Intel® C++ Compiler 16.0 User and Reference Guide
Insert a "%s ivdep" statement right before the loop at line %d to vectorize the loop.
Add "#pragma ivdep" before the specified loop. This pragma enables the vectorization of the loop at the specified line by ignoring some of the assumed cross-iteration data dependencies.
Consider the following:
void foo(float *a, int x, int n) {
int i;
for (i=0; i<n; i++) {
a[i] = a[i+x]+1;
}
return;
}
In this case, the compiler is unable to vectorize the loop because x could be -1, where each iteration is dependent on the previous iteration. If x is known to be positive, you can vectorize this loop.
If you determine it is safe to do so, you can add the pragma as follows:
void foo(float *a, int x, int n) {
int i;
#pragma ivdep
for (i=0; i<n; i++) {
a[i] = a[i+x]+1;
}
return;
}
Confirm that any arrays in the loop do not have unsafe cross-iteration dependencies. A cross-iteration dependency exists if a memory location is modified in an iteration of the loop and accessed by a read or a write statement in another iteration of the loop. Make sure that there are no such dependencies, or that any cross-iteration dependencies can be safely ignored.