waste_cpu_cycles: Use a more deterministic loop body

Previous implementation based on ldexp() was not always giving consistent
results from one run to another. Using more basic operations without extra
branches makes the execution time of the body much more predictable, leading to
more stable calibration values and precise reproduction of duty cycles.

Signed-off-by: Douglas RAILLARD <[email protected]>

src/rt-app.c

@@ -211,6 +211,12 @@ void __attribute__((noinline, optimize("O0"))) waste_cpu_cycles(unsigned long lo
 	double n;
 	unsigned long long i;
 
+	/*
+	 * Adjust the number of loops to get calibration values close to what
+	 * they used to be with the previous loop body.
+	 */
+	load_loops *= 10;
+
 	param = 0.95;
 	n = 4;
 	for (i = 0 ; i < load_loops ; i++) {
@@ -220,11 +226,7 @@ void __attribute__((noinline, optimize("O0"))) waste_cpu_cycles(unsigned long lo
 		 * documentation of "noinline" function attribute for details.
 		 */
 		asm("");
-
-		result = ldexp(param , (ldexp(param , ldexp(param , n))));
-		result = ldexp(param , (ldexp(param , ldexp(param , n))));
-		result = ldexp(param , (ldexp(param , ldexp(param , n))));
-		result = ldexp(param , (ldexp(param , ldexp(param , n))));
+		result = n/i;
 	}
 	return;
 }