[M3devel] loophole/copysign

[Tony Hosking]

Indeed.

On 5 Jul 2010, at 07:25, Jay K wrote:

> 
> 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
>>> 
>> 
>