AMD x86 manual Deriving Loop, Control For Partially, Unrolled Loops, Example 1 rolled loop

AMD Athlon™ Processor x86 Code Optimization 22007E/0 — November 1999

	no fa st er t h an three it era t ions in 1 0 cy c l es, or 6/ 10
	floating-point adds per cycle, or 1.4 times as fast as the original
	loop.
Deriving Loop	A frequently used loop construct is a counting loop. In a typical
Control For Partially	case, the loop count starts at some lower bound lo, increases by
Unrolled Loops	some fixed, positive increment inc for each iteration of the
	loop, and may not exceed some upper bound hi. The following
	example shows how to partially unroll such a loop by an
	unrolling factor of fac, and how to derive the loop control for
	the partially unrolled version of the loop.
	Example 1 (rolled loop):
	for (k = lo; k <= hi; k += inc) {
	x[k] =
	...
	}
	Example 2 (partially unrolled loop):
	for (k = lo; k <= (hi - (fac-1)*inc); k += fac*inc) {
	x[k] =
	...
	x[k+inc] =
	...
	...
	x[k+(fac-1)*inc] =
	...
	}
	/* handle end cases */
	for (k = k; k <= hi; k += inc) {
	x[k] =
	...
	}

70 Unrolling Loops