__builtin_expect详解
1 问题
在内核代码的条件分支中的判断中，经常可以看到如下两条语句：
/* ...... */
if(unlikely(!something)) //表示 !something 为真 情况很少发生
/* ...... */
if(likely(something)) //表示 something 为真 情况经常发生
/* ...... */2 FAQ/LikelyUnlikely
2.1 What are they ?
In Linux kernel code, one often find calls to likely() and unlikely(), in conditions, like :
bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx);
if (unlikely(!bvl)) {
mempool_free(bio, bio_pool);
bio = NULL;
goto out;
}In fact, these functions are hints for the compiler that allows it to correctly optimize the branch, by knowing which is the likeliest one. The definitions of these macros, found in include/linux/compiler.h are the following :
#define likely(x) __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 0)The GCC documentation explains the role of __builtin_expect() :
-- Built-in Function: long __builtin_expect (long EXP, long C)
You may use `__builtin_expect' to provide the compiler with branch
prediction information. In general, you should prefer to use
actual profile feedback for this (`-fprofile-arcs'), as
programmers are notoriously bad at predicting how their programs
actually perform. However, there are applications in which this
data is hard to collect.
The return value is the value of EXP, which should be an integral
expression. The value of C must be a compile-time constant. The
semantics of the built-in are that it is expected that EXP == C.
For example:
if (__builtin_expect (x, 0))
foo ();
would indicate that we do not expect to call `foo', since we
expect `x' to be zero. Since you are limited to integral
expressions for EXP, you should use constructions such as
if (__builtin_expect (ptr != NULL, 1))
error ();
when testing pointer or floating-point values.2.2 How does it optimize things ?
It optimizes things by ordering the generated assembly code correctly, to optimize the usage of the processor pipeline. To do so, they arrange the code so that the likeliest branch is executed without performing any jmp instruction (which has the bad effect of flushing the processor pipeline).
To see how it works, let’s compile the following simple C user space program with gcc -O2 :
#define likely(x) __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 0)
int main(char *argv[], int argc)
{
int a;
/* Get the value from somewhere GCC can't optimize */
a = atoi (argv[1]);
if (unlikely (a == 2))
a++;
else
a--;
printf ("%d\n", a);
return 0;
}Now, disassemble the resulting binary using objdump -S (comments added by me) :
080483b0 :
// Prologue
80483b0: 55 push %ebp
80483b1: 89 e5 mov %esp,%ebp
80483b3: 50 push %eax
80483b4: 50 push %eax
80483b5: 83 e4 f0 and $0xfffffff0,%esp
// Call atoi()
80483b8: 8b 45 08 mov 0x8(%ebp),%eax
80483bb: 83 ec 1c sub $0x1c,%esp
80483be: 8b 48 04 mov 0x4(%eax),%ecx
80483c1: 51 push %ecx
80483c2: e8 1d ff ff ff call 80482e4 @plt>
80483c7: 83 c4 10 add $0x10,%esp
// Test the value
80483ca: 83 f8 02 cmp $0x2,%eax
// --------------------------------------------------------
// If 'a' equal to 2 (which is unlikely), then jump,
// otherwise continue directly, without jump, so that it
// doesn't flush the pipeline.
// --------------------------------------------------------
80483cd: 74 12 je 80483e1 0x31>
80483cf: 48 dec %eax
// Call printf
80483d0: 52 push %edx
80483d1: 52 push %edx
80483d2: 50 push %eax
80483d3: 68 c8 84 04 08 push $0x80484c8
80483d8: e8 f7 fe ff ff call 80482d4 @plt>
// Return 0 and go out.
80483dd: 31 c0 xor %eax,%eax
80483df: c9 leave
80483e0: c3 ret Now, in the previous program, replace the unlikely() by a likely(), recompile it, and disassemble it again (again, comments added by me) :
080483b0 :
// Prologue
80483b0: 55 push %ebp
80483b1: 89 e5 mov %esp,%ebp
80483b3: 50 push %eax
80483b4: 50 push %eax
80483b5: 83 e4 f0 and $0xfffffff0,%esp
// Call atoi()
80483b8: 8b 45 08 mov 0x8(%ebp),%eax
80483bb: 83 ec 1c sub $0x1c,%esp
80483be: 8b 48 04 mov 0x4(%eax),%ecx
80483c1: 51 push %ecx
80483c2: e8 1d ff ff ff call 80482e4 @plt>
80483c7: 83 c4 10 add $0x10,%esp
// --------------------------------------------------
// If 'a' equal 2 (which is likely), we will continue
// without branching, so without flusing the pipeline. The
// jump only occurs when a != 2, which is unlikely.
// ---------------------------------------------------
80483ca: 83 f8 02 cmp $0x2,%eax
80483cd: 75 13 jne 80483e2 0x32>
// Here the a++ incrementation has been optimized by gcc
80483cf: b0 03 mov $0x3,%al
// Call printf()
80483d1: 52 push %edx
80483d2: 52 push %edx
80483d3: 50 push %eax
80483d4: 68 c8 84 04 08 push $0x80484c8
80483d9: e8 f6 fe ff ff call 80482d4 @plt>
// Return 0 and go out.
80483de: 31 c0 xor %eax,%eax
80483e0: c9 leave
80483e1: c3 ret 2.3 How should I use it ?
You should use it only in cases when the likeliest branch is very very very likely, or when the unlikeliest branch is very very very unlikely.
3 总结
处理器采用流水线模式，有些里面有多个逻辑运算单元，系统可以提前取多条指令进行并行处理，但遇到跳转时，则需要重新取指令，这相对于不用重新去指令就降低了速度。
因此， 引入 likely 和 unlikely ，目的是增加条件分支预测的准确性，cpu 会提前装载后面的指令，遇到条件转移指令时会提前预测并装载某个分 支的指令。
unlikely 表示你可以确认该条件是极少发生的，相反 likely 表示该条件多数情况下会发生。编译器会产生相应的代码来优化 cpu 执行效率。
程序员在编写代码时可以根据判断条件发生的概率来优化处理器的取指操作。
int x, y;
if(unlikely(x > 0))
y = 1;
else
y = -1;上面的代码中 gcc 编译的指令会预先读取 y = -1 这条指令，这适合 x 的值大于 0 的概率比较小的情况。 如果 x 的值在大部分情况下是大于 0 的，就应该用 likely(x > 0)，这样编译出的指令是预先读取 y = 1 这条指令了。这样系统在运行时就会减少重新取指了。
参考文献：
[1] https://kernelnewbies.org/FAQ/LikelyUnlikely 作者：不详
[2] http://my.oschina.net/moooofly/blog/175019 作者：摩云飞