Refactor comparison function to be branch-free

Refactored the 'cmp' function to use (a > b) - (a < b), making it
branch-free and reducing the risk of side-channel attacks caused by
branching.

Updated the function signature to match "qsort's" expected type,
eliminating the need for an explicit cast.

Change-Id: I296aa12a57e5b89abc39e6467183c8520c2d1248

Author: BennyWang1007

dudect/fixture.c

@@ -67,11 +67,11 @@ static int64_t percentile(const int64_t *a_sorted, double which, size_t size)
     return a_sorted[pos];
 }
 
-static int cmp(const int64_t *a, const int64_t *b)
+/* leverages the fact that comparison expressions return 1 or 0. */
+static int cmp(const void *aa, const void *bb)
 {
-    if (*a == *b)
-        return 0;
-    return (*a - *b) > 0 ? 1 : -1;
+    int64_t a = *(const int64_t *) aa, b = *(const int64_t *) bb;
+    return (a > b) - (a < b);
 }
 
 /* This function is used to set different thresholds for cropping measurements.
@@ -82,8 +82,7 @@ static int cmp(const int64_t *a, const int64_t *b)
  */
 static void prepare_percentiles(int64_t *exec_times, int64_t *percentiles)
 {
-    qsort(exec_times, N_MEASURES, sizeof(int64_t),
-          (int (*)(const void *, const void *)) cmp);
+    qsort(exec_times, N_MEASURES, sizeof(int64_t), cmp);
 
     for (size_t i = 0; i < NUM_PERCENTILES; i++) {
         percentiles[i] = percentile(