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

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.

• -O0 means no optimiation.

• Maximum level is 3.

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.