Re: [Gm2] translating C bit fields

[SiTex Graphics]

Looks good to me.  Control over byte alignment would be a welcome
addition.  Could there be a shorthand to set the default alignment for
an entire record?  Maybe:

TYPE
   mytype = RECORD MOD 1
      foo : CHAR;
      bar: CARDINAL;
   END;

-Scott


On Wed, Nov 23, 2011 at 1:05 PM, Dragiša Durić <address@hidden> wrote:
> Hi Gaius,
>
> It looks good.I suppose we can also use this to put "ordinary" fields "out 
> of" standard alignment ?
>
> RECORD
>  a: BOOLEAN BY 8;
>  b: CARDINAL MOD 0 BY 32?
> END;
>
> Or..?
>
> dd
>
> On Nov 23, 2011, at 3:29 PM, Gaius Mulley wrote:
>
>> Dragiša Durić <address@hidden> writes:
>>
>>> Gaius,
>>>
>>> IMO - we also need a mechanism to fix a sequence of fields to word boundary.
>>> Obviousy, a field without "BY" will fit to its default alignment position, 
>>> but
>>> what if we have
>>>
>>> type = RECORD
>>>  foo: INTEGER BY 2;
>>>  bar: CARDINAL BY 4;
>>>  butalso: CARDINAL BY 2; (* and we need to have this aligned to some 
>>> boundary…
>>> *)
>>> END;
>>>
>>> Alignment can be pretty complicated these days when on same architecture 
>>> some
>>> people decide to align 64 bit values to  byte positions divisible by 8… And
>>> other do not. I met such a situation while wrapping Gtk2 to Modula-3… gcc
>>> aligns such values to divisible-by-4 byte positions, while cm3 aligns to
>>> divisible-by-8…
>>
>> Hi Dragiša,
>>
>> ok - how about:
>>
>> type = RECORD
>>   foo: INTEGER BY 2;
>>   bar: CARDINAL BY 4;
>>   butalso: CARDINAL MOD 4 BY 2; (* and we need to have this aligned to some 
>> boundary *)
>> END;
>>
>> so we extend the ebnf to include:
>>
>> FieldAlignment := [ ByteAlignment | BitAlignment ] =:
>>
>> BitAlignment := [ 'MOD' ConstExpression ] 'BY' ConstExpression =:
>>
>> ByteAlignment := "__ATTRIBUTE__" "(" "(" Ident "(" ConstExpression ")" ")" 
>> ")"
>>               =:
>>
>> where the MOD indicates the byte alignment and BY indicates the bit
>> length?  MOD 0 indicates packed, MOD 1 the next byte.
>>
>> regards,
>> Gaius
>
>