[M3devel] loophole/copysign

[Tony Hosking]

Yes, maybe that is the best way forward.

On 5 Jul 2010, at 07:36, Jay K wrote:

> 
> another idea: let's not use bitfield ref for float/double
> 
>  - Jay
> 
> 
> 
> ----------------------------------------
>> From: jay.krell at cornell.edu
>> To: m3devel at elegosoft.com
>> Date: Mon, 5 Jul 2010 11:25:19 +0000
>> Subject: Re: [M3devel] loophole/copysign
>> 
>> 
>> Hm. it seems that it might be important to preserve the "designatorness", like in:
>> 
>> libm3/...RandomReal.m3:
>> 
>>  VAR frac, exp: INTEGER; result: LONGREAL;
>> 
>>    (* Repack as LONGREAL: *)
>>    WITH lr = LOOPHOLE (result, LongRealRep.T) DO
>>      lr.sign := 0;
>>      lr.exponent := exp;
>>      lr.significand0 := Word.Shift (Word.And (frac, 16_7fffffff),
>>                                      -(WordSize - 1 - FractionBits));
>>      lr.significand1 := r.integer (min := -16_7fffffff-1, max :=16_7fffffff);
>>    END;
>> 
>> 
>> - Jay
>> 
>> ----------------------------------------
>>> From: jay.krell at cornell.edu
>>> To: m3devel at elegosoft.com
>>> Date: Mon, 5 Jul 2010 10:42:57 +0000
>>> Subject: Re: [M3devel] loophole/copysign
>>> 
>>> 
>>> Tony, et. al.. in m3front/src/exprs/CastExpr.m3..what's the difference between a "designator" and a "value"?
>>> 
>>> 
>>> http://www-plan.cs.colorado.edu/diwan/modula3/designators.html
>>> 
>>> An identifier is a writable designator
>>> if it is declared as a variable,
>>> is a VAR or VALUE parameter,
>>> is a local of a TYPECASE
>>> or TRY EXCEPT statement,
>>> or is a WITH local that is bound to a writable designator.
>>> An identifier is a readonly designator if it is
>>> a READONLY parameter,
>>> a local of a FOR statement,
>>> or a WITH local bound to a non-designator or
>>> readonly designator.
>>> 
>>> 
>>> 
>>> I guess a designator is what I would think of a "variable" or "read only variable"?
>>> Something that either is "in memory" or can "reasonably" be put there?
>>> 
>>> 
>>> 1 + 2 is not a designator.
>>> 
>>> 
>>> Or, generally, a "variable", but that includes such similar things as parameters, "with variables", "for variables", "TYPECASE vairables", "TRY variables"
>>> 
>>> 
>>> Anything with a name??? (not functions/modules/generics -- "named data")
>>> 
>>> 
>>> Anyway, the next questions include:
>>> 
>>> 
>>> In CastExpr.m3 would it be terrible and/or wrong to treat "designators" the same as "values"?
>>> I realize, probably a deoptimization.
>>> I think this lets the backend work.
>>> 
>>> 
>>> And really, more to the point...shouldn't CastExpr.m3 use cg.loophole far more?
>>> I haven't had much luck with that. I always get the cg stack out of balance or with the wrong types, even though it seems like it should be easy.
>>> 
>>> 
>>> I have more testing to do, but classifying the loophole as V_to_S (value to structure) in place of D_to_S (designator to structure), at least if either side is one of the three float types, seems reasonable and correct, albeit slight deoptimization -- in unsafe code dealing with floating point..should be rare..
>>> 
>>> 
>>> - Jay
>>> 
>>> 
>>> 
>>> ----------------------------------------
>>>> From: jay.krell at cornell.edu
>>>> To: m3devel at elegosoft.com
>>>> Date: Mon, 5 Jul 2010 09:24:20 +0000
>>>> Subject: [M3devel] loophole/copysign
>>>> 
>>>> 
>>>> 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
>>>> 
>>> 
>> 
>