C++关键字之likely和unlikely详解_C 语言

#define likely(x) __builtin_expect(!!(x), 1) 
#define unlikely(x) __builtin_expect(!!(x), 0)

#include <cstdio>

#define likely(x)       __builtin_expect(!!(x), 1)
#define unlikely(x)     __builtin_expect(!!(x), 0)

int main(int argc, char *argv[])
{
    if (unlikely(argc > 0)) {
        puts ("Positive\n");
    } else
    {
        puts ("Zero or Negative\n");
    }
    return 0;
}

.LC0:
        .string "Positive\n"
.LC1:
        .string "Zero or Negative\n"
main:
        sub     rsp, 8
        test    edi, edi                
        jle     .L2                     ; 如果argc <= 0, 跳转到L2
        mov     edi, OFFSET FLAT:.LC0   ; 如果argc > 0, 从这里执行
        call    puts
.L3:
        xor     eax, eax
        add     rsp, 8
        ret
.L2:
        mov     edi, OFFSET FLAT:.LC1
        call    puts
        jmp     .L3

.LC0:
        .string "Positive\n"
.LC1:
        .string "Zero or Negative\n"
main:
        sub     rsp, 8
        test    edi, edi
        jg      .L6
        mov     edi, OFFSET FLAT:.LC1
        call    puts
.L3:
        xor     eax, eax
        add     rsp, 8
        ret
.L6:
        mov     edi, OFFSET FLAT:.LC0
        call    puts
        jmp     .L3

int foo(int i) {
    switch(i) {
               case 1: handle1();
                       break;
    [[likely]] case 2: handle2();
                       break;
    }
}