CS 202 - Notes 2016-03-23

Control Flow

Control flow is governed by four control flags in the processor

CF - carry flag :This is set when the operation has a carry out of the last bit. It indicates an overflow if the numbers were unsigned

ZF - zero flag :The most recent operation yielded a 0

SF - sign flag :The result of the most recent operation was negative

OF - overflow flag :Set if there was two’s compliment overflow (positive or negative)

These can be set explicitly or implicitly. They are set implicitly by almost every arithmetic operation we perform. The exception is the lea function, which was meant for address computation

we can set them explicitly using:

• cmp - compare two values (essentially subtracts the first from the second)
• test - ANDs the two arguments together. If they are the same, this will be a quick way to check if it is 0 or negative

if statements

C code:

	int simpleConditional(int x, int y){
	  int result;
	
	  if (x > y){
		result = x - y;
	  }else{
		result = y - x;
	  }
	
	  result *= 3;
	  
	  return result;
	}

Assembly:

	cmpl %esi, %edi
	jle .L2
	subl %esi, %edi
	movl %edi, %esi
	jmp .L3
	.L2:
	subl %edi, %esi
	.L3:
	leal (%rsi,%rsi,2), %eax
  ret

do while loops

C code:

	int doWhile(int x, int y){
	
	  int result;
	
	  do {
		result += x;
		y--;
	  }while (y > 0);
	
	  return result;
	}

Assembly:

	addl %edi, %eax
	subl $1, %esi
	testl %esi, %esi
	jg .L5
	rep ret

while loops

There is a very little difference between do while and while loops. A do while loop goes: init, body, update, test, body, update, test, body, update, test, etc… A while loop goes: init, test, body, update, test, body, update, test, body, update, etc… The difference is that the while loop does a test before it starts, or to put that another way, the do while does an iteration of body and update before it starts.

There are a couple of ways that we can translate a while loop into a do while loop.

jump to the middle

We start by unconditionally jumping down to the the test

guarded do

We just add a test before the do while loop that jumps over the loop

Different levels of optimization or different loops may result in one or the other of these approaches.

C code:

int whileLoop(int x, int y){
  int result;

  while (y > 0) {
    result += x;
    y--;
  }

  return result;
}

Assembly :

    jmp .L7
.L8:
    addl %edi, %eax
    subl $1, %esi
.L7:
    testl %esi, %esi
    jg .L8

for loops

This is basically pretty syntax for the same behavior as a while loop, so it can be converted the same way.

for (init; test; update){
  body;
}

init;
while(test){
  body; 
  update;
}