C / C ++位域与按位运算符相比，可以更快，更好，更便携地选择位？

Question

I need to pack some bits in a byte in this fashion:

struct  
{  
  char bit0: 1;  
  char bit1: 1;  
} a;  

if( a.bit1 ) /* etc */

or:

if( a & 0x2 ) /* etc */

From the source code clarity it's pretty obvious to me that bitfields are neater. But which option is faster? I know the speed difference won't be too much if any, but as I can use any of them, if one's faster, better.
On the other hand, I've read that bitfields are not guaranteed to arrange bits in the same order across platforms, and I want my code to be portable.

Notes: If you plan to answer 'Profile' ok, I will, but as I'm lazy, if someone already has the answer, much better.
The code may be wrong, you can correct me if you want, but remember what the point to this question is and please try and answer it too.

Answer 1

Bitfields make the code much clearer if they are used appropriately. I would use bitfields as a space saving device only. One common place I've seen them used is in compilers: Often type or symbol information consists of a bunch of true/false flags. Bitfields are ideal here since a typical program will have many thousands of these nodes created when it is compiled.

I wouldn't use bitfields to do a common embedded programming job: reading and writing device registers. I prefer using shifts and masks here because you get exactly the bits the documentation tells you you need and you don't have to worry about differences in various compilers implementation of bitfields.

As for speed, a good compiler will give the same code for bitfields that masking will.

Answer 2

I would rather use the second example in preference for maximum portability. As Neil Butterworth pointed out, using bitfields is only for the native processor. Ok, think about this, what happens if Intel's x86 went out of business tomorrow, the code will be stuck, which means having to re-implement the bitfields for another processor, say RISC chip.

You have to look at the bigger picture and ask how did OpenBSD manage to port their BSD systems to a lot of platforms using one codebase? Ok, I'll admit that is a bit over the top, and debatable and subjective, but realistically speaking, if you want to port the code to another platform, its the way to do it by using the second example you used in your question.

Not alone that, compilers for different platforms would have their own way of padding, aligning bitfields for the processor where the compiler is on. And furthermore, what about the endianess of the processor?

Never rely on bitfields as a magic bullet. If you want speed for the processor and will be fixed on it, ie no intention of porting, then feel free to use bitfields. You cannot have both!

Answer 3

C bitfields were stillborn from the moment they were invented - for unknown reason. People didn't like them and used bitwise operators instead. You have to expect other developers to not understand C bitfield code.

With regard to which is faster: Irrelevant. Any optimizing compiler (which means practically all) will make the code do the same thing in whatever notation. It's a common misconception of C programmers that the compiler will just search-and-replace keywords into assembly. Modern compilers use the source code as a blueprint to what shall be achieved and then emit code that often looks very different but achieves the intended result.

Answer 4

The first one is explicit and whatever the speed the second expression is error-prone because any change to your struct might make the second expression wrong.

So use the first.

Answer 5

If you want portability, avoid bitfields. And if you are interested in performance of specific code, there is no alternative to writing your own tests. Remember, bitfields will be using the processor's bitwise instructions under the hood.

Answer 6

I think a C programmer would tend toward the second option of using bit masks and logic operations to deduce the value of each bit. Rather than having the code littered with hex values, enumerations would be set up, or, usually when more complex operations are involved, macros to get/set specific bits. I've heard on the grapevine, that struct implemented bitfields are slower.

Answer 7

Don't read to much in the "non portable bit-fields". There are two aspects of bit fields which are implementation defined: the signedness and the layout and one unspecified one: the alignment of the allocation unit in which they are packed. If you don't need anything else that the packing effect, using them is as portable (provided you specify explicitly a signed keyword where needed) as function calls which also have unspecified properties.

Concerning the performance, profile is the best answer you can get. In a perfect world, there would be no difference between the two writings. In practice, there can be some, but I can think of as many reasons in one direction as the other. And it can be very sensitive to context (logically meaningless difference between unsigned and signed for instance) so measure in context...

To sum up, the difference is thus mostly a style difference in cases where you have truly the choice (ie not if the precise layout is important). In those case, it is a optimization (in size, not in speed) and so I'd tend first to write the code without it and add it after if needed. So, bit fields are the obvious choice (the modifications to be done are the smallest one to achieve the result and are contained to the unique place of definition instead of being spread of to all the places of use).