[M3devel] loophole/copysign

[Tony Hosking]

Surely we should instead give it the type conversion from what was stored to what is loaded.  Can you point me at the problem code in CastExpr?

On 5 Jul 2010, at 16:44, Jay K wrote:

> 
> I don't think a barrier worked.
> The thing is, I don't think a change in parse.c alone can work. It isn't being given enough information.
> Or, well, it does have enough information, but, like, it is information it never uses.
> It has some type information. It would have to notice that the most recent store to a variable was
> of a different type than a load.
> 
>  - Jay
> 
> 
> 
> ----------------------------------------
>> Subject: Re: [M3devel] loophole/copysign
>> From: hosking at cs.purdue.edu
>> Date: Mon, 5 Jul 2010 14:24:01 -0400
>> CC: m3devel at elegosoft.com
>> To: jay.krell at cornell.edu
>> 
>> 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  = (word_8) ((int_64)
>>> BIT_FIELD_REF  & -129 | (word_64) BIT_FIELD_REF <(int_64) BIT_FIELD_REF , 1, 7> << 7 & 255);
>>> = M3_CtKayy_res;
>>> return ;
>>> }
>>> 
>>> 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;
>>> D.1917;
>>> struct float_t * from.1;
>>> struct float_t * res.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
>>> 
>> 
>