This work is created by Perry Kivolowitz, Professor and Chair of Computer Science at Carthage College. It is copyright © 2021 and may be freely shared for educational purposes.
We have already covered the if and while statements. We demonstrate that a while loop is nothing more than an if statement with one additional label preceding and one unconditional branch following the code for an if statement.
A for loop is only slightly more complex.
In C++ and C, a for loop looks like this:
for (set up; decision; post step) // 1
{ // 2
// CODE BLOCK // 3
} // 4 The image on the left, below, represents from top to bottom what the parts of the for are from left to right (the post step being found at the bottom).
The image on the right, above, depicts how for loops are typically implemented. The reason for this becomes clear when we see the assembly language.
This code:
for (long i = 0; i < 10; i++) // 1
{ // 2
// CODE BLOCK // 3
} // 4 could be implemented like this in assembly language:
// Assume i is implemented using x0 // 1
// 2
mov x0, xzr // 3
// 4
1: cmp x0, 10 // 5
bge 2f // 6
// 7
// CODE BLOCK // 8
// 9
add x0, x0, 1 // 10
b 1b // 11
// 12
2: // 13 This corresponds to the flow chart on the left, above. There are 4 instructions in the loop (ignoring the code block).
The next set of assembly language corresponds to the flow chart on the right, above, where the post step and decision comes after the code block.
// Assume i is implemented using x0 // 1
// 2
mov x0, xzr // 3
b 2f // 4
// 5
1: // 6
// 7
// CODE BLOCK // 8
// 9
add x0, x0, 1 // 10
2: cmp x0, 10 // 11
blt 1b // 12
// 13 Notice this contains one fewer lines of assembly language within the loop itself (3 lines versus 4). Again, the contents of the code block are not counted.
Now let's add a continue to the code block, dividing it in two.
Here is what we would need to write to support a continue if the "conventional" ordering were used with the decision evaluation at the top:
// Assume i is implemented using x0 // 1
// 2
mov x0, xzr // 3
// 4
1: cmp x0, 10 // 5
bge 3f // 6
// 7
// FIRST PART OF CODE BLOCK // 8
// 9
// if (i == 5) // 10
// continue // 11
// 12
cmp x0, 5 // 13
beq 2f // 14
// 15
// REMAINDER OF CODE BLOCK // 16
// 17
2: add x0, x0, 1 // 18
b 1b // 19
// 20
3: // 21Here is the original code.
Below, is how a for loop is typically implemented.
// Assume i is implemented using x0 // 1
// 2
mov x0, xzr // 3
b 3f // 4
// 5
1: // 6
// 7
// FIRST PART OF CODE BLOCK // 8
// 9
// if (i == 5) // 10
// continue // 11
// 12
cmp x0, 5 // 13
beq 2f // 14
// 15
// REMAINDER OF CODE BLOCK // 16
// 17
2: add x0, x0, 1 // 18
3: cmp x0, 10 // 19
blt 1b // 20 Here is the original code.
Once again, the code moving the post step and decision evaluation to the bottom is one fewer instruction inside the loop.
for loops typically contain code ordering different from what one might expect. This is done to save an instruction within the loop. While this doesn't sound like much, consider the case where the loop is executed billions of times. In this case, saving one instruction per loop prevents the execution of a billion instructions. The shorter the code block is, the more important it is to save one instruction from within the loop.