From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Tue, 29 Apr 2008 11:48:32 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Subject: Re: [RFC][PATCH 5/8] memcg: optimize branches
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To: Li Zefan <lizf@cn.fujitsu.com>
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On Tue, 29 Apr 2008 10:04:45 +0800
Li Zefan <lizf@cn.fujitsu.com> wrote:

> KAMEZAWA Hiroyuki wrote:
> > Showing brach direction for obvious conditions.
> > 
> 
> Did you compare the compiled objects with and without this patch ?
> 
yes, on ia64. (...I attached the result of x86-64 just because it's widely used). 

> It seems gcc will take (ptr == NULL) as unlikely without your explicit
> anotation.
I didn't know that. But plz try this. I don't see what you say.
==
#include <stdio.h>

#define unlikely(x)     __builtin_expect(!!(x), 0)
//#define unlikely(x)   (x)

extern void *allocalter(int size);
extern void call(int arg);
void *foo(void)
{
        char *ptr;
        ptr = allocator(1024);
        if (unlikely(!ptr)) {
                call(1);
                call(2);
        } else {
                call(3);
                call(4);
        }
        return ptr;
}
==
When I compiled above with -O2 on ia64,
 - if unlikely is used , call(3),call(4) are on fast path.
 - if unlikely is not used, call(1), call(3) are on fast path.

In more obvious case, gcc will do some obvious optimization I agree.
But it's difficult for me to know how gcc will compile it in C-language level.

> And likely() and unlikely() should be used in some performance-
> critical path only ?
> 
I think it can be used against busy and obvious path. IMHO, it gives a hint
not only to a compiler, but also to a developper who read it.
 
Thanks,
-Kame

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