patch to qsort

["djamel anonymous" <djam8193ah at hotmail dot com>]

Hello, this time i am resending my patch that divides by two the memory 
usage of merge sort
used inside qsort(). this patch also reduces the recusrion depth by 1 level 
in many cases.
i haven't compiled the file as is because i haven't downloaded the full 
glibc source, but i
am 99% sure that it will compile.the test on a random 500k  integer elements 
vector gave
0.40 seconds with the original and 0.37 second with the patched version. 
with the vector
originally in reverse order this gave 0.25 with the original version and 
0.20 with the
patched version.
best regards.

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/* An alternative to qsort, with an identical interface.
   This file is part of the GNU C Library.
   Copyright (C) 1992,95-97,99,2000,01,02,04 Free Software Foundation, Inc.
   Written by Mike Haertel, September 1988.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

#include <alloca.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <memcopy.h>
#include <errno.h>

static void msort_with_tmp (void *b, size_t n, size_t s,
			    __compar_fn_t cmp, char *t);

static void
msort_with_tmp (void *b, size_t n, size_t s, __compar_fn_t cmp,
		char *t)
{
  char *tmp;
  char *b1, *b2;
  size_t n1, n2;

  if (n <= 2) 
    {
      if(n<=1)
        return;
      b1=b;
      b2= (char *) b + s;
      if ((*cmp) (b1, b2) > 0) 
        if (s == OPSIZ && (b1 - (char *) 0) % OPSIZ == 0) 
	  {
            *((op_t *) t) = *((op_t *) b1);
            *((op_t *) b1) = *((op_t *) b2);
            *((op_t *) b2) = *((op_t *) t);
          } 
        else 
	  {
            memcpy (t, b1, s);
            memcpy (b1, b2, s);
            memcpy (b2, t, s);
          };
      return;
    };

  n1 = n / 2;
  n2 = n - n1;
  b1 = b;
  b2 = (char *) b + (n1 * s);

  msort_with_tmp (b1, n1, s, cmp, t);
  msort_with_tmp (b2, n2, s, cmp, t);
  memcpy (t, b, n1 * s);
  b1=t;
  tmp = b;

  if (s == OPSIZ && (b1 - (char *) 0) % OPSIZ == 0)
    /* We are operating on aligned words.  Use direct word stores.  */
    while (n1 > 0 && n2 > 0)
      {
	if ((*cmp) (b1, b2) <= 0)
	  {
	    --n1;
	    *((op_t *) tmp) = *((op_t *) b1);
	    tmp += sizeof (op_t);
	    b1 += sizeof (op_t);
	  }
	else
	  {
	    --n2;
	    *((op_t *) tmp) = *((op_t *) b2);
	    tmp += sizeof (op_t);
	    b2 += sizeof (op_t);
	  }
      }
  else
    while (n1 > 0 && n2 > 0)
      {
	if ((*cmp) (b1, b2) <= 0)
	  {
	    tmp = (char *) __mempcpy (tmp, b1, s);
	    b1 += s;
	    --n1;
	  }
	else
	  {
	    tmp = (char *) __mempcpy (tmp, b2, s);
	    b2 += s;
	    --n2;
	  }
      }
  if (n1 > 0)
    memcpy (tmp, b1, n1 * s);
}

void
qsort (void *b, size_t n, size_t s, __compar_fn_t cmp)
{
  const size_t size = (n / 2) * s;

  if (size < 1024)
    {
      void *buf = __alloca (size);

      /* The temporary array is small, so put it on the stack.  */
      msort_with_tmp (b, n, s, cmp, buf);
    }
  else
    {
      /* We should avoid allocating too much memory since this might
	 have to be backed up by swap space.  */
      static long int phys_pages;
      static int pagesize;

      if (phys_pages == 0)
	{
	  phys_pages = __sysconf (_SC_PHYS_PAGES);

	  if (phys_pages == -1)
	    /* Error while determining the memory size.  So let's
	       assume there is enough memory.  Otherwise the
	       implementer should provide a complete implementation of
	       the `sysconf' function.  */
	    phys_pages = (long int) (~0ul >> 1);
	  pagesize = __sysconf (_SC_PAGESIZE);
	}

      /* Just a comment here.  We cannot compute
	   phys_pages * pagesize
	   and compare the needed amount of memory against this value.
	   The problem is that some systems might have more physical
	   memory then can be represented with a `size_t' value (when
	   measured in bytes.  */

      /* If the memory requirements are too high don't allocate memory.  */
      if (size * 6 / pagesize > (size_t) phys_pages)
	_quicksort (b, n, s, cmp);
      else
	{
	  /* It's somewhat large, so malloc it.  */
	  int save = errno;
	  char *tmp = malloc (size);
	  if (tmp == NULL)
	    {
	      /* Couldn't get space, so use the slower algorithm
		 that doesn't need a temporary array.  */
	      __set_errno (save);
	      _quicksort (b, n, s, cmp);
	    }
	  else
	    {
	      __set_errno (save);
	      msort_with_tmp (b, n, s, cmp, tmp);
	      free (tmp);
	    }
	}
    }
}
libc_hidden_def (qsort)

--- s_msort.c	2006-06-19 00:30:15.000000000 +0100
+++ new_msort.c	2006-06-20 19:45:07.000000000 +0100
@@ -36,8 +36,27 @@
   char *b1, *b2;
   size_t n1, n2;
 
-  if (n <= 1)
-    return;
+  if (n <= 2) 
+    {
+      if(n<=1)
+        return;
+      b1=b;
+      b2= (char *) b + s;
+      if ((*cmp) (b1, b2) > 0) 
+        if (s == OPSIZ && (b1 - (char *) 0) % OPSIZ == 0) 
+	  {
+            *((op_t *) t) = *((op_t *) b1);
+            *((op_t *) b1) = *((op_t *) b2);
+            *((op_t *) b2) = *((op_t *) t);
+          } 
+        else 
+	  {
+            memcpy (t, b1, s);
+            memcpy (b1, b2, s);
+            memcpy (b2, t, s);
+          };
+      return;
+    };
 
   n1 = n / 2;
   n2 = n - n1;
@@ -46,8 +65,9 @@
 
   msort_with_tmp (b1, n1, s, cmp, t);
   msort_with_tmp (b2, n2, s, cmp, t);
-
-  tmp = t;
+  memcpy (t, b, n1 * s);
+  b1=t;
+  tmp = b;
 
   if (s == OPSIZ && (b1 - (char *) 0) % OPSIZ == 0)
     /* We are operating on aligned words.  Use direct word stores.  */
@@ -86,13 +106,12 @@
       }
   if (n1 > 0)
     memcpy (tmp, b1, n1 * s);
-  memcpy (b, t, (n - n2) * s);
 }
 
 void
 qsort (void *b, size_t n, size_t s, __compar_fn_t cmp)
 {
-  const size_t size = n * s;
+  const size_t size = (n / 2) * s;
 
   if (size < 1024)
     {
@@ -118,11 +137,6 @@
 	       implementer should provide a complete implementation of
 	       the `sysconf' function.  */
 	    phys_pages = (long int) (~0ul >> 1);
-
-	  /* The following determines that we will never use more than
-	     a quarter of the physical memory.  */
-	  phys_pages /= 4;
-
 	  pagesize = __sysconf (_SC_PAGESIZE);
 	}
 
@@ -134,7 +148,7 @@
 	   measured in bytes.  */
 
       /* If the memory requirements are too high don't allocate memory.  */
-      if (size / pagesize > (size_t) phys_pages)
+      if (size * 6 / pagesize > (size_t) phys_pages)
 	_quicksort (b, n, s, cmp);
       else
 	{