The BSD qsort() performs tail recursion elimination on the second

side of the array being partitioned to save on stack space.  Greater
savings can be gained by choosing recursion for the smaller side
of the partition and eliminating recursion for the larger side.
This also results in a small but measurable performance gain.
OK otto@ schwarze@

src/lib/libc/stdlib/qsort.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: qsort.c,v 1.14 2017/01/04 15:20:30 millert Exp $ */
+/*	$OpenBSD: qsort.c,v 1.15 2017/05/17 16:58:20 millert Exp $ */
 /*-
  * Copyright (c) 1992, 1993
  *	The Regents of the University of California.  All rights reserved.
@@ -85,7 +85,7 @@ qsort(void *aa, size_t n, size_t es, int (*cmp)(const void *, const void *))
 {
 	char *pa, *pb, *pc, *pd, *pl, *pm, *pn;
 	int cmp_result, swaptype;
-	size_t d, r;
+	size_t d, r, s;
 	char *a = aa;
 
 loop:	SWAPINIT(a, es);
@@ -139,14 +139,29 @@ loop:	SWAPINIT(a, es);
 	vecswap(a, pb - r, r);
 	r = min(pd - pc, pn - pd - es);
 	vecswap(pb, pn - r, r);
-	if ((r = pb - pa) > es)
-		qsort(a, r / es, es, cmp);
-	if ((r = pd - pc) > es) { 
-		/* Iterate rather than recurse to save stack space */
-		a = pn - r;
-		n = r / es;
-		goto loop;
+	/*
+	 * To save stack space we sort the smaller side of the partition first
+	 * using recursion and eliminate tail recursion for the larger side.
+	 */
+	r = pb - pa;
+	s = pd - pc;
+	if (r < s) {
+		/* Recurse for 1st side, iterate for 2nd side. */
+		if (s > es) {
+			if (r > es)
+				qsort(a, r / es, es, cmp);
+			a = pn - s;
+			n = s / es;
+			goto loop;
+		}
+	} else {
+		/* Recurse for 2nd side, iterate for 1st side. */
+		if (r > es) {
+			if (s > es)
+				qsort(pn - s, s / es, es, cmp);
+			n = r / es;
+			goto loop;
+		}
 	}
-/*		qsort(pn - r, r / es, es, cmp);*/
 }
 DEF_STRONG(qsort);