[M3devel] better typing on SUBARRAY temporaries?

[Rodney M. Bates]

I'm not exactly sure what you are asking, but here is some light on what
you are seeing.  These temporaries are exactly the dope the compiler uses
to represent all open array values.  First a pointer to the zeroth
array element, then the "shape", as defined in M3 definition, 2.2.3, i.e.
an array of element counts for each open subscript.  For an open array
parameter, this would be the machine representation of the parameter
itself, authored in M3. (but passed by reference.)  For a heap object,
it is stored right before the elements themselves.  For a SUBARRAY
expression, it has to be a temporary.  It also has to be constructed
at the call site, as an anonymous temporary, when passing an entire fixed
array to an open array parameter

So, a good type for it might look like:


RECORD
   Elements : REF ARRAY [0..Max, ... ,0..Max] OF ElementType
; Shape : ARRAY [0..OpenDepth-1] of CARDINAL
END

Max will be like the notorious TextLiteral.MaxBytes, i.e., we don't want any
static limit here in the type of Elements, as it will be enforced dynamically,
using Shape.  But we don't want to just say REF ARRAY OF ElementType either,
as this would mean another open array inside the dope, resulting in infinite
recursion.

On 07/22/2015 12:42 AM, Jay K wrote:
> In the C backend I have a notion of "weak struct types" and "strong struct types".
>
>
> "Strong" types have fields with types and names corresponding to the original Modula-3. i.e. they debug well.
>
>
> "Weak" types have just arrays of characters (in a struct), sized/aligned to what the front end asked for. i.e. they debug poorly.
>
>
>
> Originally I had only weak types.
> Ideally I have no weak types.
> I'm down to very few weak types now.
> I'd like to finish eliminating weak types.
>
>
>
> A quick investigation shows weak types come from open arrays and jmpbufs.
> Open array temporaries from SUBARRAY specifically.
>
>
>
> Can we fix this?
>
>
>
> We have:
> m3front/src/types/OpenArrayType.m3:
>
> PROCEDURE DeclareTemp (t: Type.T): CG.Var =
>    VAR
>      p    := Reduce (t);
>      size := Target.Address.pack + OpenDepth (p) * Target.Integer.pack;
>    BEGIN
>      RETURN CG.Declare_temp (size, Target.Address.align,
>                              CG.Type.Struct, in_memory := TRUE);
>    END DeclareTemp;
>
>
> PROCEDURE Compiler (p: P) =
>    VAR size := Target.Address.pack + OpenDepth (p) * Target.Integer.pack;
>    BEGIN
>      Type.Compile (p.element);
>      CG.Declare_open_array (Type.GlobalUID(p), Type.GlobalUID(p.element), size);
>    END Compiler;
>
>
> DeclareTemp is used in SUBARRAY expressions -- truly temporaries,
> not variables authored by anyone in Modula-3.
>
>
> Can this be easily fixed?
>
>
> Thanks,
>   - Jay
>
>
>
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>

-- 
Rodney Bates
rodney.m.bates at acm.org