It's also not legal to reorder the statements inside one original iteration if it breaks the sequential dependency. It's not legal to move the memory reference *c out of the loop if it may alias with a or b.
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In other words, a is executed after a, b is after a and the call to bar happens last. Therefore, the following transformed vector form of the loop is legal because the compiler keeps the sequential behavior of each original loop iteration.
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There's no perfect parallelism among loop iterations since there's a backward dependency from a to a, but because of the SIMD directive the compiler is still allowed to pack consecutive iterations of the first statement into one vector instruction and run them in parallel. By annotating a loop with the OpenMP SIMD directive, users intend to have multiple loop iterations executed simultaneously.įor example, the following loop is annotated with the OpenMP SIMD directive. The OpenMP SIMD directive provides users a way to dictate the compiler make loops vector-friendly. Warning C4849: OpenMP 'simdlen' clause ignored in 'simd' directive
#Vector file extension code
Non-effective SIMD clauses are parsed and ignored by the compiler with a warning.įor example, use of the following code issues a warning: #pragma omp simd simdlen(8) Specify data privatization with final value from the last iteration. The linear mapping from loop induction variable to array subscription. The OpenMP SIMD directive can also take the following clauses to enhance vector-support: Directive Mycode.cpp(96) : info C5001: Omp simd loop vectorizedįor non-vector-friendly loops, the compiler issues each a message: cl -O2 -openmp:experimental mycode.cpp Mycode.cpp(90) : info C5002: Omp simd loop not vectorized due to reason '1200' Mycode.cpp(84) : info C5002: Omp simd loop not vectorized due to reason '1200'
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Visual C++ provides similar non-OpenMP loop pragmas like #pragma vector and #pragma ivdep, however with OpenMP SIMD, the compiler can do more, like: By using the simd directive before a loop, the compiler can ignore vector dependencies, make the loop as vector-friendly as possible, and respect the users’ intention to have multiple loop iterations executed simultaneously. OpenMP SIMD, introduced in the OpenMP 4.0 standard, targets making vector-friendly loops. The -openmp:experimental switch subsumes -openmp, meaning all OpenMP 2.0 features are included in its use.įor more information, see SIMD Extension to C++ OpenMP in Visual Studio. To use SIMD, compile with the -openmp:experimental switch that enables additional OpenMP features not available when using the -openmp switch.