Clever Geek Handbook

Are NOP Expressions Needed?

I wonder if empty expressions evaluate to NOP or are compiler dependent.

// Trivial example
int main()
{
    ;;
}
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It depends on the compiler, but the observed behavior should be that nothing happens. In practice, I'm pretty sure most compilers won't specify any code for an empty expression at all.

A conforming implementation executing a well-formed program must produce the same observable behavior as one of the possible executions of a corresponding abstract machine instance with the same program and the same input.

And the observed behavior is determined by:

The smallest requirements for a corresponding implementation:

  • Access to volatile objects is evaluated strictly according to the rules of an abstract machine.
  • At the end of the program, all data written to the files must be identical to one of the possible results of the program execution in accordance with the abstract semantics.
  • The dynamics of the input and output signals of interactive devices must occur in such a way that the request for output is actually delivered before the program waits for input. What an interactive device is is implementation-defined.

These collections are called the observed program behavior.

This is really the only requirement for implementation. This is often known as a "how-if" rule — the compiler can do whatever it likes as long as the observed behavior is as expected.



For what it's worth, these empty expressions are referred to as null statements:

An expression statement with a missing expression is called null.

If you really want NOP, you can try:

asm("nop");

It is, however, conditionally supported and its behavior is implementation-defined.

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or if it depends on the compiler.



It depends on the compiler ("usually"), but most sensible optimization compilers will simply ignore empty statements for efficiency, and they will usually not issue instructions NOP

.

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