Linux 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.cinttestfun(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.cinttestfun(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 >