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Re: bit-fields of type unsigned long and unsigned long long

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David Brown <david.brown@hesbynett.no> writes:
> On 23/06/2021 19:53, Tim Rentsch wrote:
>> Philipp Klaus Krause <pkk@spth.de> writes:
>>> Since bit-fields of type unsigned long and unsigned long long are
>>> useful, commonly suppoted by implementations, and commonly used, I'd
>>> like to see the standard support them.
>>>
>>> Here's  a draft of a proposal for that:
>>>
>>> http://www.colecovision.eu/stuff/proposal-unsigned-long-bit-field.html
>> 
>> Here is a possible (and simpler) alternative.
>> 
>> Type specifier for bit-fields is _Bool, int, signed, or unsigned
>> (with signed int and unsigned int being synonymous with signed
>> and unsigned, respectively).
>> 
>> Bit-fields of type _Bool act just like they do now.
>> 
>> Bit-fields of type int are either the same as an unsigned
>> bit-field or the same as a signed bit-field, with the choice
>> being implementation-defined, just like they are now.
>> 
>> Bit-fields of types signed and unsigned may use widths up to the
>> widths of intmax_t and uintmax_t, respectively.
>> 
>> The type of a signed bit-field is the first of signed, long,
>> long long, and intmax_t, that is wide enough to hold all the
>> possible values of the bit-field.
>> 
>> The type of an unsigned bit-field is the first of unsigned,
>> unsigned long, unsigned long long, and uintmax_t, that is wide
>> enough to hold all the possible values of the bit-field.
>> 
>> For compatibility with current integer promotion rules, an access
>> for an unsigned bit-field whose range of possible values are all
>> representable within the range of signed int yields a value that
>> has been promoted to type signed int, just like such things are
>> done now.  In all other cases the type of a bit-field access
>> is the type of the bit-field, as stated above.
>> 
>> (Personally I like this approach better than allowing different
>> integer types as bit-field specifiers.)
>
> This would give a very different result from the way compilers implement
> bitfields today.  In particular, consider :
>
> 	sizeof(struct { int8_t a : 1; });
>
> today, that is 1 on compilers that accept any integer type in bit
> fields.  With your proposal, it would be sizeof(int).

How does that follow?  The bit field occupies only 1 bit.  Why should
its underlying type affect the size of the structure?

I know that with gcc and clang, these structures have different sizes:
    struct { _Bool    bitfield:1; }
    struct { unsigned bitfield:1; }
but I don't see anything in the standard that suggests they should.
(I vaguely recall that it's an ABI requirement.)

> 	sizeof(struct { int64_t a : 1; });
>
> today, that is 8 (assuming 8-bit char), while with your proposal it
> would be sizeof(int).
>
> It would be conceivable that an implementation could make the types of
> the bitfields act the way you describe, while keeping the sizes (and
> alignments, padding, splitting, etc.) of the bitfields the same as
> today.  But it would be needlessly confusing, and lead to other
> differences (such as when the bitfields are used in other expressions).
>  The only sane approach to bitfield types is that the type of the
> bitfield is the type given by the programmer when they declared the
> bitfield.  (And here I agree with Keith that it is unfortunate that a
> bitfield declared as type "int" might be signed or unsigned.)

The "type of a bit-field" seems like a tricky concept.  Consider:

    struct { unsigned int bf: 1; };

N1570 6.7.2.1p5 says:
    A bit-field shall have a type that is a qualified or unqualified
    version of _Bool, signed int, unsigned int, or some other
    implementation-defined type.
which implies that bf is of type unsigned int.  But p10 says:
    A bit-field is interpreted as having a signed or unsigned integer
    type consisting of the specified number of bits.
So the type of bf is an unsigned type consisting of 1 bit, and clearly
that's not unsigned int.

I think p5 is referring to the type used to define the bit-field, and
p10 is referring to the (anonymous) type that the bit-field actually
ends up having.  IMHO it could be worded more clearly.

> I also note that your "simpler" alternative is significantly longer than
> Philipp's original proposal or the even simpler, obvious, and currently
> implemented change:
>
> "A bit-field shall have a type that is a qualified or unqualified
> version of an integer type".
>
> That's what compilers do today, that's what programmers use today, and
> it is shorter, simpler and clearer than any other version.  And it is
> consistent with C++ (which also allows enumeration types - which would
> be another nice addition to the C standard).

Any proposal should take C++ into account, both to steal ideas and to
avoid inconsistencies (like C++'s statement that if the specified width
is more than the width of the specified type, the extra bits are padding
bits).

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips Healthcare
void Void(void) { Void(); } /* The recursive call of the void */

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Thread

bit-fields of type unsigned long and unsigned long long Philipp Klaus Krause <pkk@spth.de> - 2021-06-22 16:49 +0200
  Re: bit-fields of type unsigned long and unsigned long long David Brown <david.brown@hesbynett.no> - 2021-06-22 17:18 +0200
    Re: bit-fields of type unsigned long and unsigned long long Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-22 11:17 -0700
      Re: bit-fields of type unsigned long and unsigned long long Philipp Klaus Krause <pkk@spth.de> - 2021-06-23 08:25 +0200
        Re: bit-fields of type unsigned long and unsigned long long David Brown <david.brown@hesbynett.no> - 2021-06-23 08:59 +0200
          Re: bit-fields of type unsigned long and unsigned long long Philipp Klaus Krause <pkk@spth.de> - 2021-06-23 13:12 +0200
            Re: bit-fields of type unsigned long and unsigned long long David Brown <david.brown@hesbynett.no> - 2021-06-23 14:14 +0200
              Re: bit-fields of type unsigned long and unsigned long long Philipp Klaus Krause <pkk@spth.de> - 2021-06-23 17:45 +0200
                Re: bit-fields of type unsigned long and unsigned long long Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-23 10:22 -0700
                Re: bit-fields of type unsigned long and unsigned long long Philipp Klaus Krause <pkk@spth.de> - 2021-06-23 19:52 +0200
                Re: bit-fields of type unsigned long and unsigned long long Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-23 16:12 -0700
                Re: bit-fields of type unsigned long and unsigned long long David Brown <david.brown@hesbynett.no> - 2021-06-24 10:41 +0200
                Re: bit-fields of type unsigned long and unsigned long long David Brown <david.brown@hesbynett.no> - 2021-06-24 10:32 +0200
        Re: bit-fields of type unsigned long and unsigned long long Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-23 00:39 -0700
  Re: bit-fields of type unsigned long and unsigned long long Philipp Klaus Krause <pkk@spth.de> - 2021-06-23 13:22 +0200
    Re: bit-fields of type unsigned long and unsigned long long Philipp Klaus Krause <pkk@spth.de> - 2021-06-23 18:27 +0200
  Re: bit-fields of type unsigned long and unsigned long long Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-06-23 10:53 -0700
    Re: bit-fields of type unsigned long and unsigned long long Philipp Klaus Krause <pkk@spth.de> - 2021-06-23 20:13 +0200
      Re: bit-fields of type unsigned long and unsigned long long Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-06-24 11:48 -0700
        Re: bit-fields of type unsigned long and unsigned long long Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-24 12:23 -0700
          Re: bit-fields of type unsigned long and unsigned long long David Brown <david.brown@hesbynett.no> - 2021-06-24 22:40 +0200
          Re: bit-fields of type unsigned long and unsigned long long Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-07-10 09:13 -0700
        Re: bit-fields of type unsigned long and unsigned long long Philipp Klaus Krause <pkk@spth.de> - 2021-06-24 22:14 +0200
          Re: bit-fields of type unsigned long and unsigned long long Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-07-10 09:23 -0700
    Re: bit-fields of type unsigned long and unsigned long long David Brown <david.brown@hesbynett.no> - 2021-06-24 11:02 +0200
      Re: bit-fields of type unsigned long and unsigned long long Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-24 11:06 -0700
        Re: bit-fields of type unsigned long and unsigned long long David Brown <david.brown@hesbynett.no> - 2021-06-24 23:11 +0200
          Re: bit-fields of type unsigned long and unsigned long long Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-24 15:16 -0700
            Re: bit-fields of type unsigned long and unsigned long long David Brown <david.brown@hesbynett.no> - 2021-06-25 10:43 +0200
              Re: bit-fields of type unsigned long and unsigned long long Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-25 02:02 -0700
                Re: bit-fields of type unsigned long and unsigned long long David Brown <david.brown@hesbynett.no> - 2021-06-25 13:54 +0200
                Re: bit-fields of type unsigned long and unsigned long long Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-06-25 13:07 +0100
                Re: bit-fields of type unsigned long and unsigned long long David Brown <david.brown@hesbynett.no> - 2021-06-25 15:17 +0200
                Re: bit-fields of type unsigned long and unsigned long long Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-25 08:22 -0700
                Re: bit-fields of type unsigned long and unsigned long long David Brown <david.brown@hesbynett.no> - 2021-06-25 18:22 +0200
                Re: bit-fields of type unsigned long and unsigned long long Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-25 13:28 -0700
                Re: bit-fields of type unsigned long and unsigned long long Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-06-25 19:12 +0100
                Re: bit-fields of type unsigned long and unsigned long long Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-07-10 08:58 -0700
                Re: bit-fields of type unsigned long and unsigned long long Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-06-25 08:09 -0700
              Re: bit-fields of type unsigned long and unsigned long long Philipp Klaus Krause <pkk@spth.de> - 2021-06-25 14:43 +0200
                Re: bit-fields of type unsigned long and unsigned long long David Brown <david.brown@hesbynett.no> - 2021-06-25 15:33 +0200
                Re: bit-fields of type unsigned long and unsigned long long Jakob Bohm <jb-usenet@wisemo.com.invalid> - 2021-07-06 23:18 +0200

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