Commits on Jan 2, 2019

1. Add a x86_64::cmpxchg16b intrinsic …

   This intrinsic isn't actually specified by Intel, but it's something
   gated with CPUID and can otherwise be a useful thing to have when
   building primitives!
   
   There exists an `AtomicU128` type in the standard library but it's only
   exposed currently (and it's unstable) when a platform fully supports
   128-bit atomics. The x86_64 architecture does not support it *unless*
   the `cmpxchg16b` instruction is available, and it isn't always available!
   
   This commit is also a proposal for how we can include support for
   128-bit atomics in the standard library on relevant platforms. I'm
   thinking that we'll expose this one low-level intrinsic in
   `std::arch::x86_64`, and then if desired a crate on crates.io can build
   `AtomicU128` from this API.
   
   In any case this is all unstable regardless!

   

   alexcrichton committed Jan 2, 2019
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