Example
A loop is called DOALL if all of its iterations, in any given invocation, can be executed concurrently. The Fortran code below is DOALL, and can be auto-parallelized by a compiler because each iteration is independent of the others, and the final result of array z will be correct regardless of the execution order of the other iterations.
On the other hand, the following code cannot be auto-parallelized, because the value of z(i) depends on the result of the previous iteration, z(i - 1).
This does not mean that the code cannot be parallelized. Indeed, it is equivalent to
do i = 2, n z(i) = z(1)*2**(i - 1) enddoHowever, current parallelizing compilers are not usually capable of bringing out these parallelisms automatically, and it is questionable whether this code would benefit from parallelization in the first place.
Read more about this topic: Automatic Parallelization
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