OpenACC Tutorial - Adding directives: Difference between revisions

OpenACC Tutorial - Adding directives (view source)

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+== Fixing false loop dependencies ==
+Sometimes, the compiler believes that loops cannot be parallelized despite being obvious to the programmer. One common case, in C and C++, is what is called ''[https://en.wikipedia.org/wiki/Pointer_aliasing pointer aliasing]''. Contrary to Fortran arrays, C and C++ do not formally have arrays. They have what is called pointers. Two pointers are said to be ''aliased'' if they point to the same memory. If the compiler does not know that pointers are not aliased, it must assume that they are aliased. Going back to the previous example, it becomes obvious why the compiler could not parallelize the loop. If we assume that each pointer is the same, then there is an obvious dependence between loop iterations.
+=== <tt>restrict</tt> keyword ===
+The way to tell the compiler that pointers are '''not''' going to be aliased, is by using a special keyword. In C, the keyword <tt>restrict</tt> was introduced in C99.  In C++, there is no standard way, but each compiler typically has its own keyword. Either <tt>__restrict</tt> or <tt>__restrict__</tt> can be used depending on the compiler. For Portland Group compilers, the keyword is <tt>__restrict</tt>.
+For more information on this concept, see this [https://en.wikipedia.org/wiki/Restrict Wikipedia article].
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