[M3devel] loophole/copysign

[Tony Hosking]

We really don't want to do this.  We need to figure out what needs to be generated as gcc trees.  Perhaps an anonymous struct will suffice.


On 5 Jul 2010, at 08:56, Jay K wrote:

> 
> nope.
> I know this is gross, but how about we addd CG.make_volatile that front end can call very sparingly,
> that acts just like generating a call to setjmp or vfork -- in that it makes locals, parameters, temporaries
> in current function as volatile?
> 
> CastExpr.m3 would call it for D_to_S and FloatType[src] # FloatType[dest].
> 
> It would be available as a desparate measure if we find other problems.
> To selectively inhibit optimization a function at a time.
>   Which, granted, is generally overkill.
> 
> I'm trying this now..
>  - Jay
> 
> ----------------------------------------
>> From: jay.krell at cornell.edu
>> To: m3devel at elegosoft.com
>> Subject: RE: [M3devel] loophole/copysign
>> Date: Mon, 5 Jul 2010 11:36:11 +0000
>> 
>> 
>> 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
>>>>> 
>>>> 
>>> 
>> 
>