Visible to Intel only — GUID: ewa1456929096307
Ixiasoft
Visible to Intel only — GUID: ewa1456929096307
Ixiasoft
6.1.5. Removing Loop-Carried Dependencies Caused by Accesses to Memory Arrays
You can provide more information about loop dependencies by adding the safelen(N) clause to the ivdep pragma. The safelen(N) clause specifies the maximum number of consecutive loop iterations without loop-carried dependencies. For example, #pragma ivdep safelen(32) indicates to the compiler that there are a maximum of 32 iterations of the loop before loop-carried dependencies might be introduced. That is, while #pragma ivdep promises that there are no implicit memory dependency between any iteration of this loop, #pragma safelen(32) promises that the iteration that is 32 iterations away is the closest iteration that could be dependent on this iteration.
- If all accesses to memory arrays that are inside a loop do not cause loop-carried dependencies, add the line #pragma ivdep before the loop in your kernel code.
Example kernel code:
// no loop-carried dependencies for A and B array accesses #pragma ivdep for (int i = 0; i < N; i++) { A[i] = A[i - X[i]]; B[i] = B[i - Y[i]]; }
- To specify that accesses to a particular memory array inside a loop does not cause loop-carried dependencies, add the line #pragma ivdep array (array_name) before the loop in your kernel code.
The array specified by the ivdep pragma must be a local or private memory array, or a pointer variable that points to a global, local, or private memory storage. If the specified array is a pointer, the ivdep pragma also applies to all arrays that may alias with specified pointer.
The array specified by the ivdep pragma can also be an array or a pointer member of a struct.
Example kernel code:
// No loop-carried dependencies for A array accesses // The offline compiler will insert hardware that reinforces dependency constraints for B #pragma ivdep array(A) for (int i = 0; i < N; i++) { A[i] = A[i - X[i]]; B[i] = B[i - Y[i]]; } // No loop-carried dependencies for array A inside struct #pragma ivdep array(S.A) for (int i = 0; i < N; i++) { S.A[i] = S.A[i - X[i]]; } // No loop-carried dependencies for array A inside the struct pointed by S #pragma ivdep array(S->X[2][3].A) for (int i = 0; i < N; i++) { S->X[2][3].A[i] = S.A[i - X[i]]; } // No loop-carried dependencies for A and B because ptr aliases // with both arrays int *ptr = select ? A : B; #pragma ivdep array(ptr) for (int i = 0; i < N; i++) { A[i] = A[i - X[i]]; B[i] = B[i - Y[i]]; } // No loop-carried dependencies for A because ptr only aliases with A int *ptr = &A[10]; #pragma ivdep array(ptr) for (int i = 0; i < N; i++) { A[i] = A[i - X[i]]; B[i] = B[i - Y[i]]; }