Why and when to use __noop?

The __noop intrinsic specifies that a function should be ignored and the argument list be parsed but no code be generated for the arguments. It is intended for use in global debug functions that take a variable number of arguments.

In your case the argument is an obviously side effect free expression that can be easily optimized out, so it doesn't matter.

But if the argument expression has side effects or is so complex that the compiler can't prove that it terminates normally and has no side-effects then using __noop prevents the potentially expensive evaluation of that expression.

The second benefit is that it behaves like a function call with a variable number of arguments syntactically. So substituting it for a function call doesn't affect the parsing of the program. With some other replacements (like the empty string), that might be a problem in some situations.

#define PRINT
extern int some_complicated_calculation();
PRINT("%d\n", some_complicated_calculation());

would call the function even though you don't want the result.

Using __noop, the function won't be called.

You could (assuming the compiler supports variadic macros) define PRINT to ignore the arguments; but then they won't be parsed at all, and may become invalid if you change the code around them without compiling the variant that defines PRINT to do something. Using __noop, the arguments are still parsed, so are more likely to remain valid.