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

C Standard says:

(c11, 6.7.1p6) "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."

These suggestions being implementation-defined means the implementation must define the choices being made ((c11, J.3.8 Hints) "The extent to which suggestions made by using the register storage-class specifier are effective (6.7.1)").

Here is what the documentation of some popular C compilers says.

For gcc (source):

The register specifier affects code generation only in these ways:

• When used as part of the register variable extension, see Explicit Register Variables.

• When -O0 is in use, the compiler allocates distinct stack memory for all variables that do not have the register storage-class specifier; if register is specified, the variable may have a shorter lifespan than the code would indicate and may never be placed in memory.

• On some rare x86 targets, setjmp doesn’t save the registers in all circumstances. In those cases, GCC doesn’t allocate any variables in registers unless they are marked register.

For IAR compiler for ARM (source):

Honoring the register keyword (6.7.1)

User requests for register variables are not honored