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stdlib: Handle various corner cases in the fallback heapsort for qsort

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The previous implementation did not consistently apply the rule that
the child nodes of node K are at 2 * K + 1 and 2 * K + 2, or
that the parent node is at (K - 1) / 2.

Add an internal test that targets the heapsort implementation
directly.

Reported-by: Stepan Golosunov <stepan@golosunov.pp.ru>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
This commit is contained in:
Florian Weimer 2023-11-21 16:45:35 +01:00
parent e4d8117b82
commit 55364e1f7d
3 changed files with 173 additions and 17 deletions
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1
stdlib/Makefile
View File

@ -261,6 +261,7 @@ tests := \
# tests
tests-internal := \
tst-qsort4 \
tst-strtod1i \
tst-strtod3 \
tst-strtod4 \

55
stdlib/qsort.c
View File

@ -125,29 +125,44 @@ pop (stack_node *top, char **lo, char **hi, size_t *depth)
return top;
}
/* NB: N is inclusive bound for BASE. */
/* Establish the heap condition at index K, that is, the key at K will
not be less than either of its children, at 2 * K + 1 and 2 * K + 2
(if they exist). N is the last valid index. */
static inline void
siftdown (void *base, size_t size, size_t k, size_t n,
enum swap_type_t swap_type, __compar_d_fn_t cmp, void *arg)
{
while (k <= n / 2)
/* There can only be a heap condition violation if there are
children. */
while (2 * k + 1 <= n)
{
size_t j = 2 * k;
/* Left child. */
size_t j = 2 * k + 1;
/* If the right child is larger, use it. */
if (j < n && cmp (base + (j * size), base + ((j + 1) * size), arg) < 0)
j++;
/* If k is already >= to its children, we are done. */
if (j == k || cmp (base + (k * size), base + (j * size), arg) >= 0)
break;
/* Heal the violation. */
do_swap (base + (size * j), base + (k * size), size, swap_type);
/* Swapping with j may have introduced a violation at j. Fix
it in the next loop iteration. */
k = j;
}
}
/* Establish the heap condition for the indices 0 to N (inclusive). */
static inline void
heapify (void *base, size_t size, size_t n, enum swap_type_t swap_type,
__compar_d_fn_t cmp, void *arg)
{
/* If n is odd, k = n / 2 has a left child at n, so this is the
largest index that can have a heap condition violation regarding
its children. */
size_t k = n / 2;
while (1)
{
@ -157,32 +172,38 @@ heapify (void *base, size_t size, size_t n, enum swap_type_t swap_type,
}
}
/* A non-recursive heapsort, used on introsort implementation as a fallback
routine with worst-case performance of O(nlog n) and worst-case space
complexity of O(1). It sorts the array starting at BASE and ending at
END, with each element of SIZE bytes. The SWAP_TYPE is the callback
function used to swap elements, and CMP is the function used to compare
elements. */
/* A non-recursive heapsort, used on introsort implementation as a
fallback routine with worst-case performance of O(nlog n) and
worst-case space complexity of O(1). It sorts the array starting
at BASE and ending at END (inclusive), with each element of SIZE
bytes. The SWAP_TYPE is the callback function used to swap
elements, and CMP is the function used to compare elements. */
static void
heapsort_r (void *base, void *end, size_t size, enum swap_type_t swap_type,
__compar_d_fn_t cmp, void *arg)
{
const size_t count = ((uintptr_t) end - (uintptr_t) base) / size;
if (count < 2)
size_t n = ((uintptr_t) end - (uintptr_t) base) / size;
if (n <= 1)
/* Handled by insertion sort. */
return;
size_t n = count - 1;
/* Build the binary heap, largest value at the base[0]. */
heapify (base, size, n, swap_type, cmp, arg);
/* On each iteration base[0:n] is the binary heap, while base[n:count]
is sorted. */
while (n > 0)
while (true)
{
/* Indices 0 .. n contain the binary heap. Extract the largest
element put it into the final position in the array. */
do_swap (base, base + (n * size), size, swap_type);
/* The heap is now one element shorter. */
n--;
if (n == 0)
break;
/* By swapping in elements 0 and the previous value of n (now at
n + 1), we likely introduced a heap condition violation. Fix
it for the reduced heap. */
siftdown (base, size, 0, n, swap_type, cmp, arg);
}
}

134
stdlib/tst-qsort4.c Normal file
View File

@ -0,0 +1,134 @@
/* Test the heapsort implementation behind qsort.
Copyright (C) 2023 Free Software Foundation, Inc.
This file is part of the GNU C Library.
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, see
<http://www.gnu.org/licenses/>. */
#include "qsort.c"
#include <stdio.h>
#include <support/check.h>
#include <support/support.h>
static int
cmp (const void *a1, const void *b1, void *closure)
{
const signed char *a = a1;
const signed char *b = b1;
return *a - *b;
}
/* Wrapper around heapsort_r that set ups the required variables. */
static void
heapsort_wrapper (void *const pbase, size_t total_elems, size_t size,
__compar_d_fn_t cmp, void *arg)
{
char *base_ptr = (char *) pbase;
char *lo = base_ptr;
char *hi = &lo[size * (total_elems - 1)];
if (total_elems <= 1)
/* Avoid lossage with unsigned arithmetic below. */
return;
enum swap_type_t swap_type;
if (is_aligned (pbase, size, 8))
swap_type = SWAP_WORDS_64;
else if (is_aligned (pbase, size, 4))
swap_type = SWAP_WORDS_32;
else
swap_type = SWAP_BYTES;
heapsort_r (lo, hi, size, swap_type, cmp, arg);
}
static void
check_one_sort (signed char *array, int length)
{
signed char *copy = xmalloc (length);
memcpy (copy, array, length);
heapsort_wrapper (copy, length, 1, cmp, NULL);
/* Verify that the result is sorted. */
for (int i = 1; i < length; ++i)
if (copy[i] < copy[i - 1])
{
support_record_failure ();
printf ("error: sorting failure for length %d at offset %d\n",
length, i - 1);
printf ("input:");
for (int i = 0; i < length; ++i)
printf (" %d", array[i]);
printf ("\noutput:");
for (int i = 0; i < length; ++i)
printf (" %d", copy[i]);
putchar ('\n');
break;
}
/* Verify that no elements went away or were added. */
{
int expected_counts[256];
for (int i = 0; i < length; ++i)
++expected_counts[array[i] & 0xff];
int actual_counts[256];
for (int i = 0; i < length; ++i)
++actual_counts[copy[i] & 0xff];
for (int i = 0; i < 256; ++i)
TEST_COMPARE (expected_counts[i], expected_counts[i]);
}
free (copy);
}
/* Enumerate all possible combinations of LENGTH elements. */
static void
check_combinations (int length, signed char *start, int offset)
{
if (offset == length)
check_one_sort (start, length);
else
for (int i = 0; i < length; ++i)
{
start[offset] = i;
check_combinations(length, start, offset + 1);
}
}
static int
do_test (void)
{
/* A random permutation of 20 values. */
check_one_sort ((signed char[20]) {5, 12, 16, 10, 14, 11, 9, 13, 8, 15,
0, 17, 3, 7, 1, 18, 2, 19, 4, 6}, 20);
/* A permutation that appeared during adversarial testing for the
quicksort pass. */
check_one_sort ((signed char[16]) {15, 3, 4, 2, 1, 0, 8, 7, 6, 5, 14,
13, 12, 11, 10, 9}, 16);
/* Array lengths 2 and less are not handled by heapsort_r and
deferred to insertion sort. */
for (int i = 3; i <= 8; ++i)
{
signed char *buf = xmalloc (i);
check_combinations (i, buf, 0);
free (buf);
}
return 0;
}
#include <support/test-driver.c>