Writing a method using MIPS code

Question

I am trying to understand how convert C code to MIPS code and I have having trouble understanding why the stack pointer( $sp ) needs to be manipulated before and after the procedural code.Isn't the program supposed to automatically increment the stack pointer after every instruction?

C CODE :

int myMethod (int g, h, i, j)
{ 
  int f;
  f = (g + h) - (i + j);
  return f; 
}

If we let g, h, i, j = $a0, $a1, $a2, $a3 and f = $s0 and result = $v0 then, MIPS CODE :

addi $sp, $sp, -4
  sw   $s0, 0($sp)          
  add  $t0, $a0, $a1       
  add  $t1, $a2, $a3
  sub  $s0, $t0, $t1         #SUB STATEMENT
  add  $v0, $s0, $zero
  lw   $s0, 0($sp)
  addi $sp, $sp, 4          
jr $ra

I don't really know why the stack pointer is decremented by 1 word size and then it's value loaded into the f variable, if you look at the subtraction statement you will notice that the value of f is then overwritten by the result of the subtraction so what was the use?

Answer 1

Isn't the program supposed to automatically increment the stack pointer after every instruction?

No. You're confusing the stack pointer with the program counter. The PC is incremented after every instruction.

I don't really know why the stack pointer is decremented by 1 word size

To make room on the stack for the local variable f.

and then its value loaded into the f variable, if you look at the subtraction statement you will notice that the value of f is then overwritten by the result of the subtraction so what was the use?

None. It's redundant. An optimization flag would probably have removed that.