likely/unlikely macros in Linux Kernel

http://kerneltrap.org/node/4705

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 <>
  ^^^ --- 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 <>

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 <>
  ^^^ --- 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 <>