c99语义中的存储类说明符“注册”

Question

I'm a foreigner reading C99, while a sentence (in 6.7.1) makes me confused:

• '(A declaration of an identifier for an object with storage-class specifier register suggests that access to the object be as fast as possible.) The extent to which such suggestions are effective is implementation-defined. '

How should I parse the second sentence :

• The extent to, which such suggestions are effective , is implementation-defined.
• The extent, to which such suggestions are effective , is implementation-defined.

which one is better?

Does that means an implementation has full powers to decide how to deal with register , even with a termination of translation?

Thanks.

Answer 1

Don't mix up register variables with registers of the CPU. These are not the same.

register has the purpose to allow for optimizations. Taking the address of a such a variable is forbidden and as a consequence such a variable can never alias, the compiler always masters its latest value. Thus it can realize such a variable easily as a CPU register or an assembler immediate, for example.

As a particular subcase there are const qualified register variables that can't be altered by the program directly nor behind the scenes by some other code that would access it through a pointer. Only such variables can easily be guaranteed to be constant through out the whole program execution.

Answer 2

Register is an old keyword. Basically it is a hint to an optimizer telling that it is better to use register for storing this variable. The pharse that you are mentioning means that compiler can treat this hint in any way it wants. In our days of optimizing compilers, its meaning and value for 99% is nothing.

Answer 3

No -- the extent to which the suggestions are effective, is implementation defined, but the effect of the code as a whole is not.

To put it slightly differently, when/if you put register into your code (where allowed), the compiler can completely ignore it -- and I feel obliged to add that nearly all reasonably modern compilers will .

Nonetheless, the mere presence of register won't prevent the code from compiling, unless you try to apply it somewhere it's not allowed (eg, to a global variable).

Bottom line: don't use it, but don't worry about removing it from code that already has it either. It's a waste of time and effort, but with reasonably modern compilers a completely harmless one.