Thank you for your reply. So, for example, the expression "if (likely(prev != next))" is same as "if ((prev != next) == 1)", isn't it? Then, unlikely is opposite of that. Now the things came to be clearer, thanks. And I'm sorry for my HTML mail. I didn't notice that because I used my provider's html mail site to send the mail from remote with http access. I'll care it next time. Thanks, Shinpei Kato On Thu, 04 Dec 2003 11:15:51 +0300 "Ruslan U. Zakirov" <cubic@wildgate.miee.ru> wrote: > > shinny@j02.itscom.net wrote: > > >Hi, > > > >How may I ask for your help? > >Now I'm studying the linux scheduler(sched.c). > >And I'm wondering what likey()/unlikely macros mean. > >I could find they're defined in linux/compiler.h as follows. > > > >/* > > * Generic compiler-dependent macros required for kernel > > * build go below this comment. Actual compiler/compiler version > > * specific implementations come from the above header files > > */ > > > >#define likely(x) __builtin_expect(!!(x), 1) > >#define unlikely(x) __builtin_expect(!!(x), 0) > > > >And I could find __builtin_expect()'s definition only in linux/compiler-gcc2.h as a follow. > >I don't know what's going if __GNUC_MINOR__ is over 96, though... > > > >#if __GNUC_MINOR__ < 96 > ># define __builtin_expect(x, expected_value) (x) > >#endif > > > >As a result, although it seems likely() and unlikely() do nothing, is my expectation correct? > >If there are other references about this, please let me know. > > > > > Hello. > As I've understand gcc after 96 introduced new feature which helps > programmer suggest > to compiler wishful execution branch of code. Compiler use this build in > for optimizing. > Gcc docs: > long *__builtin_expect*/ /(/long /exp/, long /c)/ / Built-in Function > > You may use |__builtin_expect| to provide the compiler with branch > prediction information. In general, you should prefer to use actual > profile feedback for this (|-fprofile-arcs|), as programmers are > notoriously bad at predicting how their programs actually perform. > However, there are applications in which this data is hard to collect. > > The return value is the value of exp, which should be an integral > expression. The value of c must be a compile-time constant. The > semantics of the built-in are that it is expected that exp == c. For > example: > > if (__builtin_expect (x, 0)) > foo (); > > > would indicate that we do not expect to call |foo|, since we expect |x| > to be zero. Since you are limited to integral expressions for exp, you > should use constructions such as > > if (__builtin_expect (ptr != NULL, 1)) > error (); > > > when testing pointer or floating-point values. > > Good luck. Ruslan. > PS: Don't send html letters in future. > > -- > Kernelnewbies: Help each other learn about the Linux kernel. > Archive: http://mail.nl.linux.org/kernelnewbies/ > FAQ: http://kernelnewbies.org/faq/ > -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
