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5205b32de3
As pointed out in the PR and the article linked below, the switch to insertion sort in the BSD qsort code is based on a misunderstanding of Knuth's TAOCP and is actually a pessimization. As demonstrated by the added test, it is trivially easy to construct pathological input which results in quadratic runtime. Without that misguided optimization, the same input runs in nearly linearithmic time. https://www.raygard.net/2022/02/26/Re-engineering-a-qsort-part-3 PR: 287089 MFC after: 1 week Reviewed by: imp Differential Revision: https://reviews.freebsd.org/D51907
248 lines
6.7 KiB
C
248 lines
6.7 KiB
C
/*-
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* SPDX-License-Identifier: BSD-3-Clause
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*
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* Copyright (c) 1992, 1993
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* The Regents of the University of California. All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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* 3. Neither the name of the University nor the names of its contributors
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* may be used to endorse or promote products derived from this software
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* without specific prior written permission.
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*
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* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*/
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#include <errno.h>
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#include <stdint.h>
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#include <stdlib.h>
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#include <string.h>
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#include "libc_private.h"
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#if defined(I_AM_QSORT_R)
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typedef int cmp_t(const void *, const void *, void *);
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#elif defined(I_AM_QSORT_R_COMPAT)
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typedef int cmp_t(void *, const void *, const void *);
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#elif defined(I_AM_QSORT_S)
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typedef int cmp_t(const void *, const void *, void *);
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#else
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typedef int cmp_t(const void *, const void *);
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#endif
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static inline char *med3(char *, char *, char *, cmp_t *, void *);
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#define MIN(a, b) ((a) < (b) ? a : b)
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/*
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* Qsort routine from Bentley & McIlroy's "Engineering a Sort Function".
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*/
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static inline void
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swapfunc(char *a, char *b, size_t es)
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{
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char t;
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do {
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t = *a;
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*a++ = *b;
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*b++ = t;
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} while (--es > 0);
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}
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#define vecswap(a, b, n) \
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if ((n) > 0) swapfunc(a, b, n)
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#if defined(I_AM_QSORT_R)
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#define CMP(t, x, y) (cmp((x), (y), (t)))
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#elif defined(I_AM_QSORT_R_COMPAT)
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#define CMP(t, x, y) (cmp((t), (x), (y)))
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#elif defined(I_AM_QSORT_S)
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#define CMP(t, x, y) (cmp((x), (y), (t)))
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#else
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#define CMP(t, x, y) (cmp((x), (y)))
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#endif
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static inline char *
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med3(char *a, char *b, char *c, cmp_t *cmp, void *thunk
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#if !defined(I_AM_QSORT_R) && !defined(I_AM_QSORT_R_COMPAT) && !defined(I_AM_QSORT_S)
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__unused
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#endif
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)
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{
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return CMP(thunk, a, b) < 0 ?
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(CMP(thunk, b, c) < 0 ? b : (CMP(thunk, a, c) < 0 ? c : a ))
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:(CMP(thunk, b, c) > 0 ? b : (CMP(thunk, a, c) < 0 ? a : c ));
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}
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/*
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* The actual qsort() implementation is static to avoid preemptible calls when
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* recursing. Also give them different names for improved debugging.
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*/
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#if defined(I_AM_QSORT_R)
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#define local_qsort local_qsort_r
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#elif defined(I_AM_QSORT_R_COMPAT)
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#define local_qsort local_qsort_r_compat
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#elif defined(I_AM_QSORT_S)
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#define local_qsort local_qsort_s
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#endif
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static void
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local_qsort(void *a, size_t n, size_t es, cmp_t *cmp, void *thunk)
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{
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char *pa, *pb, *pc, *pd, *pl, *pm, *pn;
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size_t d1, d2;
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int cmp_result;
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/* if there are less than 2 elements, then sorting is not needed */
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if (__predict_false(n < 2))
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return;
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loop:
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if (n < 7) {
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for (pm = (char *)a + es; pm < (char *)a + n * es; pm += es)
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for (pl = pm;
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pl > (char *)a && CMP(thunk, pl - es, pl) > 0;
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pl -= es)
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swapfunc(pl, pl - es, es);
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return;
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}
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pm = (char *)a + (n / 2) * es;
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if (n > 7) {
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pl = a;
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pn = (char *)a + (n - 1) * es;
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if (n > 40) {
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size_t d = (n / 8) * es;
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pl = med3(pl, pl + d, pl + 2 * d, cmp, thunk);
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pm = med3(pm - d, pm, pm + d, cmp, thunk);
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pn = med3(pn - 2 * d, pn - d, pn, cmp, thunk);
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}
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pm = med3(pl, pm, pn, cmp, thunk);
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}
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swapfunc(a, pm, es);
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pa = pb = (char *)a + es;
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pc = pd = (char *)a + (n - 1) * es;
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for (;;) {
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while (pb <= pc && (cmp_result = CMP(thunk, pb, a)) <= 0) {
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if (cmp_result == 0) {
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swapfunc(pa, pb, es);
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pa += es;
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}
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pb += es;
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}
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while (pb <= pc && (cmp_result = CMP(thunk, pc, a)) >= 0) {
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if (cmp_result == 0) {
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swapfunc(pc, pd, es);
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pd -= es;
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}
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pc -= es;
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}
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if (pb > pc)
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break;
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swapfunc(pb, pc, es);
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pb += es;
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pc -= es;
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}
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pn = (char *)a + n * es;
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d1 = MIN(pa - (char *)a, pb - pa);
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vecswap(a, pb - d1, d1);
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/*
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* Cast es to preserve signedness of right-hand side of MIN()
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* expression, to avoid sign ambiguity in the implied comparison. es
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* is safely within [0, SSIZE_MAX].
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*/
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d1 = MIN(pd - pc, pn - pd - (ssize_t)es);
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vecswap(pb, pn - d1, d1);
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d1 = pb - pa;
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d2 = pd - pc;
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if (d1 <= d2) {
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/* Recurse on left partition, then iterate on right partition */
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if (d1 > es) {
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local_qsort(a, d1 / es, es, cmp, thunk);
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}
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if (d2 > es) {
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/* Iterate rather than recurse to save stack space */
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/* qsort(pn - d2, d2 / es, es, cmp); */
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a = pn - d2;
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n = d2 / es;
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goto loop;
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}
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} else {
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/* Recurse on right partition, then iterate on left partition */
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if (d2 > es) {
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local_qsort(pn - d2, d2 / es, es, cmp, thunk);
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}
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if (d1 > es) {
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/* Iterate rather than recurse to save stack space */
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/* qsort(a, d1 / es, es, cmp); */
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n = d1 / es;
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goto loop;
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}
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}
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}
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#if defined(I_AM_QSORT_R)
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void
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(qsort_r)(void *a, size_t n, size_t es, cmp_t *cmp, void *thunk)
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{
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local_qsort_r(a, n, es, cmp, thunk);
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}
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#elif defined(I_AM_QSORT_R_COMPAT)
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void
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__qsort_r_compat(void *a, size_t n, size_t es, void *thunk, cmp_t *cmp)
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{
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local_qsort_r_compat(a, n, es, cmp, thunk);
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}
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#elif defined(I_AM_QSORT_S)
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errno_t
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qsort_s(void *a, rsize_t n, rsize_t es, cmp_t *cmp, void *thunk)
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{
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if (n > RSIZE_MAX) {
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__throw_constraint_handler_s("qsort_s : n > RSIZE_MAX", EINVAL);
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return (EINVAL);
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} else if (es > RSIZE_MAX) {
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__throw_constraint_handler_s("qsort_s : es > RSIZE_MAX",
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EINVAL);
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return (EINVAL);
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} else if (n != 0) {
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if (a == NULL) {
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__throw_constraint_handler_s("qsort_s : a == NULL",
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EINVAL);
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return (EINVAL);
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} else if (cmp == NULL) {
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__throw_constraint_handler_s("qsort_s : cmp == NULL",
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EINVAL);
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return (EINVAL);
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} else if (es <= 0) {
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__throw_constraint_handler_s("qsort_s : es <= 0",
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EINVAL);
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return (EINVAL);
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}
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}
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local_qsort_s(a, n, es, cmp, thunk);
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return (0);
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}
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#else
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void
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qsort(void *a, size_t n, size_t es, cmp_t *cmp)
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{
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local_qsort(a, n, es, cmp, NULL);
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}
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#endif
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