🚀 go-pugleaf

>       SUM = 0.0
>       DO 10 I = 1, 1000000
>       SUM = SUM + VEC(I)
> 10    CONTINUE
>
> A data cache is *no use at all* for this problem.  You will get a
> cache miss on every data access.  

	Well, not quite.  True, you're going to miss on all the VEC(I)
references, but you should hit on all the I and SUM references.  Rewrite
the loop as follows (and no, I don't care if I got the test-at-the-top or
test-at-the-bottom backwards)

10	IF (I .GT. 1000000) GOTO 20	# fetch I
	I = I + 1			# fetch I, store I
	SUM = SUM + VEC(I)		# fetch I, fetch SUM, store SUM
	GOTO 10
20	CONTINUE

and you can count 4 data fetches (as pseudo-commented above), all of which
should be cache hits.  This far outnumbers the 1 cache miss per loop due to
the main data array accesses.  Some FORTRAN compilers have even been known
to put constants in memory and refer to them indirectly; if that is true
here, you get another two cache hits for the 1 and the 1000000.  You could
argue, of course, that I and SUM might be in registers, but that's a whole
other argument.
-- 
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
{att,philabs,cmcl2,rutgers,hombre}!phri!roy -or- roy@alanine.phri.nyu.edu
"The connector is the network"