Loop inversion

void pre_inversion() { while (/* condition */) { /* loop body */ } }

is equivalent to:

void post_inversion() { if (/* condition */) { do { /* loop body */ } while (/* condition */); } }

No change in performance occurs for initial and non-final iterations through the loop, including when the loop is not entered because the condition is false. However, the final iteration has 2 fewer jump instructions when the loop is entered because the do-while loop does not need to jump to the start of the while loop to evaluate the loop condition.

int i, a[100]; i = 0; while (i < 100) { a[i] = 0; i++; }

is equivalent to:

int i, a[100]; i = 0; if (i < 100) { do { a[i] = 0; i++; } while (i < 100); }

Despite the seemingly greater complexity of the second example, it may actually run faster on modern CPUs because they use an instruction pipeline. By nature, any jump in the code causes a pipeline stall, which is a detriment to performance.

Additionally, loop inversion allows safe loop-invariant code motion.

i := 0 L1: if i >= 100 goto L2 a[i] := 0 i := i + 1 goto L1 L2:

If i had been initialized at 100, the instructions executed at runtime would have been:

if i >= 100 goto L2

Let us assume that i had been initialized to some value less than 100. Now let us look at the instructions executed at the moment after i has been incremented to 99 in the loop:

goto L1 if i < 100 a[i] := 0 i := i + 1 goto L1 if i >= 100 goto L2 <<at L2>>

Now, let's look at the optimized version:

i := 0 if i >= 100 goto L2 L1: a[i] := 0 i := i + 1 if i < 100 goto L1 L2:

Again, let's look at the instructions executed if i is initialized to 100:

if i >= 100 goto L2

We didn't waste any cycles compared to the original version. Now consider the case where i has been incremented:

if i < 100 goto L1 a[i] := 0 i := i + 1 if i < 100 <<at L2>>

As you can see, two gotos (and thus, two pipeline stalls) have been eliminated in the execution.