Re: Seeking Endorsement: Boost.int128

Matt Borland via Boost <[email protected]> Wed, 29 Apr 2026 12:42:49 +0000

Newsgroups	gmane.comp.lib.boost.devel
Message-ID	<KSpbX1p7rwDqHJi4xrmJl00n3WwnmPcg1d_6M1_G-Nv8cwgkkPPG51Ff9fX-IaVWT1AFz1JJbPBesPR4LiNh9G_tphQkk_HOu9taYYnWCf0=@mattborland.com>




On Wednesday, April 29th, 2026 at 1:59 AM, Rainer Deyke via Boost <[email protected]> wrote:

> On 4/28/26 19:02, Matt Borland via Boost wrote:
> > Dear All,
> >
> > I am seeking endorsement for review of my library int128 [1]. The library requires only C++14, is header only, and has no mandatory dependencies. What does int128 provide? Two portable and performant types: a 128-bit signed integer and a 128-bit unsigned integer, as well as a standard library for them. The performance of these types have been tuned and tested on a variety of architectures to include: x64, x32, s390x, ARM64, ARM32, PPC64LE. Both of the types and most of their library functions also work inside CUDA kernels. Benchmarks against Boost.Multiprecision, Absl, MSVC's software 128-bit integers, and built-ins, are available [2].
> 

> I have mixed feelings about this.  What I really want is a true
> std::int128_t using native CPU instructions that is a true integral type
> and can be used in switch statements.  The proposed library looks good
> from a performance perspective, but that could all change when true 128
> bit integer CPU instructions arrive, and as a class type it can't be
> used with switch statements.
> 


Here is the paper that I referenced earlier about the std::int_least128_t proposal being replaced by _BitInt: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2026/p3666r3.html. Hopefully an unsigned _BitInt(128) does as you describe, but I have not looked at the existing implementations to see if that's true. The max required width is only 64-bits, so we may very well end up in a situation where 128-bit still is not cross-platform.

> Despite my reservations, I endorse this library.  It's available now, it
> runs on current CPUs, and I can always hide it behind a typedef so that
> I can switch to another int128 implementation when a true native one
> becomes available.
> 


Thank you!

Matt

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