Compiler optimization

A compiler with enabled optimization (via flags) will take more time for the generation, but the execution of generated binary will most likely have a better performance.

Hint: If you are as lazy as I am you can use a web tool (https://godbolt.org/) to take a look on the generated assembly code for a choosen compiler.

Examples
For the following examples I have choosen GCC 9.2 (x86-64) as compiler.

Useless stuff removal

Source code Without optimization With optimization (-O) 
      void function()
      {
        int i;
        i = 42;
      }
      
      function():
        push  rbp
        mov   rbp, rsp
        mov   DWORD PTR [rbp-4], 42
        nop
        pop   rbp
        ret
      
      function():
        ret

Pre-calculation

Source code Without optimization With optimization (-O) 
      int function()
      {
        int i = 0;
        i++;
        return i;
      }
      
      function():
        push  rbp
        mov   rbp, rsp
        mov   DWORD PTR [rbp-4], 0
        add   DWORD PTR [rbp-4], 1
        mov   eax, DWORD PTR [rbp-4]
        pop   rbp
        ret
      
      function():
        mov   eax, 1
        ret