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atomics: C11 -> C++20 #168

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Oct 12, 2019
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atomics: C11 -> C++20 #168

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atomics: C11 -> C++20
RalfJung Oct 12, 2019
37a0693
mention C11 in case someone wonders
RalfJung Oct 12, 2019
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17 changes: 10 additions & 7 deletions src/atomics.md
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# Atomics

Rust pretty blatantly just inherits C11's memory model for atomics. This is not
Rust pretty blatantly just inherits the memory model for atomics from C++20. This is not
due to this model being particularly excellent or easy to understand. Indeed,
this model is quite complex and known to have [several flaws][C11-busted].
Rather, it is a pragmatic concession to the fact that *everyone* is pretty bad
at modeling atomics. At very least, we can benefit from existing tooling and
research around C.
research around the C/C++ memory model.
(You'll often see this model referred to as "C11" or "C/C++11". C and C++ share their
memory model and those were the first versions but they have received some bugfixes
since then.)

Trying to fully explain the model in this book is fairly hopeless. It's defined
in terms of madness-inducing causality graphs that require a full book to
properly understand in a practical way. If you want all the nitty-gritty
details, you should check out [C's specification (Section 7.17)][C11-model].
details, you should check out the [C++20 draft specification (Section 31)][C++-model].
Still, we'll try to cover the basics and some of the problems Rust developers
face.

The C11 memory model is fundamentally about trying to bridge the gap between the
The C++ memory model is fundamentally about trying to bridge the gap between the
semantics we want, the optimizations compilers want, and the inconsistent chaos
our hardware wants. *We* would like to just write programs and have them do
exactly what we said but, you know, fast. Wouldn't that be great?
Expand Down Expand Up @@ -113,7 +116,7 @@ programming:

# Data Accesses

The C11 memory model attempts to bridge the gap by allowing us to talk about the
The C++ memory model attempts to bridge the gap by allowing us to talk about the
*causality* of our program. Generally, this is by establishing a *happens
before* relationship between parts of the program and the threads that are
running them. This gives the hardware and compiler room to optimize the program
Expand Down Expand Up @@ -148,7 +151,7 @@ propagated to other threads. The set of orderings Rust exposes are:
* Acquire
* Relaxed

(Note: We explicitly do not expose the C11 *consume* ordering)
(Note: We explicitly do not expose the C++ *consume* ordering)

TODO: negative reasoning vs positive reasoning? TODO: "can't forget to
synchronize"
Expand Down Expand Up @@ -252,4 +255,4 @@ relaxed operations can be cheaper on weakly-ordered platforms.


[C11-busted]: http://plv.mpi-sws.org/c11comp/popl15.pdf
[C11-model]: http://www.open-std.org/jtc1/sc22/wg14/www/standards.html#9899
[C++-model]: http://eel.is/c++draft/atomics.order