[转] Kernel : likely/unlikely macros

Ever wondered what the likely and unlikely macros in the linux kernel are ?

The macros are defined as :

#define likely(x)       __builtin_expect((x),1)
#define unlikely(x)     __builtin_expect((x),0)

The __builtin_expect is a method that gcc (versions >= 2.96) offer for programmers to indicate branch prediction information to the compiler. The return value of __builtin_expect is the first argument (which could only be an integer) passed to it.

To check it out how it could be beneficial, an excerpt from "info gcc" :

     if (__builtin_expect (x, 0))
                foo ();


     [This] would indicate that we do not expect to call `foo', since we
     expect `x' to be zero. 

Based on this information the compiler generates intelligent code, such that the most expected result is favored.

---

Let us consider it with a simple example function :

[kedar@ashwamedha ~]$ cat abc.c
int
testfun(int x)
{
        if(__builtin_expect(x, 0)) {
                              ^^^--- We instruct the compiler, "else" block is more probable
                x = 5;
                x = x * x;
        } else {
                x = 6;
        }
        return x;
}
 
[kedar@ashwamedha ~]$ gcc -O2 -c abc.c
[kedar@ashwamedha ~]$ objdump  -d abc.o
 
abc.o:     file format elf32-i386
 
Disassembly of section .text:
 
00000000 :
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   8b 45 08                mov    0x8(%ebp),%eax
   6:   85 c0                   test   %eax,%eax
   8:   75 07                   jne    11 < testfun+0x11 >
                                ^^^ --- The compiler branches the "if" block and keeps "else" sequential
   a:   b8 06 00 00 00          mov    $0x6,%eax
   f:   c9                      leave
  10:   c3                      ret
  11:   b8 19 00 00 00          mov    $0x19,%eax
  16:   eb f7                   jmp    f < testfun+0xf >

And let us see what happens if we make the "if" block more likely.

[kedar@ashwamedha ~]$ cat abc.c
int
testfun(int x)
{
        if(__builtin_expect(x, 1)) {
                              ^^^ --- We instruct the compiler, "if" block is more probable
                x = 5;
                x = x * x;
        } else {
                x = 6;
        }
        return x;
}
                                                                                                    
[kedar@ashwamedha ~]$ gcc -O2 -c abc.c
[kedar@ashwamedha ~]$ objdump  -d abc.o
                                                                                                    
abc.o:     file format elf32-i386
                                                                                                    
Disassembly of section .text:
                                                                                                    
00000000 :
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   8b 45 08                mov    0x8(%ebp),%eax
   6:   85 c0                   test   %eax,%eax
   8:   74 07                   je     11 < testfun+0x11 >
                                ^^^ --- The compiler branches the "else" block and keeps "if" sequential 
   a:   b8 19 00 00 00          mov    $0x19,%eax
   f:   c9                      leave
  10:   c3                      ret
  11:   b8 06 00 00 00          mov    $0x6,%eax
  16:   eb f7                   jmp    f < testfun+0xf >



likely和unlikely是用来编译优化的，其实都可以没有。
我们知道很多cpu里面有告诉缓存，且有预读机制，likely和unlikely就是增加执行判断语句时的命中率
如果是if(lilely(a))，说明a条件发生的可能性大，那么a为真的语句在编译成二进制的时候就应该紧跟在前面程序的后面，这样就会被cache预读取进去，增加程序执行速度。 unlikely则是正好相反。

kernel里面的定义：

#ifndef __LINUX_COMPILER_H
#define __LINUX_COMPILER_H

/* Somewhere in the middle of the GCC 2.96 development cycle, we implemented
    a mechanism by which the user can annotate likely branch directions and
    expect the blocks to be reordered appropriately.   Define __builtin_expect
    to nothing for earlier compilers.   */

#if __GNUC__ == 2 && __GNUC_MINOR__ < 96
#define __builtin_expect(x, expected_value) (x)
#endif

#define likely(x) __builtin_expect((x),1)
#define unlikely(x) __builtin_expect((x),0)

#endif /* __LINUX_COMPILER_H */

可以看到在2.96以前版本中，gcc并没有实现lilely和unlikely。在gcc以后版本中，__builtin_expect是gcc的内置函数。