Re: [Gm2] translating C bit fields

[Dragiša Durić]

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