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 <>
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