C++ C由四个8位整数组成无符号32位整数

您建议的单个表达式产生15条指令，而不是20条指令 如果我对反汇编的解释是正确的-很好！

不-这是代码中未定义/实现定义的行为的问题，如果编写正确，则无关紧要.我还建议在参数中使用指针表示法(因为C将数组作为指针传递)，如果函数不更改它们，则将参数声明为const.它帮助编译器进行优化(甚至从const correctness中提取)

uint32_t foo(const uint8_t *buf)
{
    uint32_t large = 0;
    large |= ((uint32_t)buf[0]) << 24;
    large |= ((uint32_t)buf[1]) << 16;
    large |= (uint32_t)buf[2] << 8;
    large |= buf[3] << 0;
    return large;
}


uint32_t bar(const uint8_t *buf)
{
    return (uint32_t) buf[0] << 24 | (uint32_t) buf[1] << 16 | (uint32_t) buf[2] << 8 | buf[3];
}

两者都会生成相同的机器码:

foo:
.L__stack_usage = 0
        mov r30,r24
        mov r31,r25
        ld r22,Z
        ldd r23,Z+1
        ldd r24,Z+2
        ldd r25,Z+3
        rcall __bswapsi2
        ret
bar:
.L__stack_usage = 0
        mov r30,r24
        mov r31,r25
        ld r22,Z
        ldd r23,Z+1
        ldd r24,Z+2
        ldd r25,Z+3
        rcall __bswapsi2
        ret

https://godbolt.org/z/b7o4114EP个

另外，AVR编译器假定为little endian，您可以从大端表示法"合成"uint32_t数字.

如果字符顺序匹配，我会建议使用memcpy

memcpy(&large, buff, sizeof(large));

优化编译器不会调用memcpy

uint32_t bar(uint8_t buf[4])
{
    uint32_t large;
    memcpy(&large, buf, sizeof(large));
    return large
}

bar:
.L__stack_usage = 0
        mov r30,r24
        mov r31,r25
        ld r22,Z
        ldd r23,Z+1
        ldd r24,Z+2
        ldd r25,Z+3
        ret

但更有趣的是，如果buf数据是大端字节序，则使用100s可以使代码更高效

uint32_t foo(const uint8_t *buf)
{
    union 
    {
        uint32_t large;
        uint8_t small[4];
    }d = {.small = {[0] = buf[0], [1] = buf[1], [2] = buf[2], [3] = buf[3]}};
    return d.large;
}

uint32_t bar(const uint8_t *buf)
{
    union 
    {
        uint32_t large;
        uint8_t small[4];
    }d = {.small = {[0] = buf[3], [1] = buf[2], [2] = buf[1], [3] = buf[0]}};
    return d.large;
}

生成的代码如下:

foo:
.L__stack_usage = 0
        mov r30,r24
        mov r31,r25
        ldd r23,Z+1
        ld r22,Z
        ldd r24,Z+2
        ldd r25,Z+3
ret
bar:
.L__stack_usage = 0
        mov r30,r24
        mov r31,r25
        ldd r23,Z+2
        ldd r22,Z+3
        ldd r24,Z+1
        ld r25,Z
ret