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.
int x= 0;
std::cout << y.load(std::memory_order_acquire) << " ";
std::cout << x << std::endl;
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.
I made this mistake in my presentation "Multithreading done right?" in Berlin. In Moscow, I did it right. I never claimed that the C++ memory model is a piece of cake.
Now it's time for CppMem. Let's see what CppMem finds out.
int x= 0;
atomic_int y= 0;
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, and Slavko Radman.
My special thanks to Embarcadero
My special thanks to PVS-Studio
My special thanks to Tipi.build
I'm happy to give online seminars or face-to-face seminars worldwide. Please call me if you have any questions.
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
- German Seminar Page: www.ModernesCpp.de
- Mentoring Page: www.ModernesCpp.org
which i am going to present in college.