[M3devel] loophole/copysign

[Tony Hosking]

We shouldn't need a barrier here. That is for memory operations, whereas these need not be.  I would hate to make this change.  Why can't we produce gcc trees that accomplish what we need?

On 5 Jul 2010, at 05:24, Jay K wrote:

> 
> Our codegen is remarkably low level. That is, lower level earlier than C.
> 
> 
> gcc/m3cg -ftree-dump-all
> 
> 
> As early as LongFloat.mc.003t.original, the first file dumped, we have:
> 
> LongFloat__CopySign (M3_CtKayy_x, M3_CtKayy_y)
> {
>   xreel M3_CtKayy__result;
>   xreel M3_CtKayy_res;
> 
>     xreel M3_CtKayy__result;
>     xreel M3_CtKayy_res;
>   M3_CtKayy_res = M3_CtKayy_x;
>   BIT_FIELD_REF <M3_CtKayy_res, 8, 56> = (word_8) ((int_64) 
>   BIT_FIELD_REF <M3_CtKayy_res, 8, 56> & -129 | (word_64) BIT_FIELD_REF <(int_64) BIT_FIELD_REF <M3_CtKayy_y, 8, 56>, 1, 7> << 7 & 255);
>   <retval> = M3_CtKayy_res;
>   return <retval>;
> }
> 
> compared to C where as test_copysign.c.t69.copyrename3, the last file dumped, we have:
> 
> copy_sign_f (from, to)
> {
>   float res;
>   float D.1918;
>   <unnamed type> D.1917;
>   struct float_t * from.1;
>   struct float_t * res.0;
> 
> <bb 0>:
>   res = to_1;
>   res.0_4 = (struct float_t *) &res;
>   from.1_5 = (struct float_t *) &from;
>   D.1917_6 = from.1_5->sign;
>   res.0_4->sign = D.1917_6;
>   D.1918_7 = res;
>   return D.1918_7;
> 
> }
> 
> 
> See, you know, from gcc's point of view, we don't have any records/structs/unions.
> Just integers and offsets from them mostly.
> 
> 
> The right fix is to build up types.
> That way also debugging with gdb will have a chance.
> Perhaps not a small amount of work. But maybe not too bad.
> 
> 
> For now my inclination is in m3front to insert a barrier between the store and the load associated with loopholes.
> At least if one type but not the other is floating point.
> I don't know if that will work, but maybe.
> 
> Or maybe have m3front actually call loophole for this case and again, either a barrier or make the load and/or
> store volatile.
> 
>  - Jay
>