[M3devel] loophole/copysign

[Jay K]

CastExpr.m3 has precious few calls to CG.Loophole, including none for this case.
cm3cg -y output for m3core/LongReal.mc CopySign has no calls to loophole.
We store into a local as one type and read it back as another type.

 - Jay

----------------------------------------
> From: hosking at cs.purdue.edu
> Date: Mon, 5 Jul 2010 17:33:43 -0400
> To: jay.krell at cornell.edu
> CC: m3devel at elegosoft.com
> Subject: Re: [M3devel] loophole/copysign
>
> 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
> >>>
> >>
> >
>