Translation of Loop

While loop
Take an example of this C code. This code is inside some function.

int a = 4;
int b = 8;
int d = 0;
while(b > a)
{
    d = a + 2;
    b--;
}

Generated assembly code:

    movl    $4, -4(%ebp)
    movl    $8, -8(%ebp)
    movl    $0, -12(%ebp)
    jmp .L2
.L3:
    movl    -4(%ebp), %eax
    addl    $2, %eax
    movl    %eax, -12(%ebp)
    subl    $1, -8(%ebp)
.L2:
    movl    -8(%ebp), %eax
    cmpl    -4(%ebp), %eax
jg	.L3

Location of local variables of the stack (local variables are explained here)

a => -4(%ebp)
b => -8(%ebp)
d => -12(%ebp)

The use of registers as temporary memory is described here
Comments on generated assembly code

# a = 4
    movl    $4, -4(%ebp)

# b = 8
    movl    $8, -8(%ebp)

# d = 0
    movl    $0, -12(%ebp)

# jump to label .L2. The condition for the while loop is evaluated at .L2
    jmp .L2

# The label .L3. This is the start instruction inside while {}
.L3:

# tmp = a
    movl    -4(%ebp), %eax

# tmp = tmp + 2
    addl    $2, %eax

# d = tmp
    movl    %eax, -12(%ebp)

# b = b - 1
    subl    $1, -8(%ebp)

# The instruction for evaluating the condition of while starts here
.L2:

# tmp = b
    movl    -8(%ebp), %eax

# compare a to tmp
cmpl    -4(%ebp), %eax

# jump to start of loop block if the above tmp greater than a in above comparison
    jg  .L3

For Loop
Take example of this C code:

int a = 4;
int b = 8;
int d = 0;
for(b = 9; b > a; b--)
{
    d = a + 2;
}

Generated assembly code:

    movl    $4, -4(%ebp)
    movl    $8, -8(%ebp)
    movl    $0, -12(%ebp)
    movl    $9, -8(%ebp)
    jmp	.L2
.L3:
    movl    -4(%ebp), %eax
    addl    $2, %eax
    movl    %eax, -12(%ebp)
    subl    $1, -8(%ebp)
.L2:
    movl    -8(%ebp), %eax
    cmpl    -4(%ebp), %eax
    jg  .L3

The generated code is almost the same as the while loop. Here are few things to notice:

• The init condition of the loop is generated before the code for if-block.
• The update statement of the loop is generated at the end of the code of the if-block.