From: Ian Lance Taylor <iant@google.com>
Date: Fri, 7 Mar 2014 15:52:48 +0000 (+0000)
Subject: * sort.c (backtrace_qsort): Use middle element as pivot.
X-Git-Tag: releases/gcc-4.9.0~541
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=dbe2084238e45a60aa6e6b0469bd9e0c4ef15b2f;p=thirdparty%2Fgcc.git

* sort.c (backtrace_qsort): Use middle element as pivot.

From-SVN: r208403
---

diff --git a/libbacktrace/ChangeLog b/libbacktrace/ChangeLog
index abbea3ed6e8b..8a1a8e74a323 100644
--- a/libbacktrace/ChangeLog
+++ b/libbacktrace/ChangeLog
@@ -1,3 +1,7 @@
+2014-03-07  Ian Lance Taylor  <iant@google.com>
+
+	* sort.c (backtrace_qsort): Use middle element as pivot.
+
 2014-03-06  Ian Lance Taylor  <iant@google.com>
 
 	* sort.c: New file.
diff --git a/libbacktrace/sort.c b/libbacktrace/sort.c
index 88f923142247..8c4760f8aa31 100644
--- a/libbacktrace/sort.c
+++ b/libbacktrace/sort.c
@@ -69,6 +69,12 @@ backtrace_qsort (void *basearg, size_t count, size_t size,
   if (count < 2)
     return;
 
+  /* The symbol table and DWARF tables, which is all we use this
+     routine for, tend to be roughly sorted.  Pick the middle element
+     in the array as our pivot point, so that we are more likely to
+     cut the array in half for each recursion step.  */
+  swap (base, base + (count / 2) * size, size);
+
   mid = 0;
   for (i = 1; i < count; i++)
     {