Explain Loop inversion in Java

 Loop inversion is a compiler optimization technique where a while loop is replaced by an if block containing a do..while loop.

How does loop inversion improve performance?

Let me explain with an example.

int i = 0;
int a[] = new int[10];
while (i < 10) {
	a[i] = 0;
	i++;
}

Post loop inversion optimization, above code will be transformed like below.

int i = 0;
int a[] = new int[10];

if (i < 10) {
	do {
		a[i] = 0;
		i++;
	} while (i < 10);
}

How this transformation improve performance?

When the value of i is 10, the processor need not execute a goto instruction to come out of the loop (which is required in the first case). This improves performance. By nature, any jump in the code causes a pipeline stall, which is a detriment to performance.

 

Reference

https://en.wikipedia.org/wiki/Pipeline_stall

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