[m-dev.] question about data representation decisions

[Peter Wang]

On Fri, 30 Mar 2018 17:31:02 +1100 (AEDT), "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
> 
> I propose that we switch to a different approach. Instead of
> recording in interface files the *information needed to reach
> the right decisions* about data type representations, we should
> record *the decisions themselves*. This would require adding
> a new kind of item that can occur only in compiler generated
> interface files, but we agreed several months ago that that is ok.
> 
> I see two advantages of the approach I am proposing.
> 
> First, it has a much simpler correctness argument about
> ensuring consistency: the compiler decides the representation
> of a du type when it compiles the module that defines that type,
> and every compiler invocation on any other module uses the
> record of that decision. If the defining module changes,
> the decision may change, but then so will the interface file
> containing that decision, which will require a recompile
> of all other modules dependent on it.

Agreed.

> Second, and more subtly, in the scenario above, we *won't*
> have to export the bar == float equivalence as a global equivalence;
> we would export it only as something that applies to the fields of foo
> that are bars. Occurrences of bars in other contexts would not be
> affected.

It also affects whether arguments of type bar are passed via float
registers, if used.

Peter