Re: [Caml-list] obj magic performance and other questions

[Guillaume Yziquel <guillaume.yziquel@citycable.ch>]

Le Tuesday 15 Mar 2011 à 12:05:31 (-0400), Nicolas Ojeda Bar a écrit :
> Hello,
> 
> This is not very efficient. I can use Obj.magic to access that same field as
> follows:
> 
> let rec2_b x =
> Obj.obj (Obj.field (Obj.field (Obj.repr x.children 0) 0))

Seems that you messed up some parentheses.

> My question is: is this use of Obj safe? and is this compiled to (two) simple
> array accesses? Or is Obj.obj doing something behind the scenes?

yziquel@seldon:~/sandbox/ocaml$ cat a.ml 
let f x = Obj.obj (Obj.field (Obj.field (Obj.repr x) 0) 0)
yziquel@seldon:~/sandbox/ocaml$ ocamlopt -c -S -dlambda a.ml 
(seq
  (let
    (f/1030
       (function x/1031
         (id (array.unsafe_get (array.unsafe_get (id x/1031) 0) 0))))
    (setfield_imm 0 (global A!) f/1030))
  0a)

So it should be compiled to array accesses. However, they are not so
simple array accesses due to OCaml data layout. Moreover, there's this
'id' around. Let's see:

yziquel@seldon:~/sandbox/ocaml$ cat a.s

[...]

camlA__f_1030:
	subq	$8, %rsp
.L103:
	movq	%rax, %rdi
	movzbq	-8(%rdi), %rax

So the stack has been managed, and the %rax registers now contains the
tag of the 'x' argument to 'f'.

	cmpq	$254, %rax

This checks that the tag is or isn't the tag of an array of doubles.
(Obj.double_array_tag is 254). So we shouldn't jump on the ext 'je'.

	je	.L102
	movq	(%rdi), %rbx

The register %rbx now contains the first field of 'x'.

	jmp	.L101

[...]

.L101:
	movzbq	-8(%rbx), %rax

This looks at the tag of the first field of 'x', and checks below if it
is an array of doubles.

	cmpq	$254, %rax
	je	.L100
	movq	(%rbx), %rax

%rax now contains the first field of the first field of 'x'.

	addq	$8, %rsp
	ret

And 'f' returns with what you want.

So, aside from checking if tag is 254, it's essentially two direct array
accesses, and the 'id' function of Obj.repr and Obj.obj is erased.

	external repr : 'a -> t = "%identity"
	external obj : t -> 'a = "%identity"
	external field : t -> int -> t = "%obj_field"

You could probably hack the compiler to avoid the tag checking if you
really wanted to avoid the 254 test. That'd require another "%..." to be
implemented I guess.

> Thanks!
> N

-- 
     Guillaume Yziquel