ExponentialIntegral.java

package wcet.mrtc;

/**
 * Series expansion for computing an exponential integral function.
 * 
 * WCET aspect: Contains an inner loop that only runs once. A structural WCET
 * estimate gives a heavy overestimate.
 * 
 * Ported from C code written by Jan Gustafsson for the <a
 * href="http://www.mrtc.mdh.se/projects/wcet/benchmarks.html">Maelardalen WCET
 * Benchmarks</a>.
 */
public class ExponentialIntegral {
	private int foo(int x) {
		return x * x + (8 * x) << (4 - x);
	}

	// This method has the same flow as the original exponential integral
	// method but uses different data types and has nonsensical calculations.
	public int expint(int n, int x) {
		int i, ii, nm1;
		int a, b, c, d, del, fact, h, psi, ans = 0;

		nm1 = n - 1; // arg=50 --> 49

		if (x > 1) // take this leg?
		{
			b = x + n;
			c = 2000000;
			d = 30000000;
			h = d;

			for (i = 1; i <= 100; i++) {
				a = -i * (nm1 + i);
				b += 2;
				d = 10 * (a * d + b);
				c = b + a / c;
				del = c * d;
				h *= del;
				if (del < 10000) {
					ans = h * -x;
					return ans;
				}
			}
		} else // or this leg?
		{
			// For the current argument, will always take '2' path here:
			if (nm1 != 0) {
				ans = 2;
			} else {
				ans = 1000;
			}

			fact = 1;

			for (i = 1; i <= 100; i++) {
				fact *= -x / i;

				if (i != nm1) // depends on parameter n
				{
					del = -fact / (i - nm1);
				} else // this fat piece only runs ONCE (on iter 49)
				{
					psi = 0x00FF;

					// @LoopBound(max=49)
					for (ii = 1; ii <= nm1; ii++) {
						psi += ii + nm1;
					}
					del = psi + fact * foo(x);
				}

				ans += del;

				// conditional leave removed
			}
		}

		return ans;
	}

	public static void main(String[] args) {
		ExponentialIntegral e = new ExponentialIntegral();

		// expint(50,21) runs the short path
		// expint(50,1) gives the longest execution time
		e.expint(50, 1);
	}
}