Re: [Gm2] translating C bit fields

[Dragiša Durić]

Good thinking..

We need to have syntax for packing,
and also a way to control byte boundaries (MOD 0, MOD 2, MOD 4, MOD 8) of 
fields and whole record.
I don't see MOD even-number as needed, but before I met my Gtk2 problem, I 
didn't think MOD 8 can be an issue too.. So, systematize and let it in gm2.

dd 

On Nov 23, 2011, at 11:19 PM, SiTex Graphics wrote:

> 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
>> 
>> 
> 
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