[M3devel] a trouble with passing records by value..

[Jay K]

t1 must still be passed in registers. The ABI can't be changed by that.

 - Jay

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> From: hosking at cs.purdue.edu
> Date: Tue, 31 Aug 2010 09:15:32 -0400
> To: jay.krell at cornell.edu
> CC: m3devel at elegosoft.com
> Subject: Re: [M3devel] a trouble with passing records by value..
>
> What happens if you take the address of t inside ActionLookup?
> What happens if you take the address of t1 inside main?
>
> On 31 Aug 2010, at 07:25, Jay K wrote:
>
> >
> > Given something like this:
> >
> > jbook2:p247 jay$ more 1.c
> > #include 
> >
> > typedef struct { long code; long value; } T1;
> >
> > void ActionLookup(T1 t, long code, long value);
> >
> > void ActionLookup(T1 t, long code, long value)
> > {
> > assert(t.code == code);
> > assert(t.value == value);
> > }
> >
> > int main()
> > {
> > T1 t1 = {2,2};
> > ActionLookup(t1, 2, 2);
> > return 0;
> > }
> > j
> >
> >
> > on some platforms, such as AMD64_DARWIN, T1 is passed in registers. Good.
> >
> >
> > However, one of the unfortunate aspects of our Modula-3 system is that when you reference e.g. t1.value,
> > the backend isn't told you are accessing the "value" "field" of "t1", and it figures out where that is,
> > but rather 64bits at offset 64bits into t1. Therefore t1 must have an address. Therefore it can't be in registers.
> > Darn.
> >
> >
> > If m3cg were higher level this could be better.
> >
> >
> > There should be a viable compromise where the parameter is passed in registers, and only "homed"
> > to some stack location if its address is used -- e.g. to pass unused parameters in registers.
> > But given the inefficiency of field accesses, I'm not sure it is worth trying?
> >
> >
> > Maybe we should have M3CG include field references?
> >
> >
> > There is this basic problem that the interface between m3front and m3cc isn't really at the
> > right level for m3cc. But it is probably for m3front. m3front wants a lower level code generator.
> >
> >
> > - Jay
> >
>