Is it useless to use the `register` keyword with modern compilers, when optimizing?

Question

The C register keyword gives the compiler a hint to prefer storing a variable in a register rather than, say, on the stack. A compiler may ignore it if it likes

Answer 1

For GCC, register has had no effect on code generation at all, not even a hint, at all optimization levels, for all supported CPU architectures, for over a decade.

The reasons for this are largely historical. GCC 2.95 and older had two register allocators, one ("stupid") used when not optimizing, and one ("local, global, reload") used when optimizing. The "stupid" allocator did try to honor register, but the "local, global, reload" allocator completely ignored it. (I don't know what the original rationale for that design decision was; you'd have to ask Richard Kenner.) In version 3.0, the "stupid" allocator was scrapped in favor of adding a fast-and-sloppy mode to "local, global, reload". Nobody bothered to write the code to make that mode pay attention to register, so it doesn't.

As of this writing, the GCC devs are in the process of replacing "local, global, reload" with a new allocator called "IRA and LRA", but it, too, completely ignores register.

However, the (C-only) rule that you cannot take the address of a register variable is still enforced, and the keyword is used by the explicit register variable extension, which allows you to dedicate a specific register to a variable; this can be useful in programs that use a lot of inline assembly.