[M3devel] loophole/copysign

[Jay K]

I think this is still a good idea but I don't think relevant here.
There's no offset or type mismatch to trigger that path, in this code, I'm pretty sure.

I think a temporary would still work, in the "LV" D_to_S variant.
But a temporary is wrong for the non-LV case, due the need to preserve writes to the original.
But I didn't read through how the compiler choses LV or not-LV so not yet keen on making that sort of change.

 - Jay


----------------------------------------
> From: hosking at cs.purdue.edu
> Date: Mon, 5 Jul 2010 14:25:17 -0400
> To: jay.krell at cornell.edu
> CC: m3devel at elegosoft.com
> Subject: Re: [M3devel] loophole/copysign
>
> 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
> >>>>
> >>>
> >>
> >
>