title: 256-bit arithmetic in Clang (extended integers)
body: <p>I'm in the design phase of a project that needs to do a lot of simple 256-bit integer arithmetic (add, sub, mul, div only) and need something that is reasonably well optimised for these four operations.</p>
  <p>I'm already familiar with GMP, NTL and most of the other heavyweight bignum implementations. However, the overhead of these implementations is pushing me towards doing my own low-level implementation - which I <em>really</em> don't want to do; this stuff is notoriously hard to get right.</p>
  <p>In my research I noticed the new extended integer types in Clang - I am a gcc user - and I was wondering if anyone has any experience of the extended integers in real-life, in-anger implementations? Are they optimised for the &quot;obvious&quot; bit sizes (256, 512, etc)?</p>
  <p>I'm working in C on x-64 under linux (currently Ubuntu, though open to other distributions if necessary). I mostly use gcc for production work.</p>
  <p>Edited to add:
  @phuclv identified a previous answer <a href="https://stackoverflow.com/questions/5242819/c-128-256-bit-fixed-size-integer-types">C++ 128/256-bit fixed size integer types</a>. (Thanks @phuclv.) This q/a focuses on c++ support; I was hoping to identify whether anyone had any specific experience with the new Clang types.</p>