From 3791eca4cb692e8c2699027ea137a69a6ef22c11 Mon Sep 17 00:00:00 2001
From: millert <>
Date: Wed, 17 May 2017 16:58:20 +0000
Subject: [PATCH] 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 | 35 +++++++++++++++++++++++++----------
 1 file changed, 25 insertions(+), 10 deletions(-)

diff --git a/src/lib/libc/stdlib/qsort.c b/src/lib/libc/stdlib/qsort.c
index f16f9d0e..c97979ef 100644
--- a/src/lib/libc/stdlib/qsort.c
+++ b/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);