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
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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
