From: Xavier Leroy <Xavier.Leroy@inria.fr>
To: Goswin von Brederlow <goswin-v-b@web.de>
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] How to read different ints from a Bigarray?
Date: Wed, 28 Oct 2009 18:09:13 +0100 [thread overview]
Message-ID: <4AE87AB9.5020607@inria.fr> (raw)
In-Reply-To: <87eiond3of.fsf@frosties.localdomain>
Goswin von Brederlow wrote:
> I'm working on binding s for linux libaio library (asynchron IO) with
> a sharp eye on efficiency. That means no copying must be done on the
> data, which in turn means I can not use string as buffer type.
>
> The best type for this seems to be a (int, int8_unsigned_elt,
> c_layout) Bigarray.Array1.t. So far so good.
That's a reasonable choice.
> Now I define helper functions:
>
> let get_uint8 buf off = buf.{off}
> let set_uint8 buf off x = buf.{off} <- x
>
> But I want more:
>
> get/set_int8 - do I use Obj.magic to "convert" to int8_signed_elt?
Not at all. If you ask OCaml's typechecker to infer the type of
get_uint8, you'll see that it returns a plain OCaml "int" (in the
0...255 range). Likewise, the "x" parameter to "set_uint8" has type
"int" (of which only the 8 low bits are used).
Repeat after me: "Obj.magic is not part of the OCaml language".
> And endian correcting access for larger ints:
>
> get/set_big_uint16
> get/set_big_int16
> get/set_little_uint16
> get/set_little_int16
> get/set_big_uint24
> ...
> get/set_little_int56
> get/set_big_int64
> get/set_little_int64
The "56" functions look like a bit of overkill to me :-)
> What is the best way there? For uintXX I can get_uint8 each byte and
> shift and add them together. But that feels inefficient as each access
> will range check
Not necessarily. OCaml 3.11 introduced unchecked accesses to
bigarrays, so you can range-check yourself once, then perform
unchecked accesses. Use with caution...
> and the shifting generates a lot of code while cpus
> can usualy endian correct an int more elegantly.
>
> Is it worth the overhead of calling a C function to write optimized
> stubs for this?
The only way to know is to benchmark both approaches :-( My guess is
that for 16-bit accesses, you're better off with a pure Caml solution,
but for 64-bit accesses, a C function could be faster.
- Xavier Leroy
next prev parent reply other threads:[~2009-10-28 17:10 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-10-28 13:54 Goswin von Brederlow
2009-10-28 14:16 ` Sylvain Le Gall
2009-10-28 15:00 ` [Caml-list] " Goswin von Brederlow
2009-10-28 15:17 ` Sylvain Le Gall
2009-10-28 17:57 ` [Caml-list] " Goswin von Brederlow
2009-10-28 18:19 ` Sylvain Le Gall
2009-10-28 21:05 ` [Caml-list] " Goswin von Brederlow
2009-10-28 21:26 ` Sylvain Le Gall
2009-10-28 22:48 ` [Caml-list] " blue storm
2009-10-29 9:50 ` Goswin von Brederlow
2009-10-29 10:34 ` Goswin von Brederlow
2009-10-29 12:20 ` Richard Jones
2009-10-29 17:07 ` Goswin von Brederlow
2009-10-30 20:30 ` Richard Jones
2009-11-01 15:11 ` Goswin von Brederlow
2009-11-01 19:57 ` Richard Jones
2009-11-02 16:11 ` Goswin von Brederlow
2009-11-02 16:33 ` Mauricio Fernandez
2009-11-02 20:27 ` Richard Jones
2009-11-03 13:18 ` Goswin von Brederlow
2009-11-02 20:48 ` Goswin von Brederlow
2009-10-29 20:40 ` Florian Weimer
2009-10-29 21:04 ` Gerd Stolpmann
2009-10-29 23:43 ` Goswin von Brederlow
2009-10-30 0:48 ` Gerd Stolpmann
2009-10-29 23:38 ` Goswin von Brederlow
2009-10-28 15:37 ` [Caml-list] " Olivier Andrieu
2009-10-28 16:05 ` Sylvain Le Gall
2009-10-28 15:43 ` [Caml-list] " Gerd Stolpmann
2009-10-28 16:06 ` Sylvain Le Gall
2009-10-28 18:09 ` [Caml-list] " Goswin von Brederlow
2009-10-28 17:09 ` Xavier Leroy [this message]
2009-10-28 19:05 ` Goswin von Brederlow
2009-10-29 17:05 ` Goswin von Brederlow
2009-10-29 18:42 ` Christophe TROESTLER
2009-10-29 19:03 ` Goswin von Brederlow
2009-10-29 18:48 ` Sylvain Le Gall
2009-10-29 23:25 ` [Caml-list] " Goswin von Brederlow
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