Re: 128-bit integer - nonsensical documentation?

[David Brown <david at westcontrol dot com>]

On 26/08/15 13:04, Kostas Savvidis wrote:
> The online documentation contains the attached passage as part of the
> "C-Extensionsâ chapter. There are no actual machines which have an "
> integer mode wide enough to hold 128 bitsâ as the document puts it.
> This would be a harmless confusion if it didnât go on to say ââ long
> long integer less than 128 bits wideâ (???!!!) Whereas in reality
> "long long intâ 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>.

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