But we can improve and further improve the acquire-release semantics of the last post. Why should x be atomic? There is no reason. That was my first but incorrect assumption. See why?
A typical misunderstanding in applying the acquire-release semantic is to assume that the acquire operation is waiting for the release operation. So based on this assumption, you may think that x has not be an atomic variable. So we can further optimize the program.
The program has a data race on x and has, therefore, undefined behavior. If y.store(11,std::memory_order_release) (line 12) is executed before y.load(std::memory_order_acquire) (line 16), the acquire-release semantic guarantees that x= 2000 (line 11) is executed before the reading of x in line 17. But if not. In this case, the reading of x will be executed simultaneously as the writing of x. So we have concurrent access to a shared variable, one of them is a write. That’s, per definition, a data race.
The table puts it in a nutshell.
Now it’s time for CppMem. Let’s see what CppMem finds out.
The data race occurs if one thread writes x= 2000 and the other reads x. The graph shows a dr symbol (data race) on the arrow.
The ultimate step in the process of ongoing optimization is still missing. In the next post, I will use the relaxed semantic.
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Thanks, in particular, to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, Dendi Suhubdy, Sudhakar Belagurusamy, Richard Sargeant, Rusty Fleming, John Nebel, Mipko, Alicja Kaminska, Slavko Radman, and David Poole.
|My special thanks to Embarcadero|
|My special thanks to PVS-Studio|
|My special thanks to Tipi.build|
|My special thanks to Take Up Code|
I’m happy to give online seminars or face-to-face seminars worldwide. Please call me if you have any questions.
- Embedded Programmierung mit modernem C++ 12.12.2023 – 14.12.2023 (Präsenzschulung, Termingarantie)
Standard Seminars (English/German)
Here is a compilation of my standard seminars. These seminars are only meant to give you a first orientation.
- C++ – The Core Language
- C++ – The Standard Library
- C++ – Compact
- C++11 and C++14
- Concurrency with Modern C++
- Design Pattern and Architectural Pattern with C++
- Embedded Programming with Modern C++
- Generic Programming (Templates) with C++
- Clean Code with Modern C++
- Phone: +49 7472 917441
- Mobil:: +49 176 5506 5086
- Mail: schulung@ModernesCpp.de
- German Seminar Page: www.ModernesCpp.de
- Mentoring Page: www.ModernesCpp.org
Modernes C++ Mentoring,