c - Packed struct bitfield vs #define'd bitmasks

Question

I have an AVR program which stores a set (usually less than 8) bit flags in a static status variable (which is contained inside a struct holding various other state fields of the module).

if it more or less efficient to do it like this:

#define STATUS_ENABLED 0x01

struct DeviceState {
    uint8_t status;
}

static struct DeviceState myState;

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

Or do it with a packed bitfield:

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

使用 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;

产生以下汇编代码：

    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 

所以同样的指令（地址除外！）。

Answer 2

位字段版本是不可移植的，并且位的实际位置定义不明确。除此之外，应该没有区别。因此，请使用前一个版本，因为它是 100% 可移植的。

Answer 3

打包位域允许您在 8 位内存空间中存储其他变量，因此如果您要向 DeviceStatus 添加更多变量，则内存效率会更高。