c - 压缩结构位域与#define'd 位掩码

Question

我有一个 AVR 程序，它在一个静态状态变量中存储一组(通常少于 8 个)位标志(它包含在一个包含模块的各种其他状态字段的结构中)。

如果这样做效率更高或更低:

#define STATUS_ENABLED 0x01

struct DeviceState {
    uint8_t status;
}

static struct DeviceState myState;

//and somewhere in the program...
myState.status |= STATUS_ENABLED;

或者用打包的位域来做:

struct DeviceState {
    uint8_t enabled : 1;
}

static struct DeviceState myState;

//and somewhere in the program...
myState.enabled = 1; // could use TRUE/FALSE if defined

Answer 1

对于 avr-gcc 4.3.3，似乎在实现上没有区别:

#define STATUS_ENABLE

struct DeviceState {
    uint8_t status;
    uint8_t enabled : 1;
}

static struct DeviceState myState;

//and this code...
myState.status |= STATUS_ENABLED;
myState.enabled = 1;

生成以下汇编代码:

    myState.status |= STATUS_ENABLE;
00003746  LDS R24,0x20B5            Load direct from data space 
00003748  ORI R24,0x01              Logical OR with immediate 
00003749  STS 0x20B5,R24            Store direct to data space 

    myState.enabled = TRUE;
0000374B  LDS R24,0x20B4            Load direct from data space 
0000374D  ORI R24,0x01              Logical OR with immediate 
0000374E  STS 0x20B4,R24            Store direct to data space 

所以相同的指令(除了地址!)。