From: Martin Sebor <msebor@gmail.com>
To: David Brown <david@westcontrol.com>,
Kostas Savvidis <ksavvidis@gmail.com>,
gcc-help@gcc.gnu.org
Subject: Re: 128-bit integer - nonsensical documentation?
Date: Wed, 26 Aug 2015 16:02:00 -0000 [thread overview]
Message-ID: <55DDE30A.1080506@gmail.com> (raw)
In-Reply-To: <55DDAD64.3040607@westcontrol.com>
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-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 â⦠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>.
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
next prev parent reply other threads:[~2015-08-26 16:02 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-08-26 11:04 Kostas Savvidis
2015-08-26 11:44 ` Jeffrey Walton
2015-08-26 12:13 ` David Brown
2015-08-26 16:02 ` Martin Sebor [this message]
2015-08-27 7:12 ` David Brown
2015-08-27 9:32 ` Jonathan Wakely
2015-08-27 9:42 ` Marc Glisse
2015-08-27 9:43 ` Jonathan Wakely
2015-08-27 15:09 ` Martin Sebor
2015-08-28 6:54 ` David Brown
2015-08-28 15:30 ` Martin Sebor
2015-08-26 12:22 ` Jonathan Wakely
2015-08-26 12:32 ` Kostas Savvidis
2015-08-26 12:39 ` Jonathan Wakely
2015-08-26 12:47 ` Jeffrey Walton
2015-08-26 12:47 ` David Brown
2015-08-26 12:48 ` Jeffrey Walton
2015-08-26 12:51 ` Marc Glisse
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