Re: [Caml-list] Alignment of data

[Goswin von Brederlow <goswin-v-b@web.de>]

Christophe Papazian <christophe.papazian@gmail.com> writes:

> Dear users and developers of OCAML,
>
> I am working on some ppc architecture, and I realize that I have a
> (very) big slowdown due to bad alignment of data by ocamlopt. I need
> to have my data aligned in memory depending of the size of the data :
> floats are to be aligned on 8 bytes, int on 4 bytes, etc....
> BUT, after verification, I remark that ocamlopt doesn't align as I
> need. I tried to use ARCH_ALIGN_DOUBLE, but it doesn't seem to be what
> I thought, and doesn't change anything for my needs. Is there ANY way
> to obtain what I need easily or at least quickly ?
>
> You can use the following code to test your alignment on your
> architecture :
> [ compile with ocamlopt align_stubs.c align.ml -o align ]
>
> ######### align.ml #########
> open Obj
>
> external get_addr : 'a -> int * string = "get_addr"
>
> let rec align acc r =
>   if r mod 2 = 1 then acc else align (acc*2) (r/2)
>
> let get_addr_print v = let a,b = get_addr v in Printf.printf "%6X %s
> \n" a b; a

That will cut of the upper bits of my address. Not important for
alignment but bad practice.

> let rec get_align acc = function
>     h::q as l -> get_align (acc lor get_addr_print l) q
>   | [] -> acc
>
> let  f block s l =
>   let r =
>     if block then (* if the element is a block, consider it like a
> pointer *)
>       List.fold_left (fun r e -> r lor get_addr_print e) 0 l
>     else get_align 0 l
>   in
>   Printf.printf "%s are aligned on %i bytes\n%!" s (align 1 r)
>
> let build_list v l = List.map (fun i -> Array.make i v) l
>
> let main =
>   f false "Chars" ['a';'b';'c';'d';'e'];
>   f false "Integers" [0;1;2;3;4];
>   f true "Floats" [0.;1./.3.;2./.5.;3./.7.;4./.9.];
>   f true "Int Arrays" (build_list  37 [3;4;5;6;7]);
>   f true "Float Arrays" (build_list  (1./.3.) [2;3;4;5;6]);
>   f true "Other Float Arrays" [Array.make 1 max_float;Array.make 2
> 0.;Array.make 3 0.;Array.make 37 0.;Array.make 17 0.];
>
> ####### align_stubs.c ########
>
> #include <stdio.h>
>
> #include <caml/memory.h>
> #include <caml/mlvalues.h>
> #include <caml/custom.h>
> #include <caml/alloc.h>
>
> CAMLprim
> value get_addr(value v)
> {
>   CAMLparam1 (v);
>   char *repr = malloc(9);
>   value res = alloc_tuple(2);
>   Field(res,0) = Val_int((unsigned int) v);
>   sprintf(repr,"%8X", *((int*)v));

Again cutting of upper bits. I have a 64bit cpu so up to 16 hex digits
for an address.

>   Field(res,1) = (caml_copy_string(repr));
>   CAMLreturn(res);
> }
>
> ######### Results ##########
>
>  1D8C0       C3
>  1D8CC       C5
>  1D8D8       C7
>  1D8E4       C9
>  1D8F0       CB
> Chars are aligned on 4 bytes
>  1D878        1
>  1D884        3
>  1D890        5
>  1D89C        7
>  1D8A8        9
> Integers are aligned on 4 bytes
>  1D85C        0
>  7612C 55555555
>  76114 9999999A
>  760FC DB6DB6DB
>  760E4 1C71C71C
> Floats are aligned on 4 bytes
>  74A2C       4B
>  74A18       4B
>  74A00       4B
>  749E4       4B
>  749C4       4B
> Int Arrays are aligned on 4 bytes
>  732C0 55555555
>  732A4 55555555
>  73280 55555555
>  73254 55555555
>  73220 55555555
> Float Arrays are aligned on 4 bytes
>  71928 FFFFFFFF
>  71940        0
>  71960        0
>  71988        0
>  71AC0        0
> Other Float Arrays are aligned on 8 bytes
>
> You can see the addresses in memory of each element of the lists and
> it's internal representation (to check
> if the memory pointer really point to the right value : you can even
> see that 31 bit ocaml integer (and Chars) i have a C representation of
> 2*i+1).
> It seems that small values
> are on the minor heap, and large values are on major heap.
> Note that the last array is correctly aligned, but it's just a matter
> of luck : If I change
> something else before this line in my code, I usually get the last
> array aligned on 4 bytes.
> (But I can't find a way to obtain a float array aligned on 8 bytes
> with the use of "build_list")

Everything is aligned to a value. I don't think there is a special alloc
call for the GC that gives you double alignement. Nothing in
caml/alloc.h anyway.

> Si if you have any idea of how to get floats and floats arrays aligned
> on 8 bytes both on major and minor heap, please answer me !
>
> Thank you very much
>
> 	Christophe

You need to write a new function

CAMLextern value caml_alloc_double_array (mlsize_t),

or similar that ensures alignment on 8 byte for double even for 32bit
systems.

You should also check the CAMLextern value caml_copy_double (double);
that it does the same.


An alternative might be to use a Bigarray.

MfG
        Goswin