Bug 872497 - Remove problematic "optimization" in NS_QuickSort. r=jlebar

This change allows us to avoid O(n^2) performance when partially sorted data
is given to NS_QuickSort.  We no longer attempt an insertion sort if the input
appears pre-sorted.  This brings the expected performance back to O(n*log(n))
but also eliminates the possibility of O(n) best case performance.  Attempts
to achieve O(n) performance should be the responsibility of callers as they
are in a better position to evaluate the costs/benefit trade-off of looking
for special cases that can be sorted quicker.

xpcom/glue/nsQuickSort.cpp

@@ -104,10 +104,10 @@ void NS_QuickSort (
     )
 {
 	char *pa, *pb, *pc, *pd, *pl, *pm, *pn;
-	int d, r, swaptype, swap_cnt;
+	int d, r, swaptype;
 
 loop:	SWAPINIT(a, es);
-	swap_cnt = 0;
+	/* Use insertion sort when input is small */
 	if (n < 7) {
 		for (pm = (char *)a + es; pm < (char *)a + n * es; pm += es)
 			for (pl = pm; pl > (char *)a && cmp(pl - es, pl, data) > 0;
@@ -115,6 +115,7 @@ loop:	SWAPINIT(a, es);
 				swap(pl, pl - es);
 		return;
 	}
+	/* Choose pivot */
 	pm = (char *)a + (n / 2) * es;
 	if (n > 7) {
 		pl = (char *)a;
@@ -131,10 +132,16 @@ loop:	SWAPINIT(a, es);
 	pa = pb = (char *)a + es;
 
 	pc = pd = (char *)a + (n - 1) * es;
+	/* loop invariants:
+	 * [a, pa) = pivot
+	 * [pa, pb) < pivot
+	 * [pb, pc + es) unprocessed
+	 * [pc + es, pd + es) > pivot
+	 * [pd + es, pn) = pivot
+	 */
 	for (;;) {
 		while (pb <= pc && (r = cmp(pb, a, data)) <= 0) {
 			if (r == 0) {
-				swap_cnt = 1;
 				swap(pa, pb);
 				pa += es;
 			}
@@ -142,7 +149,6 @@ loop:	SWAPINIT(a, es);
 		}
 		while (pb <= pc && (r = cmp(pc, a, data)) >= 0) {
 			if (r == 0) {
-				swap_cnt = 1;
 				swap(pc, pd);
 				pd -= es;
 			}
@@ -151,23 +157,16 @@ loop:	SWAPINIT(a, es);
 		if (pb > pc)
 			break;
 		swap(pb, pc);
-		swap_cnt = 1;
 		pb += es;
 		pc -= es;
 	}
-	if (swap_cnt == 0) {  /* Switch to insertion sort */
-		for (pm = (char *)a + es; pm < (char *)a + n * es; pm += es)
-			for (pl = pm; pl > (char *)a && cmp(pl - es, pl, data) > 0;
-			     pl -= es)
-				swap(pl, pl - es);
-		return;
-	}
-
+	/* Move pivot values */
 	pn = (char *)a + n * es;
 	r = XPCOM_MIN(pa - (char *)a, pb - pa);
 	vecswap(a, pb - r, r);
 	r = XPCOM_MIN<size_t>(pd - pc, pn - pd - es);
 	vecswap(pb, pn - r, r);
+	/* Recursively process partitioned items */
 	if ((r = pb - pa) > (int)es)
         NS_QuickSort(a, r / es, es, cmp, data);
 	if ((r = pd - pc) > (int)es) {