C 表达式中的汇编指令

asm asm-qualifiers ( AssemblerTemplate 
     : OutputOperands 
     [ : InputOperands
     [ : Clobbers ] ])

asm asm-qualifiers ( AssemblerTemplate 
      : 
      : InputOperands
      : Clobbers
      : GotoLabels)

asm("inc %0\n\tinc %0" : "=r"(res) : "0"(res));

/APP
# 11 "asm.c" 1
  inc %rax
  inc %rax
# 0 "" 2
/NO_APPs

[ [asmSymbolicName] ] constraint (cvariablename)

int64_t res;
int64_t num = 1;

asm("movq %[num], %[res]" : [res] "=r"(res) : [num] "m"(num));
asm("movq %1, %0" : "=r"(res) : "m"(num));
asm("movq %1, %0" : "=m"(res) : "m"(num));
asm("movq %1, %0" : "=r"(res) : "r"(num));

// 对应的汇编代码, 只保留asm表达式中的代码
# 13 "asm.c" 1
  movq -16(%rbp), %rax // asm-1
 # 0 "" 2
/NO_APP

/APP
 # 15 "asm.c" 1
  movq -16(%rbp), %rax // asm-2
 # 0 "" 2
/NO_APP

/APP
 # 17 "asm.c" 1
  movq -16(%rbp), -8(%rbp) // asm-3
 # 0 "" 2
/NO_APP

/APP
 # 19 "asm.c" 1
  movq %rax, %rax // asm-4
 # 0 "" 2
/NO_APP

[ [asmSymbolicName] ] constraint (cexpression)

int64_t res = 3;
int64_t num = 1;
asm("addq %1, %0" : "=g"(res) : "0"(num));

// 输入输出位置相同
  movq $3, -8(%rbp)
  movq $1, -16(%rbp)
  movq -16(%rbp), %rax
/APP
# 32 "asm.c" 1
  addq %rax, %rax
# 0 "" 2
/NO_APP

 ```c
 asm volatile("movc3 %0, %1, %2"
   : /* No outputs. */
   : "r"(from), "r"(to), "g"(count)
   : "%rbx", "%rcx", "%rdx", "memory");

 /APP
 # 25 "asm.c" 1
   movc3 %rax, %r8, -72(%rbp)
 # 0 "" 2
 /NO_APP
 ```

 可以看到使用到了 rax 寄存器，然后修改程序在 Clobbers 增加 %rax, 结果如下

 ```c
 asm volatile("movc3 %0, %1, %2"
   : /* No outputs. */
   : "r"(from), "r"(to), "g"(count)
   : "%rax", "%rbx", "%rcx", "%rdx", "memory");

 /APP
 # 25 "asm.c" 1
   movc3 %r8, %r9, -72(%rbp)
 # 0 "" 2
 /NO_APP
 ```

// 避免编译器的优化，声明此处内存可能发生破坏
#define barrier() asm volatile("" ::: "memory")
// 在32位的CPU下，lock 指令为锁总线，加上一条内存操作指令就达到了内存屏障的作用，64位的cpu已经有新增的 *fence 指令可以使用
// mb() 执行一个内存屏障作用的指令，为指定CPU操作；破坏列表声明 cc memory 指示避免编译器进行优化
#ifdef CONFIG_X86_32
#define mb() asm volatile(ALTERNATIVE("lock; addl $0,-4(%%esp)", "mfence", \
        X86_FEATURE_XMM2) ::: "memory", "cc")
#define rmb() asm volatile(ALTERNATIVE("lock; addl $0,-4(%%esp)", "lfence", \
        X86_FEATURE_XMM2) ::: "memory", "cc")
#define wmb() asm volatile(ALTERNATIVE("lock; addl $0,-4(%%esp)", "sfence", \
        X86_FEATURE_XMM2) ::: "memory", "cc")
#else
#define mb() asm volatile("mfence":::"memory")
#define rmb() asm volatile("lfence":::"memory")
#define wmb() asm volatile("sfence" ::: "memory")
#endif

DECLARE_PER_CPU(struct task_struct *, current_task);

#define this_cpu_read_stable(var) percpu_stable_op("mov", var)

static __always_inline struct task_struct *get_current(void)
{
  return this_cpu_read_stable(current_task);
}

#define percpu_stable_op(op, var)   \
({       \
  typeof(var) pfo_ret__;    \
  switch (sizeof(var)) {    \
  case 8:      \
    asm(op "q "__percpu_arg(P1)",%0" \
    : "=r" (pfo_ret__)   \
    : "p" (&(var)));   \
    break;     \
  }      \
  pfo_ret__;     \
})

asm("mov" "q ""%%""gs" ":" "%" "P1"",%0" : "=r" (pfo_ret__) : "p" (&(current_task)));
// 变换一下为
asm("movq %%gs:%P1, %0" : "=r"(pfo_ret__) : "p"(&(current_task)));