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Date: Sun, 08 Mar 2015 20:37:57 +0100
From: magnum <john.magnum@...hmail.com>
To: john-dev@...ts.openwall.com
Subject: Re: 256/128 bit integer arithmatic

On 2015-03-08 16:26, Sayantan Datta wrote:
> Hi all,
> 
> Is there any way to natively perform 128 bit or 256 bit integer arithmetic,
> such as add, divide, subtract ? Maybe with AVX/SSE registers and using them
> without SIMD?
> 
> Regards,
> Sayantan
> 

gcc provides int128 for 64-bit builds (but not for 32-bit builds). Look
at our mpz_int128.h which mimics GMP but actually uses saturated 128-bit
integer (the latter is a lot faster - but GMP can go way beyond 128 bits).

They are emulated though, probably similar to using hi/lo 64-bit structs.

One caveat is you can't have constants larger than 64-bit. So to load a
128-bit constant you'd need to do something like this

uint128_t foo = 0xabcd0123deadcafeULL << 64 + 0xdeadcafeabcd0123ULL

magnum

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