[cfe-dev] different signedness for bitfield 'char' and plain 'char'

[Richard Smith]

On Tue, Jul 30, 2013 at 12:27 PM, Robert Lytton <robert at xmos.com> wrote:

>  Hi Richard,
>
> I only have access to a working draft of the C++11 standard
> (N3242=11-0012, date 2011-02-28)
> All I can find is as quoted before:
>
>     9.6:p3
>      It is implementation-defined whether a plain (neither explicitly
> signed nor unsigned) char, short, int or long bit-field is signed or
> unsigned.
>

This text was removed by DR739:

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#739


> I've tried finding any further statement but have not been successful.
> Please could you direct me to the relevant clause.
>
> robert
>
>  ------------------------------
> *From:* metafoo at gmail.com [metafoo at gmail.com] on behalf of Richard Smith [
> richard at metafoo.co.uk]
> *Sent:* 30 July 2013 20:21
>
> *To:* Robert Lytton
> *Cc:* cfe-dev at cs.uiuc.edu; benny.kra at gmail.com
> *Subject:* Re: [cfe-dev] different signedness for bitfield 'char' and
> plain 'char'
>
>  On Tue, Jul 30, 2013 at 12:12 PM, Robert Lytton <robert at xmos.com> wrote:
>
>>  Hi Richard,
>>
>>
>> > What standard? C++ requires that bit-fields have the same signedness as
>> their underlying type. If 'char' is unsigned, then bit-fields of type
>> 'char' are also unsigned.
>>  Sorry, I was thinking of the C99 standard.
>> But I actually was half quoting c++ 2011 standard.
>> Either way, I was wrong - but still not clear.
>>
>> The XCore ABI (currently implemented by the XCore llvm-gcc frontend)
>> expects:
>>
>>     'char' to be unsigned.
>>     but the bit field 'char' to be signed.
>>
>>  Does this infringe the C99 & C++11 standards?
>>
>
>  This is fine in C99, but not permitted by C++11 (and arguably, because
> the relevant DR applied to all versions of C++, is also not permitted in
> any earlier version of C++ either).
>
>  Note that the x86_64 ABI also claims that plain (neither signed nor
> unsigned) bit-fields are unsigned, but it is simply wrong. Maybe the XCore
> ABI document is wrong too?
>
>   I'll keep reading but I'm no lawyer.
>>
>> robert
>>
>> C++
>> 9.6:p3
>> It is implementation-defined whether a plain (neither explicitly signed
>> nor unsigned) char, short, int or long bit-field is signed or unsigned.
>>
>> C99
>> 6.2.5:p15
>> The three types char, signed char, and unsigned char are collectively
>> called the character types. The implementation shall define char to have
>> the same range, representation, and behavior as either signed char or
>> unsigned char.
>> 6.7.2:p5
>> Each of the comma-separated sets designates the same type, except that
>> for bit-fields, it is implementation-defined whether the specifier int
>> designates the same type as signed int or the same type as unsigned int.
>> 6.7.2.1:p9
>> A bit-field is interpreted as a signed or unsigned integer type
>> consisting of the specified number of bits. 105
>> 105) As specified in 6.7.2 above, if the actual type specifier used is
>> int or a typedef-name defined as int, then it is implementation-defined
>> whether the bit-field is signed or unsigned.
>>
>>
>>  ------------------------------
>> *From:* metafoo at gmail.com [metafoo at gmail.com] on behalf of Richard Smith
>> [richard at metafoo.co.uk]
>> *Sent:* 30 July 2013 19:19
>> *To:* Robert Lytton
>> *Cc:* cfe-dev at cs.uiuc.edu; benny.kra at gmail.com
>> *Subject:* Re: [cfe-dev] different signedness for bitfield 'char' and
>> plain 'char'
>>
>>   On Tue, Jul 30, 2013 at 10:51 AM, Robert Lytton <robert at xmos.com>wrote:
>>
>>>  Hi,
>>>
>>> I have altered isSignedCharDefault()  to make 'char' unsigned for the
>>> xcore target too.
>>> (The signedness of 'char' is an ABI issue as far a the standard is
>>> concerned.)
>>>
>>> My problem now is that char-bitfields are also being made unsigned.
>>> According to the standard the members of a bitfield are always signed
>>> unless explicitly marked unsigned.
>>>
>>
>>  What standard? C++ requires that bit-fields have the same signedness as
>> their underlying type. If 'char' is unsigned, then bit-fields of type
>> 'char' are also unsigned.
>>
>>
>>>  Hence I need:
>>>     'char' to be unsigned.
>>>     but the bit field 'char' to be signed.
>>>
>>> viz:
>>>     // ABI requires plain 'char' to be unsigned
>>>     char c = (char)-1;
>>>     assert(c >= 0);
>>>
>>>     // members of a bitfield are always signed unless explicitly marked
>>> unsigned
>>>     struct char_sign { char x : 1; };
>>>     char_sign.x = 1;
>>>     assert(char_sign.x < 0);
>>>
>>>
>>> Is this possible in clang?
>>> If not, could anyone like to point me in the right direction to where
>>> the fix would be necessary?
>>>
>>> Thank you
>>>
>>> Robert
>>>
>>>
>>>
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>>>
>>>
>>
>
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