[M3devel] m3front scanner div wierdness?

[Rodney M. Bates]

On 06/28/2016 10:54 PM, Jay K wrote:
> Does anyone understand this stuff in m3front/Scanner.m3:
>
>   Here vs. LocalHere?
>   SameFile?
>
>
>   I understand only this nuance:
>    offset MOD MaxLines
>
>    MaxLines  = 100000;
>
>
>   is to crudely handle that when asserts fail,
>   they pack the line number in with the assertion failure code,
>   potentially loosing bits.
>
>
>   I don't think this is a good design, they should just be separate INTEGERS,
>   but this is besides the point.
>

This is just pure speculation, but I am very confident of it.  These
offsets have a very high occurrence count.  There is code all over m3front
that copies Scanner.offset into various data structures.  So the small space
saving of one INTEGER instead of two would be multiplied by a big number.
I remember working in Modula-3 on a company-paid computer with 16 Meg of ram.
Today, I have 8 Gig in the one I bought, and could easily afford more, if I
thought I needed it.

Two integers would be cleaner, but this design is not too bad *if* you know
the MOD/DIV invariant.  It is commented at Scanner.m3:54, but only for one
field.  As pure documentation, there really should be a distinct type name
(say FileNoAndLineNo?) for all values that use this representation, even
though it just equates to INTEGER.  There are a lot of variables lying around
all over the front end  that use this invariant, but are just declared as
INTEGER.  That's maintainer-hostile.


>
>   What doesn't makes sense to me is the machinations around file name.
>
>
> Here:
>      file := files [offset DIV MaxLines];
>
> vs. LocalHere:
>      file := local_files [fnum];
>
>
> LocalHere makes sense. Here does not.
>
>
> PROCEDURE SameFile (a, b: INTEGER): BOOLEAN =
>    BEGIN
>      RETURN (a DIV MaxLines) = (b DIV MaxLines);
>    END SameFile;
>
>
>
> Shouldn't this just be a = b?
>

As coded, this will return TRUE if a and b are different line numbers within
the same file.  The name "SameFile" at least suggests that is what is intended.
A good example of a place where it would have been clearer if a & b were
declared as the type name I proposed above.

>
> Well, anyway, this SameFile function isn't called.
>
> Here and LocalHere are used.
>
>
> I'm looking here because I want to add a temporary measure
> such that the file names are leaf-only.
>
>
> In particular, because generic modules have target names in their paths
> and I want to temporarly remove target names from output, so I can prove
> that a few targets are identical.
>
>
> I guess, really, I propose the interface to assertion failures be expanded to take the line number separate from the failure code.
> This has to percolate quite a bit through the system -- the backends and the runtime.
>
>
> And then this Here vs. LocalHere difference should fall away.
> But still, what is it trying to do?
>
>
> Thank you,
>   - Jay
>
>
>   		 	   		
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-- 
Rodney Bates
rodney.m.bates at acm.org