Date: Tue, 17 Mar 2015 09:07:35 +0100 From: magnum <john.magnum@...hmail.com> To: john-dev@...ts.openwall.com Subject: Re: 256/128 bit integer arithmatic On 2015-03-17 06:45, Solar Designer wrote: >> I tested against unsigned __int128. Are they any different from __uint128_t >> from performance or portability perspective? > > I'd expect them to be the same performance. As to portability, I don't > know. __uint128_t worked for me even on gcc 3.4.5. GCC has had __int128_t and __uint128_t for 64-bit targets since 3.1. There were *no* corresponding macro (like __SIZEOF_INT128__) that showed it. Since gcc 4.6 or so, this is replaced with __int128 (you'd use 'unsigned __int128' for unsigned). There *is* now a __SIZEOF_INT128__ defined. The __int128_t syntax still works but I think it's deprecated. As far as I know there is no difference in implementation, just the name. Jumbo's autoconf tests this and sets HAVE___INT128_T and HAVE___INT128 accordingly (and it may set HAVE_INT128 with other compilers). I use it like this in mpz_int128.h (which is not included at all if none of the three is set, hence the last catch-all): #undef int128_t #define int128_t our_int128_t #undef uint128_t #define uint128_t our_uint128_t #if HAVE___INT128 typedef __int128 int128_t; typedef unsigned __int128 uint128_t; #elif HAVE_INT128 typedef int128 int128_t; typedef unsigned int128 uint128_t; #else /* HAVE__INT128_T */ typedef __int128_t int128_t; typedef __uint128_t uint128_t; #endif #define UINT128_MAX ((uint128_t)-1) That last line is because we can't (even in gcc-5 afaik) express constants larger than ULL. We could set a positive number using shifts but the above works fine. magnum
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