[M3devel] [modula3/cm3] Packed set literal generation issue (#21)

[Rodney M. Bates]

On 09/24/2017 07:04 AM, JC Chu wrote:
>> Are you aware that the BITS 32 here has no effect on the size of the global variable?
>
>
>
> Yes.  I was looking for a bit set type that’s guaranteed to have the same representation as some integer type, such as BITS BITSIZE(INTEGER) FOR [0..-1 + BITSIZE(INTEGER)] for INTEGER.  But that guarantee doesn’t seem to exist, according to the language definition.  Seems like the correct way is to use Word interface (but then Long (the corresponding interface for LONGINT) doesn’t seem to be usable).
>
>

Yes, the language considers scalar variable sizes (locals, globals, parameters) to be
the implementation's business.  However, for low-level coding, you can wrap what you
want inside a record, usually with only one field, and access the field:

  TYPE SetT = SET OF [0..31];
  TYPE IntSizedSetT = RECORD S : BITS BITSIZE(INTEGER) FOR SetT END;
  VAR Set := IntSizedSetT { SetT { 0 } };
  VAR Int : INTEGER;

  BEGIN
     Int := LOOPHOLE(Set.S, INTEGER);
   END

Any mistakes or failure of the size self-adaptation code will give a
compile-time error on the LOOPHOLE, due to unequal sizes.

I have used this technique a number of times in low-level code to make
it self-adapt to different word sizes, and more complicated variations
to adapt to different endianness.  The compiler and runtime do this in
some places. In some other places, they just make assumptions about
what the compiler will do that happen to be correct currently, but are
not guaranteed.  I think not all of them would give compile-time errors
if/when the assumptions changed.  I have fixed a few of them, but not
systematically.

I don't think Word and Long will help, since, as far as types are concerned,
Word.T = INTEGER and Long.T = LONGINT.  They mainly just provide operators
that apply unsigned interpretation to the bits and do other bit-twiddling.
I do use Word.T instead of INTEGER where appropriate in declarations just
as documentation of the way they will be used.  Otherwise (anyway?) it gets
tedious keeping track of when to use '+' and when to use Word.Plus.

Unfortunately, the compiler currently insists every set must be a full native
INTEGER size, whether you try to size it yourself with BITS or let the
compiler do it.  This looks easy to fix, but would require thorough vetting
to be sure all the various paths in the front and back ends can handle it.

BTW, the compiler looks to be padding every record out to a full byte
length, which won't affect this technique but limits some other things.

Also BTW, in working on the fix of the assertion failure you reported,
I find there is another assertion failure when trying to pass
BITS n FOR SetT as an actual parameter to a SetT formal.For VALUE and
READONLY, this should work, as the types are assignable.


>
> — JC
>
>
>
> *From:* Rodney M. Bates [mailto:notifications at github.com]
> *Sent:* Wednesday, September 20, 2017 5:35
> *To:* modula3/cm3 <cm3 at noreply.github.com>
> *Cc:* jcchu <jcchu at acm.org>; Author <author at noreply.github.com>
> *Subject:* Re: [modula3/cm3] Packed set literal generation issue (#21)
>
>
>
> Fixed on github, commit 12f50b4cde746056b943bd8aaa105c3fcb46b1a8
>
> Deleted the assertion, which is neither true nor needed.
>
> But see below:
>
> On 09/16/2017 11:54 PM, jcchu wrote:
>> See the fragment below.
>>
>> VAR r := SET OF [0..31] { 0 }; (* OK *)
>> s := BITS 32 FOR SET OF [0..31] { 0 }; (* -- assertion failure in “SetExpr.m3” *)
>
> Are you aware that the BITS 32 here has no effect on the size of the global variable?
>
> Quoting from 2.2.5, Packed types:
>
> TYPE T = BITS n FOR Base
>
> where Base is a type and n is an integer-valued constant expression.
> The values of type T are the same as the values of type Base, but variables of type T
> that occur in records, objects, or arrays
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> will occupy exactly n bits and be packed adjacent to the preceding field or element.
>
> s is not inside a record, object, or array, so the BITS specification has no effect on
> the memory allocated by the compiler.
>
> There are, however some more obscure differences:
>
> 1) Although T and BITS n FOR T are are assignable to each other,
> BITS n FOR T and BITS m FOR T, (n#m) have no subtype or assignability
> relationship, so to assign one to the other would require two assignment
> steps, with a T as the intermediate type.
>
> 2) Anywhere type equality, as opposed to assignability, is required, T and
> BITS n FOR T will not match. For example, you cannot pass one of these to
> the other as a VAR parameter.
>
>
>
>
>
>>
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-- 
Rodney Bates
rodney.m.bates at acm.org