How do I force multi-thread parallelization? How to use compiler directives?
The compiler will be very conservative about multithreading loops. If there is the slightest possibility of data dependencies, it will refuse to do it if -autopar is used. Function calls within loops, if statements that depend on variables which change in the loop, and many other features will be considered “dangerous” and inhibit parallelization. The reason is that such features have a potential to make the result dependent on the order in which the loop iterations are carried out, and therefore go against a parallel execution. However, often you know more than the compiler. You might be certain that a function call does not alter the value of variables that are shared with other loop iterations. If this is the case, there is ways to tell the compiler to parallelize anyhow. This is done via compiler directives that look like comments, but if compiled with the proper flags, will guide the compiler in parallelizing the code. The most common one a OpenMP compiler directives.
The compiler will be very conservative about multithreading loops automatically. If there is the slightest possibility of data dependencies, it will refuse to do it if -xautopar is used. Function calls within loops, if statements that depend on variables which change in the loop, and many other features will be considered “dangerous” and inhibit parallelization. The reason is that such features have a potential to make the result dependent on the order in which the loop iterations are carried out, and therefore go against a parallel execution. However, often you know more than the compiler. You might be certain that a function call does not alter the value of variables that are shared with other loop iterations. If this is the case, there is ways to tell the compiler to parallelize anyhow. This is done via compiler directives that look like comments, but if compiled with the proper flags, will guide the compiler in parallelizing the code. The most common one a OpenMP compiler directive
