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SubjectRe: [patch 1/13] Qsort
Hi,

I was just reading over the patch, and had a quick question/comment upon
the SWAP macro defined below. I think it's possible to do a tiny bit
better (better, of course, being subjective), as follows:

#define SWAP(a, b, size) \
do { \
register size_t __size = (size); \
register char * __a = (a), * __b = (b); \
do { \
*__a ^= *__b; \
*__b ^= *__a; \
*__a ^= *__b; \
__a++; \
__b++; \
} while ((--__size) > 0); \
} while (0)

What do you think? :)

-Vadim Lobanov

On Sat, 22 Jan 2005, Andreas Gruenbacher wrote:

> Add a quicksort from glibc as a kernel library function, and switch
> xfs over to using it. The implementations are equivalent. The nfsacl
> protocol also requires a sort function, so it makes more sense in
> the common code.
>
> Signed-off-by: Andreas Gruenbacher <agruen@suse.de>
> Acked-by: Olaf Kirch <okir@suse.de>
>
> Index: linux-2.6.11-rc2/include/linux/kernel.h
> ===================================================================
> --- linux-2.6.11-rc2.orig/include/linux/kernel.h
> +++ linux-2.6.11-rc2/include/linux/kernel.h
> @@ -93,6 +93,8 @@ extern int sscanf(const char *, const ch
> __attribute__ ((format (scanf,2,3)));
> extern int vsscanf(const char *, const char *, va_list);
>
> +extern void qsort(void *, size_t, size_t, int (*)(const void *,const void *));
> +
> extern int get_option(char **str, int *pint);
> extern char *get_options(const char *str, int nints, int *ints);
> extern unsigned long long memparse(char *ptr, char **retptr);
> Index: linux-2.6.11-rc2/lib/Kconfig
> ===================================================================
> --- linux-2.6.11-rc2.orig/lib/Kconfig
> +++ linux-2.6.11-rc2/lib/Kconfig
> @@ -30,6 +30,9 @@ config LIBCRC32C
> require M here. See Castagnoli93.
> Module will be libcrc32c.
>
> +config QSORT
> + bool "Quick Sort"
> +
> #
> # compression support is select'ed if needed
> #
> Index: linux-2.6.11-rc2/lib/Makefile
> ===================================================================
> --- linux-2.6.11-rc2.orig/lib/Makefile
> +++ linux-2.6.11-rc2/lib/Makefile
> @@ -25,6 +25,7 @@ obj-$(CONFIG_CRC_CCITT) += crc-ccitt.o
> obj-$(CONFIG_CRC32) += crc32.o
> obj-$(CONFIG_LIBCRC32C) += libcrc32c.o
> obj-$(CONFIG_GENERIC_IOMAP) += iomap.o
> +obj-$(CONFIG_QSORT) += qsort.o
>
> obj-$(CONFIG_ZLIB_INFLATE) += zlib_inflate/
> obj-$(CONFIG_ZLIB_DEFLATE) += zlib_deflate/
> Index: linux-2.6.11-rc2/lib/qsort.c
> ===================================================================
> --- /dev/null
> +++ linux-2.6.11-rc2/lib/qsort.c
> @@ -0,0 +1,249 @@
> +/* Copyright (C) 1991, 1992, 1996, 1997, 1999 Free Software Foundation, Inc.
> + This file is part of the GNU C Library.
> + Written by Douglas C. Schmidt (schmidt@ics.uci.edu).
> +
> + 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. */
> +
> +/* If you consider tuning this algorithm, you should consult first:
> + Engineering a sort function; Jon Bentley and M. Douglas McIlroy;
> + Software - Practice and Experience; Vol. 23 (11), 1249-1265, 1993. */
> +
> +# include <linux/module.h>
> +# include <linux/slab.h>
> +# include <linux/string.h>
> +
> +MODULE_LICENSE("GPL");
> +
> +/* Byte-wise swap two items of size SIZE. */
> +#define SWAP(a, b, size) \
> + do \
> + { \
> + register size_t __size = (size); \
> + register char *__a = (a), *__b = (b); \
> + do \
> + { \
> + char __tmp = *__a; \
> + *__a++ = *__b; \
> + *__b++ = __tmp; \
> + } while (--__size > 0); \
> + } while (0)
> +
> +/* Discontinue quicksort algorithm when partition gets below this size.
> + This particular magic number was chosen to work best on a Sun 4/260. */
> +#define MAX_THRESH 4
> +
> +/* Stack node declarations used to store unfulfilled partition obligations. */
> +typedef struct
> + {
> + char *lo;
> + char *hi;
> + } stack_node;
> +
> +/* The next 5 #defines implement a very fast in-line stack abstraction. */
> +/* The stack needs log (total_elements) entries (we could even subtract
> + log(MAX_THRESH)). Since total_elements has type size_t, we get as
> + upper bound for log (total_elements):
> + bits per byte (CHAR_BIT) * sizeof(size_t). */
> +#define CHAR_BIT 8
> +#define STACK_SIZE (CHAR_BIT * sizeof(size_t))
> +#define PUSH(low, high) ((void) ((top->lo = (low)), (top->hi = (high)), ++top))
> +#define POP(low, high) ((void) (--top, (low = top->lo), (high = top->hi)))
> +#define STACK_NOT_EMPTY (stack < top)
> +
> +
> +/* Order size using quicksort. This implementation incorporates
> + four optimizations discussed in Sedgewick:
> +
> + 1. Non-recursive, using an explicit stack of pointer that store the
> + next array partition to sort. To save time, this maximum amount
> + of space required to store an array of SIZE_MAX is allocated on the
> + stack. Assuming a 32-bit (64 bit) integer for size_t, this needs
> + only 32 * sizeof(stack_node) == 256 bytes (for 64 bit: 1024 bytes).
> + Pretty cheap, actually.
> +
> + 2. Chose the pivot element using a median-of-three decision tree.
> + This reduces the probability of selecting a bad pivot value and
> + eliminates certain extraneous comparisons.
> +
> + 3. Only quicksorts TOTAL_ELEMS / MAX_THRESH partitions, leaving
> + insertion sort to order the MAX_THRESH items within each partition.
> + This is a big win, since insertion sort is faster for small, mostly
> + sorted array segments.
> +
> + 4. The larger of the two sub-partitions is always pushed onto the
> + stack first, with the algorithm then concentrating on the
> + smaller partition. This *guarantees* no more than log (total_elems)
> + stack size is needed (actually O(1) in this case)! */
> +
> +void
> +qsort(void *const pbase, size_t total_elems, size_t size,
> + int(*cmp)(const void *,const void *))
> +{
> + register char *base_ptr = (char *) pbase;
> +
> + const size_t max_thresh = MAX_THRESH * size;
> +
> + if (total_elems == 0)
> + /* Avoid lossage with unsigned arithmetic below. */
> + return;
> +
> + if (total_elems > MAX_THRESH)
> + {
> + char *lo = base_ptr;
> + char *hi = &lo[size * (total_elems - 1)];
> + stack_node stack[STACK_SIZE];
> + stack_node *top = stack + 1;
> +
> + while (STACK_NOT_EMPTY)
> + {
> + char *left_ptr;
> + char *right_ptr;
> +
> + /* Select median value from among LO, MID, and HI. Rearrange
> + LO and HI so the three values are sorted. This lowers the
> + probability of picking a pathological pivot value and
> + skips a comparison for both the LEFT_PTR and RIGHT_PTR in
> + the while loops. */
> +
> + char *mid = lo + size * ((hi - lo) / size >> 1);
> +
> + if ((*cmp) ((void *) mid, (void *) lo) < 0)
> + SWAP (mid, lo, size);
> + if ((*cmp) ((void *) hi, (void *) mid) < 0)
> + SWAP (mid, hi, size);
> + else
> + goto jump_over;
> + if ((*cmp) ((void *) mid, (void *) lo) < 0)
> + SWAP (mid, lo, size);
> + jump_over:;
> +
> + left_ptr = lo + size;
> + right_ptr = hi - size;
> +
> + /* Here's the famous ``collapse the walls'' section of quicksort.
> + Gotta like those tight inner loops! They are the main reason
> + that this algorithm runs much faster than others. */
> + do
> + {
> + while ((*cmp) ((void *) left_ptr, (void *) mid) < 0)
> + left_ptr += size;
> +
> + while ((*cmp) ((void *) mid, (void *) right_ptr) < 0)
> + right_ptr -= size;
> +
> + if (left_ptr < right_ptr)
> + {
> + SWAP (left_ptr, right_ptr, size);
> + if (mid == left_ptr)
> + mid = right_ptr;
> + else if (mid == right_ptr)
> + mid = left_ptr;
> + left_ptr += size;
> + right_ptr -= size;
> + }
> + else if (left_ptr == right_ptr)
> + {
> + left_ptr += size;
> + right_ptr -= size;
> + break;
> + }
> + }
> + while (left_ptr <= right_ptr);
> +
> + /* Set up pointers for next iteration. First determine whether
> + left and right partitions are below the threshold size. If so,
> + ignore one or both. Otherwise, push the larger partition's
> + bounds on the stack and continue sorting the smaller one. */
> +
> + if ((size_t) (right_ptr - lo) <= max_thresh)
> + {
> + if ((size_t) (hi - left_ptr) <= max_thresh)
> + /* Ignore both small partitions. */
> + POP (lo, hi);
> + else
> + /* Ignore small left partition. */
> + lo = left_ptr;
> + }
> + else if ((size_t) (hi - left_ptr) <= max_thresh)
> + /* Ignore small right partition. */
> + hi = right_ptr;
> + else if ((right_ptr - lo) > (hi - left_ptr))
> + {
> + /* Push larger left partition indices. */
> + PUSH (lo, right_ptr);
> + lo = left_ptr;
> + }
> + else
> + {
> + /* Push larger right partition indices. */
> + PUSH (left_ptr, hi);
> + hi = right_ptr;
> + }
> + }
> + }
> +
> + /* Once the BASE_PTR array is partially sorted by quicksort the rest
> + is completely sorted using insertion sort, since this is efficient
> + for partitions below MAX_THRESH size. BASE_PTR points to the beginning
> + of the array to sort, and END_PTR points at the very last element in
> + the array (*not* one beyond it!). */
> +
> + {
> + char *end_ptr = &base_ptr[size * (total_elems - 1)];
> + char *tmp_ptr = base_ptr;
> + char *thresh = min(end_ptr, base_ptr + max_thresh);
> + register char *run_ptr;
> +
> + /* Find smallest element in first threshold and place it at the
> + array's beginning. This is the smallest array element,
> + and the operation speeds up insertion sort's inner loop. */
> +
> + for (run_ptr = tmp_ptr + size; run_ptr <= thresh; run_ptr += size)
> + if ((*cmp) ((void *) run_ptr, (void *) tmp_ptr) < 0)
> + tmp_ptr = run_ptr;
> +
> + if (tmp_ptr != base_ptr)
> + SWAP (tmp_ptr, base_ptr, size);
> +
> + /* Insertion sort, running from left-hand-side up to right-hand-side. */
> +
> + run_ptr = base_ptr + size;
> + while ((run_ptr += size) <= end_ptr)
> + {
> + tmp_ptr = run_ptr - size;
> + while ((*cmp) ((void *) run_ptr, (void *) tmp_ptr) < 0)
> + tmp_ptr -= size;
> +
> + tmp_ptr += size;
> + if (tmp_ptr != run_ptr)
> + {
> + char *trav;
> +
> + trav = run_ptr + size;
> + while (--trav >= run_ptr)
> + {
> + char c = *trav;
> + char *hi, *lo;
> +
> + for (hi = lo = trav; (lo -= size) >= tmp_ptr; hi = lo)
> + *hi = *lo;
> + *hi = c;
> + }
> + }
> + }
> + }
> +}
> +EXPORT_SYMBOL(qsort);
> Index: linux-2.6.11-rc2/fs/xfs/support/qsort.c
> ===================================================================
> --- linux-2.6.11-rc2.orig/fs/xfs/support/qsort.c
> +++ /dev/null
> @@ -1,155 +0,0 @@
> -/*
> - * Copyright (c) 1992, 1993
> - * The Regents of the University of California. All rights reserved.
> - *
> - * Redistribution and use in source and binary forms, with or without
> - * modification, are permitted provided that the following conditions
> - * are met:
> - * 1. Redistributions of source code must retain the above copyright
> - * notice, this list of conditions and the following disclaimer.
> - * 2. Redistributions in binary form must reproduce the above copyright
> - * notice, this list of conditions and the following disclaimer in the
> - * documentation and/or other materials provided with the distribution.
> - * 3. Neither the name of the University nor the names of its contributors
> - * may be used to endorse or promote products derived from this software
> - * without specific prior written permission.
> - *
> - * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
> - * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
> - * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
> - * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
> - * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
> - * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
> - * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
> - * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
> - * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
> - * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
> - * SUCH DAMAGE.
> - */
> -
> -#include <linux/kernel.h>
> -#include <linux/string.h>
> -
> -/*
> - * Qsort routine from Bentley & McIlroy's "Engineering a Sort Function".
> - */
> -#define swapcode(TYPE, parmi, parmj, n) { \
> - long i = (n) / sizeof (TYPE); \
> - register TYPE *pi = (TYPE *) (parmi); \
> - register TYPE *pj = (TYPE *) (parmj); \
> - do { \
> - register TYPE t = *pi; \
> - *pi++ = *pj; \
> - *pj++ = t; \
> - } while (--i > 0); \
> -}
> -
> -#define SWAPINIT(a, es) swaptype = ((char *)a - (char *)0) % sizeof(long) || \
> - es % sizeof(long) ? 2 : es == sizeof(long)? 0 : 1;
> -
> -static __inline void
> -swapfunc(char *a, char *b, int n, int swaptype)
> -{
> - if (swaptype <= 1)
> - swapcode(long, a, b, n)
> - else
> - swapcode(char, a, b, n)
> -}
> -
> -#define swap(a, b) \
> - if (swaptype == 0) { \
> - long t = *(long *)(a); \
> - *(long *)(a) = *(long *)(b); \
> - *(long *)(b) = t; \
> - } else \
> - swapfunc(a, b, es, swaptype)
> -
> -#define vecswap(a, b, n) if ((n) > 0) swapfunc(a, b, n, swaptype)
> -
> -static __inline char *
> -med3(char *a, char *b, char *c, int (*cmp)(const void *, const void *))
> -{
> - return cmp(a, b) < 0 ?
> - (cmp(b, c) < 0 ? b : (cmp(a, c) < 0 ? c : a ))
> - :(cmp(b, c) > 0 ? b : (cmp(a, c) < 0 ? a : c ));
> -}
> -
> -void
> -qsort(void *aa, size_t n, size_t es, int (*cmp)(const void *, const void *))
> -{
> - char *pa, *pb, *pc, *pd, *pl, *pm, *pn;
> - int d, r, swaptype, swap_cnt;
> - register char *a = aa;
> -
> -loop: SWAPINIT(a, es);
> - swap_cnt = 0;
> - 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) > 0;
> - pl -= es)
> - swap(pl, pl - es);
> - return;
> - }
> - pm = (char *)a + (n / 2) * es;
> - if (n > 7) {
> - pl = (char *)a;
> - pn = (char *)a + (n - 1) * es;
> - if (n > 40) {
> - d = (n / 8) * es;
> - pl = med3(pl, pl + d, pl + 2 * d, cmp);
> - pm = med3(pm - d, pm, pm + d, cmp);
> - pn = med3(pn - 2 * d, pn - d, pn, cmp);
> - }
> - pm = med3(pl, pm, pn, cmp);
> - }
> - swap(a, pm);
> - pa = pb = (char *)a + es;
> -
> - pc = pd = (char *)a + (n - 1) * es;
> - for (;;) {
> - while (pb <= pc && (r = cmp(pb, a)) <= 0) {
> - if (r == 0) {
> - swap_cnt = 1;
> - swap(pa, pb);
> - pa += es;
> - }
> - pb += es;
> - }
> - while (pb <= pc && (r = cmp(pc, a)) >= 0) {
> - if (r == 0) {
> - swap_cnt = 1;
> - swap(pc, pd);
> - pd -= es;
> - }
> - pc -= 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) > 0;
> - pl -= es)
> - swap(pl, pl - es);
> - return;
> - }
> -
> - pn = (char *)a + n * es;
> - r = min(pa - (char *)a, pb - pa);
> - vecswap(a, pb - r, r);
> - r = min((long)(pd - pc), (long)(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;
> - }
> -/* qsort(pn - r, r / es, es, cmp);*/
> -}
> Index: linux-2.6.11-rc2/fs/xfs/Makefile
> ===================================================================
> --- linux-2.6.11-rc2.orig/fs/xfs/Makefile
> +++ linux-2.6.11-rc2/fs/xfs/Makefile
> @@ -142,7 +142,6 @@ xfs-y += $(addprefix linux-2.6/, \
> xfs-y += $(addprefix support/, \
> debug.o \
> move.o \
> - qsort.o \
> uuid.o)
>
> xfs-$(CONFIG_XFS_TRACE) += support/ktrace.o
> Index: linux-2.6.11-rc2/fs/xfs/support/qsort.h
> ===================================================================
> --- linux-2.6.11-rc2.orig/fs/xfs/support/qsort.h
> +++ /dev/null
> @@ -1,41 +0,0 @@
> -/*
> - * Copyright (c) 2000-2002 Silicon Graphics, Inc. All Rights Reserved.
> - *
> - * This program is free software; you can redistribute it and/or modify it
> - * under the terms of version 2 of the GNU General Public License as
> - * published by the Free Software Foundation.
> - *
> - * This program is distributed in the hope that it would be useful, but
> - * WITHOUT ANY WARRANTY; without even the implied warranty of
> - * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> - *
> - * Further, this software is distributed without any warranty that it is
> - * free of the rightful claim of any third person regarding infringement
> - * or the like. Any license provided herein, whether implied or
> - * otherwise, applies only to this software file. Patent licenses, if
> - * any, provided herein do not apply to combinations of this program with
> - * other software, or any other product whatsoever.
> - *
> - * You should have received a copy of the GNU General Public License along
> - * with this program; if not, write the Free Software Foundation, Inc., 59
> - * Temple Place - Suite 330, Boston MA 02111-1307, USA.
> - *
> - * Contact information: Silicon Graphics, Inc., 1600 Amphitheatre Pkwy,
> - * Mountain View, CA 94043, or:
> - *
> - * http://www.sgi.com
> - *
> - * For further information regarding this notice, see:
> - *
> - * http://oss.sgi.com/projects/GenInfo/SGIGPLNoticeExplan/
> - */
> -
> -#ifndef QSORT_H
> -#define QSORT_H
> -
> -extern void qsort (void *const pbase,
> - size_t total_elems,
> - size_t size,
> - int (*cmp)(const void *, const void *));
> -
> -#endif
> Index: linux-2.6.11-rc2/fs/xfs/linux-2.6/xfs_linux.h
> ===================================================================
> --- linux-2.6.11-rc2.orig/fs/xfs/linux-2.6/xfs_linux.h
> +++ linux-2.6.11-rc2/fs/xfs/linux-2.6/xfs_linux.h
> @@ -64,7 +64,6 @@
> #include <sema.h>
> #include <time.h>
>
> -#include <support/qsort.h>
> #include <support/ktrace.h>
> #include <support/debug.h>
> #include <support/move.h>
> Index: linux-2.6.11-rc2/fs/Kconfig
> ===================================================================
> --- linux-2.6.11-rc2.orig/fs/Kconfig
> +++ linux-2.6.11-rc2/fs/Kconfig
> @@ -306,6 +306,7 @@ config FS_POSIX_ACL
>
> config XFS_FS
> tristate "XFS filesystem support"
> + select QSORT
> help
> XFS is a high performance journaling filesystem which originated
> on the SGI IRIX platform. It is completely multi-threaded, can
>
> --
> Andreas Gruenbacher <agruen@suse.de>
> SUSE Labs, SUSE LINUX PRODUCTS GMBH
>
> -
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