{"id":4837,"date":"2016-08-03T07:11:18","date_gmt":"2016-08-03T07:11:18","guid":{"rendered":"https:\/\/www.modernescpp.com\/index.php\/ongoing-optimization\/"},"modified":"2023-06-26T12:48:05","modified_gmt":"2023-06-26T12:48:05","slug":"ongoing-optimization","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/ongoing-optimization\/","title":{"rendered":"Ongoing Optimization"},"content":{"rendered":"

Now it’s time to put the theory into practice. The job is relatively easy. A small program should undergo ongoing optimization.<\/p>\n

<\/p>\n

The program<\/h2>\n

<\/p>\n

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 1\r\n 2\r\n 3\r\n 4\r\n 5\r\n 6\r\n 7\r\n 8\r\n 9\r\n10\r\n11\r\n12\r\n13\r\n14\r\n15\r\n16\r\n17\r\n18\r\n19\r\n20\r\n21\r\n22\r\n23\r\n24<\/pre>\n<\/td>\n
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\/\/ ongoingOptimization.cpp<\/span>\r\n\r\n#include <iostream><\/span>\r\n#include <thread><\/span>\r\n\r\nint<\/span> x= 0;\r\nint<\/span> y= 0;\r\n\r\nvoid<\/span> writing(){\r\n  x= 2000;\r\n  y= 11;\r\n}\r\n\r\nvoid<\/span> reading(){ \r\n  std::cout << \"y: \"<\/span> << y << \" \"<\/span>;\r\n  std::cout << \"x: \"<\/span> << x << std::endl;\r\n}\r\n\r\nint<\/span> main(){\r\n  std::thread<\/span> thread1(writing);\r\n  std::thread<\/span> thread2(reading);\r\n  thread1.join();\r\n  thread2.join();\r\n}\r\n<\/pre>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n
 <\/p>\n
The program is quite simple. It consists of two threads thread1<\/span> and thread2.thread1 <\/span>writes the values x and y. thread 2 <\/span>reads the values x and y in the opposite direction.  The execution of the program is not so thrilling either.<\/p>\n
\"ongoingOptimization\"<\/p>\n
But you see, even this simple program has different results. So I finally came to my key idea.<\/p>\n<\/p>\n
In the process of the ongoing optimization of the program, I have two questions.<\/p>\n
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  1. Is the program well-defined? Precisely, is there a data race<\/a>?<\/li>\n
  2. Which values for x and y are possible?<\/li>\n<\/ol>\n
    The first question is often very challenging to answer. So I will reason in the first step about the program and verify in the second step my reasoning with CppMem<\/a>.  If I have answered the first question, the second answer can easily be derived. I will provide the possible values in a table.<\/p>\n
    \"schabloneEng\"<\/p>\n
     <\/p>\n
    But still, one question isn’t answered yet. What do I mean by ongoing optimization? I mean, by ongoing optimization, that I try to improve the program by weakening the C++ memory model. So in my journey, I discuss the following stations.<\/p>\n