PREFETCH and NOPREFETCH

Syntax

cDEC$ PREFETCH [var1[: hint1[: distance1]] [,var2[: hint2[: distance2]]]...]

cDEC$ NOPREFETCH [var1[,var2]...]

c	Is one of the following: C (or c), !, or *. (See Syntax Rules for Compiler Directives.)
var	Is an optional memory reference.
hint	Is an optional integer initialization expression with the value 0, 1, 2, or 3. These are the same as the values for hint in the intrinsic subroutine MM_PREFETCH. To use this argument, you must also specify var.
distance	Is an optional integer initialization expression with a value greater than 0. It indicates the number of loop iterations to perform before the prefetch. To use this argument, you must also specify var and hint.

To use these directives, compiler option O2 or O3 must be set.

This directive affects the DO loop it precedes.

If you specify PREFETCH with no arguments, all arrays accessed in the DO loop will be prefetched.

If a loop includes expression A(j), placing cDEC$ PREFETCH A in front of the loop instructs the compiler to insert prefetches for A(j + d) within the loop. The d is determined by the compiler.

Example

cDEC$ NOPREFETCH c

cDEC$ PREFETCH a

do i = 1, m

b(i) = a(c(i)) + 1

enddo

The following example is valid on IA-64 architecture:

sum = 0.d0

do j=1,lastrow-firstrow+1

i = rowstr(j)

iresidue = mod( rowstr(j+1)-i, 8 )

sum = 0.d0

CDEC$ NOPREFETCH a,p,colidx

do k=i,i+iresidue-1

sum = sum + a(k)*p(colidx(k))

enddo

CDEC$ NOPREFETCH colidx

CDEC$ PREFETCH a:1:40

CDEC$ PREFETCH p:1:20

do k=i+iresidue, rowstr(j+1)-8, 8

sum = sum + a(k )*p(colidx(k ))

& + a(k+1)*p(colidx(k+1)) + a(k+2)*p(colidx(k+2))

& + a(k+3)*p(colidx(k+3)) + a(k+4)*p(colidx(k+4))

& + a(k+5)*p(colidx(k+5)) + a(k+6)*p(colidx(k+6))

& + a(k+7)*p(colidx(k+7))

enddo

q(j) = sum

enddo