Pragmas are directives that provide instructions to the compiler for use in specific cases. For example, you can use the novector pragma to specify that a loop should never be vectorized. The keyword #pragma is standard in the C++ language, but individual pragmas are machine-specific or operating system-specific, and vary by compiler.
Some pragmas provide the same functionality as do compiler options. Pragmas override behavior specified by compiler options.
You enter pragmas into your C++ source code using the following syntax:
#pragma <pragma name>
The vector always directive instructs the compiler to override any efficiency heuristic during the decision to vectorize or not, and will vectorize non-unit strides or very unaligned memory accesses.
Example of the vector always directive
#pragma vector always
for(i=0; i<=N; i++)
{
a[32*i]=b[99*i];
}