Performance questions, -inline, ...

[Kuba Ober <ober.14@osu.edu>]

I haven't looked at assembly output yet, but I've run into some unexpected
behavior in my benchmarks.

This was compiled by ocamlopt -inline 100 -unsafe, the results and code are 
below (MIPS is obtained by dividing 50 million iterations by (Unix.times 
()) . Unix.tms_utime it took to run). I haven't included the timing etc. code 
(it's part of a larger benchmark).

What I wonder is why vector-to-vector add is so much faster than (constant) 
scalar to vector add. Vectors are preinitialized each time with a 1.0000, 
1.0001, ... sequence.

Also, the very bad performance from generic vector-to-vector *with* inlining 
is another puzzler, whereas generic add of scalar-to-scalar performs 
similarly to straight-coded one.

Cheers, Kuba

* add1: add scalar to scalar   120 MIPS
* add3: add scalar to vector   250 MIPS
* add5: add vector to vector   320 MIPS
* add2: generic add scalar to scalar   100 MIPS
* add4: generic add vector to vector   38 MIPS

let start = 1.3

(* generic scalar operation *)
let op1 op const nloop =
	let accum = ref start in
	for i = 1 to nloop do
		accum := op !accum const
	done

(* generic vector operation *)
let op2 op const a b (nloop : int) =
	let len = Array.length a in
	for j = 0 to len-1 do
		for i = 0 to len-1 do
			b.(i) <- op a.(i) b.(i)
		done;
	done

(** addition **)
let add1 nloop =
	let accum = ref start in
	for i = 1 to nloop do
		accum := !accum +. addconst
	done
let add2 = op1 ( +. ) addconst
let add3 a b nloop =
	let len = Array.length a in
	for j = 0 to len-1 do
		for i = 0 to len-1 do
			b.(i) <- a.(i) +. addconst
		done;
	done
let add4 = op2 ( +. ) addconst
let add5 a b nloop =
	let len = Array.length a in
	for j = 0 to len-1 do
		for i = 0 to len-1 do
			b.(i) <- a.(i) +. b.(i)
		done;
	done