ddot.c

#include "clapack.h"

doublereal ddot_(integer *n, doublereal *dx, integer *incx, doublereal *dy, 
	integer *incy)
{
    /* System generated locals */
    integer i__1;
    doublereal ret_val;

    /* Local variables */
    integer i__, m, ix, iy, mp1;
    doublereal dtemp;

/*     .. Scalar Arguments .. */
/*     .. */
/*     .. Array Arguments .. */
/*     .. */

/*  Purpose */
/*  ======= */

/*     forms the dot product of two vectors. */
/*     uses unrolled loops for increments equal to one. */
/*     jack dongarra, linpack, 3/11/78. */
/*     modified 12/3/93, array(1) declarations changed to array(*) */


/*     .. Local Scalars .. */
/*     .. */
/*     .. Intrinsic Functions .. */
/*     .. */
    /* Parameter adjustments */
    --dy;
    --dx;

    /* Function Body */
    ret_val = 0.;
    dtemp = 0.;
    if (*n <= 0) {
	return ret_val;
    }
    if (*incx == 1 && *incy == 1) {
	goto L20;
    }

/*        code for unequal increments or equal increments */
/*          not equal to 1 */

    ix = 1;
    iy = 1;
    if (*incx < 0) {
	ix = (-(*n) + 1) * *incx + 1;
    }
    if (*incy < 0) {
	iy = (-(*n) + 1) * *incy + 1;
    }
    i__1 = *n;
    for (i__ = 1; i__ <= i__1; ++i__) {
	dtemp += dx[ix] * dy[iy];
	ix += *incx;
	iy += *incy;
/* L10: */
    }
    ret_val = dtemp;
    return ret_val;

/*        code for both increments equal to 1 */


/*        clean-up loop */

L20:
    m = *n % 5;
    if (m == 0) {
	goto L40;
    }
    i__1 = m;
    for (i__ = 1; i__ <= i__1; ++i__) {
	dtemp += dx[i__] * dy[i__];
/* L30: */
    }
    if (*n < 5) {
	goto L60;
    }
L40:
    mp1 = m + 1;
    i__1 = *n;
    for (i__ = mp1; i__ <= i__1; i__ += 5) {
	dtemp = dtemp + dx[i__] * dy[i__] + dx[i__ + 1] * dy[i__ + 1] + dx[
		i__ + 2] * dy[i__ + 2] + dx[i__ + 3] * dy[i__ + 3] + dx[i__ + 
		4] * dy[i__ + 4];
/* L50: */
    }
L60:
    ret_val = dtemp;
    return ret_val;
} /* ddot_ */