *all* divide insns (/ u/ mod umod /mod u/mod um/mod mu/mod sm/rem
> fm/mod and whatever I forgot), not just um/mod and those words derived
> from it. Secondly, we get what we want if we just use the machine's
> (or C's) divide operator. So it seems the actual problem (if there is
> one at all) is that OpenBIOS' mu/mod does not behave as the hardware
> would. Related, / and the other single-precision division words do
> not need to call mu/mod (which is very expensive), they can much cheaper
> be implemented as C words (cheaper in execution time that is -- it costs
> more code :-) )

Actually this got me thinking, since you're right in that integer 
division should require a manual trap. This took me into OpenBIOS's 
bundled version of libgcc and __udivmoddi4.c:

__uint128_t __udivmodti4(__uint128_t num, __uint128_t den, __uint128_t 
*rem_p)
{
   __uint128_t quot = 0, qbit = 1;

   if ( den == 0 ) {
     __divide_error();
     return 0;                   /* If trap returns... */
   }

..
..
}

A quick grep of the source shows that __divide_error() is manually set 
to the unexpected exception trap handler in start.S:

_GLOBAL(__divide_error):
trap_error:
         mflr    r3
         LOAD_REG_FUNC(r4, unexpected_excep)
         mtctr r4
         bctr

Heh. So actually all we need to do is provide an empty function for 
__divide_error() and we magically get the behaviour we want. Revised 
patch to follow shortly.


ATB,

Mark.