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What's the point of std::atomic for types that the CPU can't atomically manipulate?

By : Mano Mando
Date : September 22 2020, 05:00 PM
wish of those help Sometimes you have to write code that can work on multiple platforms and atomic operations might be supported without locks on some platforms and it might not be on others. Using std::atomic gives you the best of both worlds -- optimum performance where the platform can support it and sane behavior where the platform can't. A side benefit is cleaner semantics and less risk of inadvertently holding the longer for more or less time than intended.
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Read a non-atomic variable, atomically?

By : Natasha
Date : March 29 2020, 07:55 AM
it helps some times Since you tagged x86, this will be x86-specific.
An increment is essentially three parts, read, add, write. The increment is not atomic, but all three steps (the add doesn't count I suppose, it's not observable anyway) of it are as long as the variable does not cross a cache line boundary (this condition is weaker than having to be aligned to its natural alignment, it has been like this as of P6, before that quadwords had to be aligned).

Atomically accessing a variable which isn't a `std::atomic`

By : Nicolas Prieur
Date : March 29 2020, 07:55 AM
like below fixes the issue It seems like this is indeed a gap in the language and there's a Proposal to fix it for c++ 20:

Atomically update multiple volatile and j.u.c.atomic variables

By : Muhammad Zia Umer
Date : March 29 2020, 07:55 AM
Any of those help If you need to assign two values atomically, chaning volatile int to AtomicInteger will not solve your race condition problem.
To solve your issue, you basically have two options:

std::atomic<int> - load and reset to 0 atomically?

By : keita ebrima
Date : March 29 2020, 07:55 AM
it should still fix some issue Having std::atomic how can I atomically load value and reset to 0? So If I do this operation from two threads, only one receive value, another should receive 0.

Partial updates to complex C11 atomic types and non-atomic read optimizations

By : helbee
Date : March 29 2020, 07:55 AM
This might help you No this doesn't fit into C11's model for atomics, and for good reasons. _Atomic is only syntactically a qualifier, semantically an _Atomic is a new type. This is reflected by the fact that the standard allows that size and alignement of such types are different from those for the base.
In your case of a wide atomic type, a permitted implementation of the atomic type is to add a hidden field to the struct that serves as a lock. Generally, such types are implemented as "not lock-free" that is with some hidden state (within the struct or seperately) that controls access to the data.
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