Re: likely, unlikely and builtin_expect()

[JuanJo Ciarlante <jjo@xxxxxxxxxxxxxx>]

Hola Diego!

On Sat, Oct 07, 2006 at 11:36:37AM -0300, Diego Woitasen wrote:
> I understand the meaning of likely() and unlikely() in linux/compiler.h,
> but I don't undertand the meaning of !! in the definition.
> 
> #define likely(x)       __builtin_expect(!!(x), 1)
> #define unlikely(x)     __builtin_expect(!!(x), 0)
> 
> why not this?:
> 
> #define likely(x)       __builtin_expect((x), 1)
> #define unlikely(x)     __builtin_expect((x), 0)

Think of __builtin_expect as a comparison, then
 (x)==1  will be true only for x being exactly 1 , but
!!(x)==1 will be true  any x!=0 (those '!' act as
"booleanizers"), and you want [un]likely to follow
same C lang rules (TRUE is anything but 0).

Regards!
-- 
--Juanjo

#  Juan Jose Ciarlante (JuanJo) jjo ;at; mendoza.gov.ar                     #
#  GnuPG Public Key: gpg --keyserver wwwkeys.eu.pgp.net --recv-key 66727177 #
#   Key fingerprint: 0D2F 3E5D 8B5C 729E 0560  F453 A3F7 E249 6672 7177     #

--
Kernelnewbies: Help each other learn about the Linux kernel.
Archive:       http://mail.nl.linux.org/kernelnewbies/
FAQ:           http://kernelnewbies.org/faq/