Is the register keyword still used?

Question

Just came across the register keyword in C++ and I wondered as this seems a good idea (keeping certain variables in a register) surely the compiler does this by default?

Answer 1

The keyword "register" has been deprecated since the 2011 C++ standard; see "Remove Deprecated Use of the register Keyword". It should therefore not be used.

In my own experiments I found that debug code generated by gcc (v8.1.1) does differ if the "register" keyword is used; the generated assembly code allocates designated variables to registers. Benchmarks even showed that this code ran faster (than code without "register"). This is irrelevant, however, as release (optimised) code showed no differences (ie, using "register" had no effect). Vacbob states here that if any optimization is enabled, then gcc ignores "register". My own tests confirm this.

So, in summary, don't use "register" and if debug code appears to run faster when "register" is used, bear in mind that the optimized release code will not.