3.2.3 Storeback

Privatization of variables can be very useful for improving the parallelism in the program. However, if the private variable is referenced outside the loop then the compiler needs to verify that it has the right value. Consider the following example:

Example 3-9  Parallelized Loop Using Storeback

for (i=1; i < 1000; i++) {
    t = 2 * a[i];           /* S1 */
    b[i] = t;               /* S2 */
}
x = t;                      /* S3 */

In this example, the value of t referenced in statement S3 is the final value of t computed by the loop. After the variable t has been privatized and the loop has finished executing, the right value of t needs to be stored back into the original variable. This process is called storeback, and is accomplished by copying the value of t on the final iteration back to the original location of variable t. In many cases the compiler can do this automatically, but sometimes the last value cannot be computed easily.

Example 3-10  Loop That Cannot Use Storeback

for (i=1; i < 1000; i++) {
    if (c[i] > x[i] ) {         /* C1 */
            t = 2 * a[i];           /* S1 */
            b[i] = t;               /* S2 */
    }
}
x = t*t;                       /* S3 */

For correct execution, the value of t in statement S3 is not, usually the value of t on the final iteration of the loop. In fact, it is the last iteration for which the condition C1 is true. Computing the final value of t can be difficult in general. In cases like this example, the compiler will not parallelize the loop.