prefervector

Date:: 01-09-2012

NAME

prefervector - indicates loop to vectorize within a loop nest

SYNOPSIS

#pragma _CRI prefervector

!DIR$ PREFERVECTOR

IMPLEMENTATION

Cray Linux Environment (CLE)

DESCRIPTION

Scope: Local

The prefervector directive directs the compiler to vectorize the loop that immediately follows the directive if the loop contains more than one loop in the nest that can be vectorized. The directive states a vectorization preference and does not guarantee that the loop has no memory- dependence hazard.

EXAMPLES

Example 1: prefervector directive

The following example illustrates the use of the prefervector directive for C/C++ :

float a[1000], b[100][1000];

void
f(int m, int n)
{
  int i, j;

  #pragma _CRI prefervector
  for (i = 0; i < n; i++) {
    for (j = 0; j < m; j++) {
      a[i] += b[j][i];
    }
  }
}

In this example, both loops can be vectorized, but the directive directs the compiler to vectorize the outer for loop. Without the directive and without any knowledge of n and m, the compiler would vectorize the inner loop.

Example 2: PREFERVECTOR directive

The following example illustrates the use of the PREFERVECTOR directive for Fortran:

!DIR$ PREFERVECTOR
                             DO I = 1, N
!DIR$ IVDEP

                                     DO J = 1, M
                                             A(I) = A(I) + B(J,I)
                                     END DO
                             END DO

In this example, both loops can be vectorized, but the compiler generates vector code for the outer DO I loop. Note that the DO I loop is vectorized even though the inner DO J loop was specified with an IVDEP directive: