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It matters in single-pass compilers. You can't allocate a variable in a register if its address is ever taken, but by the time a single-pass compiler knows that information it has already spit out all of the assembly for the function.

A register isn't in external memory, so isn't addressable as such. That part makes sense since if the compiler actually follows your suggestion it can't be addressed.

Thinking about it, storing arrays in registers would possibly make sense on systems like the 8051 where you actually have a bunch of general purpose register banks, but those don't exist in x86.

It was always only a suggestion to the compiler, to hold this variable in a register.

Compilers got so good at optimization that there is little point using it.

If a variable is held in a register you can't access it with a pointer. So if your intention is it should be in a register you can't take the address.

It once told the compiler to hold the value in a register because the compiler wasn't very smart at all.