Compiler and Optimizer

Compiler

The job of the compiler is to turn the human-readable code to something machine can understand.

First, it checks the syntax of the code and analyze it.
If there is no error, it proceeds to produce intermediate code, i.e. assembly code.

Machine code: directly executed by CPU, not human-readable

Assembly code: human-readable form of the machine code

Optimizer optimizes the code. So what is optimization? Check the following example:

// Stupid_add.c
int main(void) {
  int x = 1;
  x = x + 1;
  x = x + 1;
  return x;
}

We can use gcc -O options to indicate optimization.

-O means to Optimize.

The number followed is the optimization level.

Try the difference of :

gcc –S stupid_add.c –O0 –o add_O0.s
gcc –S stupid_add.c –O1 –o add_O1.s

add_O0.s

add_O1.s

You can discover that, with more optimization level, more compact the code is. The repeated steps are condensed into one single command instead.