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C/C++ register keyword usefulness

05/14/08

Permalink 12:36:56 pm, by lano1106, 286 words, 2939 views   English (CA)
Categories: C++

C/C++ register keyword usefulness

I have received a comment for this blog post to the effect that using the keyword 'register' was useless and all compilers will ignore it. I do not agree on this. If you search on the Internet, you will find out that a lot of people speculate and have their opinion on the subject but the truth is that unless you are a compiler writer, you have no idea whether a given compiler is doing something with the 'register' keyword or not. Even if you know the answer for a specific compiler, you cannot generalize for the other compilers. Here is what the C++ standard in section 7.1.1:

A register specifier has the same semantics as an auto specifier together with a hint to the implementation that the object so declared will be heavily used. [Note: the hint can be ignored and in most implementations it will be ignored if the address of the object is taken. -end note]

It is true that today compiler optimizers are very clever and can do a decent job at managing a CPU registers alone but lets imagine how the algorithm managing the registers used by the compiler works. Registers are a scarce resource and the compiler must analyze each variable usage and assign to each variable a score based on various criterias. If the compiler needs to swap back a variable to memory to reuse a register, it would choose the variable having the lowest score. What happens if more than one variable have the same low score? In such situation, a compiler could use the 'register' keyword hint to take a decision.

Situations where the 'register' keyword could be used:

  • temporary variables used in bitwise logical operations
  • loop counters

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