CODEWITHSUNDEEP
c++visual-c++x86-64stdatomiccompare-and-swap
J. Tully
J. Tully

Reputation: 129

CMPXCHG16B and MSVC Implementation of <atomic> not the default?

It seems that for CMPXCHG16B to be used, one has to define _STD_ATOMIC_ALWAYS_USE_CMPXCHG16B = 1 so that these instructions are used.

Why is this the default? I would have never found out about this unless I read the whole atomic.h header either.

What other global defines in the STL are there? Is there a list to review so one can reliably be aware of these implementation details?

Upvotes: 4

Views: 543

Answers (1)

Alex Guteniev
Alex Guteniev

Reputation: 13699

_STD_ATOMIC_ALWAYS_USE_CMPXCHG16B was recently introduced in Visual Studio 2019 (the PR)

Visual Studio 2019 still supports older OSes such as Windows Vista, and Windows 7. These OSes can run on old AMD Opteron CPUs that don't have this instruction.

Even if _STD_ATOMIC_ALWAYS_USE_CMPXCHG16B = 0, there's runtime detection that uses CMPXCHG16B if it is available. But in this case the instructions is not inlined, and there's also a branch, so it is less efficient than defining _STD_ATOMIC_ALWAYS_USE_CMPXCHG16B = 1.

Please also note that CMPXCHG16B is used for atomic_ref, but not for atomic due to ABI compatibility. (It was possible to introduce for atomic_ref, since there was no pre-C++20 atomic_ref to be ABI-compatible with).

In vNext version (the next major, ABI breaking version), atomic should use CMPXCHG16B as well. There's also hope that old CPUs/OSes support will be dropped, and the use of CMPXCHG16B would become unconditional. (See https://github.com/microsoft/STL/issues/1151).

I would have never found out about this unless I read the whole atomic.h header either.

What other global defines in the STL are there? Is there a list to review so one can reliably be aware of these implementation details?

I'm afraid there's no comprehensive list, although some are documented.

The excuse for _STD_ATOMIC_ALWAYS_USE_CMPXCHG16B in particular could be that whole atomic_ref is not documented, and as C++20 feature it has experimental status in Visual Studio 2019.

Upvotes: 6

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