128-bit integer - nonsensical documentation?

Martin Sebor msebor@gmail.com
Wed Aug 26 16:02:00 GMT 2015


On 08/26/2015 06:13 AM, David Brown wrote:
> On 26/08/15 13:04, Kostas Savvidis wrote:
>> The online documentation contains the attached passage as part of the
>> "C-Extensions” chapter. There are no actual machines which have an"
>> integer mode wide enough to hold 128 bits” as the document puts it.
>> This would be a harmless confusion if it didn’t go on to say “… long
>> long integer less than 128 bits wide” (???!!!) Whereas in reality
>> "long long int” is 64 bits everywhere i have seen.
>>
>> KS
>>
>> -------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>>
>>   6.8 128-bit integers
>>
>> As an extension the integer scalar type __int128 is supported for
>> targets which have an integer mode wide enough to hold 128 bits.
>> Simply write __int128 for a signed 128-bit integer, or unsigned
>> __int128 for an unsigned 128-bit integer. There is no support in GCC
>> for expressing an integer constant of type __int128 for targets with
>> long long integer less than 128 bits wide.
>>
>
> You can use __int128 integers on any platform that supports them (which
> I think is many 64-bit targets), even though "long long int" is
> typically 64-bit.  The documentation says you can't express an integer
> /constant/ of type __int128 without 128-bit long long's.  It is perhaps
> not very clear, but it makes sense.
>
> Thus you can write (using C++'s new digit separator for clarity):
>
> __int128 a = 0x1111'2222'3333'4444'5555'6666'7777'8888LL;
>
> to initialise a 128-bit integer - but /only/ if "long long" supports
> 128-bit values.  On a platform that has __int128 but 64-bit long long's,
> there is no way to write the 128-bit literal.  Thus you must use
> something like this:
>
> __int128 a = (((__int128) 0x1111'2222'3333'4444LL) << 32)
> 	| 0x5555'6666'7777'8888LL;
>
> This is, I believe, the main reason that __int128 integers are an
> "extension", but are not an "extended integer type" - and therefore
> there is no int128_t and uint128_t defined in <stdint.h>.

It's the other way around. If __int128_t were an extended integer
type then intmax_t would need to be at least as wide. The width
of intmax_t is constrained by common ABIs to be that of long long,
which precludes defining extended integer types with greater
precision.

>
> Maybe what we need is a "LLL" suffix for long long long ints :-)

The standard permits integer constants that aren't representable
in any of the standard integer types to have an extended integer
type so a new suffix isn't strictly speaking necessary for
extended integer type constants.

Martin



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