Assembly: what are semantic NOPs?

Question

I was wondering what are "semantic NOPs" in assembly?

Answer 1

Code that isn't an actual nop but doesn't affect the behavior of the program.

In C, the following sequence could be thought of as a semantic NOP:

{
    // Since none of these have side affects, they are effectively no-ops
    int x = 5;
    int y = x * x;
    int z = y / x;
}

Answer 2

They are instructions that have no effect, like a NOP, but take more bytes. Useful to get code aligned to a cache line boundary. An instruction like lea edi,[edi+0] is an example, it would take 7 NOPs to fill the same number of bytes but takes only 1 cycle instead of 7.