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