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OpenACC Tutorial - Adding directives: Difference between revisions
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=== <tt>restrict</tt> keyword === <!--T:26--> | === <tt>restrict</tt> keyword === <!--T:26--> | ||
One 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 for this purpose. In C++, there is no standard way yet, 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 an explanation as to why there is no standard way to do this in C++, you can read [http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3988.pdf this paper]. This concept is important not only for OpenACC, but for any C/C++ programming, since many more optimizations can be done by compilers when pointers are guaranteed not to be aliased. Note that the keyword goes ''after'' the pointer, since it refers to the pointer, and not to the type. In other words, you would declare <code>float * __restrict A;</code> rather than <code>float __restrict * A;</code>. | One 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 for this purpose. In C++, there is no standard way yet, 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 and NVidia compilers, the keyword is <tt>__restrict</tt>. For an explanation as to why there is no standard way to do this in C++, you can read [http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3988.pdf this paper]. This concept is important not only for OpenACC, but for any C/C++ programming, since many more optimizations can be done by compilers when pointers are guaranteed not to be aliased. Note that the keyword goes ''after'' the pointer, since it refers to the pointer, and not to the type. In other words, you would declare <code>float * __restrict A;</code> rather than <code>float __restrict * A;</code>. | ||
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=== Loop directive with independent clause === <!--T:29--> | === Loop directive with independent clause === <!--T:29--> | ||
Another way to tell the compiler that loops iterations are independent is to specify it explicitly by using a different directive: <tt>loop</tt>, with the clause <tt>independent</tt>. This is a ''prescriptive'' directive. Like any prescriptive directive, this tells the compiler what to do, and overrides any compiler analysis. The initial example above would become: | Another way to tell the compiler that loops iterations are independent is to specify it explicitly by using a different directive: <tt>loop</tt>, with the clause <tt>independent</tt>. This is a ''prescriptive'' directive. Like any prescriptive directive, this tells the compiler what to do, and overrides any compiler analysis. The initial example above would become: | ||