jump label

jump label"/>

jump label

这段代码的反汇编如下所示
static inline bool test_idle_cores(int cpu, bool def)
{struct sched_domain_shared *sds;if (static_branch_unlikely(&sched_smt_present)) {sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));if (sds)return READ_ONCE(sds->has_idle_cores);}return def;
}0000000000000884 <test_idle_cores>:
{884:       12001c21        and     w1, w1, #0xff
#define JUMP_LABEL_NOP_SIZE             AARCH64_INSN_SIZEstatic __always_inline bool arch_static_branch(struct static_key *key,bool branch)
{asm_volatile_goto(888:       d503201f        nop
}88c:       2a0103e0        mov     w0, w1890:       d65f03c0        retsds = rcu_dereference(per_cpu(sd_llc_shared, cpu));894:       90000003        adrp    x3, 0 <__per_cpu_offset>898:       91000063        add     x3, x3, #0x089c:       90000002        adrp    x2, 0 <sd_llc_shared>8a0:       91000042        add     x2, x2, #0x08a4:       f860d860        ldr     x0, [x3, w0, sxtw #3]8a8:       f8606840        ldr     x0, [x2, x0]if (sds)8ac:       b4ffff00        cbz     x0, 88c <test_idle_cores+0x8>return READ_ONCE(sds->has_idle_cores);8b0:       b9400800        ldr     w0, [x0, #8]8b4:       7100001f        cmp     w0, #0x08b8:       1a9f07e1        cset    w1, ne  // ne = any8bc:       17fffff4        b       88c <test_idle_cores+0x8>为什么会是这样呢，我们看看static_branch_likely的实现	 
如果使能jump_label的话是先如下，如果没有的话就等同普通的unlikely	 
#define static_branch_unlikely(x)						\
({										\bool branch;								\if (__builtin_types_compatible_p(typeof(*x), struct static_key_true))	\branch = arch_static_branch_jump(&(x)->key, false);		\else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \branch = arch_static_branch(&(x)->key, false);			\else									\branch = ____wrong_branch_error();				\unlikely_notrace(branch);							\
})由于本例中的sched_smt_present 默认为false，所以我们看看	 arch_static_branch的实现static __always_inline bool arch_static_branch(struct static_key *key,bool branch)
{asm_volatile_goto("1:	nop					\n\t""	.pushsection	__jump_table, \"aw\"	\n\t""	.align		3			\n\t""	.long		1b - ., %l[l_yes] - .	\n\t""	.quad		%c0 - .			\n\t""	.popsection				\n\t":  :  "i"(&((char *)key)[branch]) :  : l_yes);return false;
l_yes:return true;
}可以和上面的汇编对比下。这里也可以看出static_branch_unlikely 是一个宏，会被展开，最后就是一行unlikely_notrace
可见static_branch_unlikely 是包含unlikely_notrace.
简单理解就是
仔细看代码就会发现 static_branch_likely 不等于 ！static_branch_unlikely 同样！static_branch_likely 也不等于static_branch_unlikely.
可以通过反汇编查询上面两种情况的区别. 性能上也是有差异的，取决于key的初始值是true还是false.
那什么时候用！static_branch_unlikely这个组合呢？
那就是当key是false时，static_branch_unlikely 对应的就是nop，但是代码中又不需要这个条件成立时，可以自己体会下.