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Diffstat (limited to 'lib/libc/tests/stdlib/qsort_bench.c')
| -rw-r--r-- | lib/libc/tests/stdlib/qsort_bench.c | 113 |
1 files changed, 113 insertions, 0 deletions
diff --git a/lib/libc/tests/stdlib/qsort_bench.c b/lib/libc/tests/stdlib/qsort_bench.c new file mode 100644 index 000000000000..5f2cfae40140 --- /dev/null +++ b/lib/libc/tests/stdlib/qsort_bench.c @@ -0,0 +1,113 @@ +/*- + * Copyright (c) 2025 Dag-Erling Smørgrav <des@FreeBSD.org> + * + * SPDX-License-Identifier: BSD-2-Clause + */ + +#include <atf-c.h> +#include <stdbool.h> +#include <stdio.h> +#include <stdlib.h> +#include <time.h> +#include <unistd.h> + +/*- + * Measures qsort(3) runtime with pathological input and verify that it + * stays close to N * log2(N). + * + * Thanks to Vivian Hussey for the proof of concept. + * + * The input we construct is similar to a sweep from 0 to N where each + * half, except for the first element, has been reversed; for instance, + * with N = 8, we get { 0, 3, 2, 1, 4, 8, 7, 6 }. This triggers a bug in + * the BSD qsort(3) where it will switch to insertion sort if the pivots + * are sorted. + * + * This article goes into more detail about the bug and its origin: + * + * https://www.raygard.net/2022/02/26/Re-engineering-a-qsort-part-3 + * + * With this optimization (the `if (swap_cnt == 0)` block), qsort(3) needs + * roughly N * N / 4 comparisons to sort our pathological input. Without + * it, it needs only a little more than N * log2(N) comparisons. + */ + +/* we stop testing once a single takes longer than this */ +#define MAXRUNSECS 10 + +static bool debugging; + +static uintmax_t ncmp; + +static int +intcmp(const void *a, const void *b) +{ + ncmp++; + return ((*(int *)a > *(int *)b) - (*(int *)a < *(int *)b)); +} + +static void +qsort_bench(int log2n) +{ + uintmax_t n = 1LLU << log2n; + int *buf; + + /* fill an array with a pathological pattern */ + ATF_REQUIRE(buf = malloc(n * sizeof(*buf))); + buf[0] = 0; + buf[n / 2] = n / 2; + for (unsigned int i = 1; i < n / 2; i++) { + buf[i] = n / 2 - i; + buf[n / 2 + i] = n - i; + } + + ncmp = 0; + qsort(buf, n, sizeof(*buf), intcmp); + + /* check result and free array */ + if (debugging) { + for (unsigned int i = 1; i < n; i++) { + ATF_REQUIRE_MSG(buf[i] > buf[i - 1], + "array is not sorted"); + } + } + free(buf); + + /* check that runtime does not exceed N² */ + ATF_CHECK_MSG(ncmp / n < n, + "runtime %ju exceeds N² for N = %ju", ncmp, n); + + /* check that runtime does not exceed N log N by much */ + ATF_CHECK_MSG(ncmp / n <= log2n + 1, + "runtime %ju exceeds N log N for N = %ju", ncmp, n); +} + +ATF_TC_WITHOUT_HEAD(qsort_bench); +ATF_TC_BODY(qsort_bench, tc) +{ + struct timespec t0, t1; + uintmax_t tus; + + for (int i = 10; i <= 30; i++) { + clock_gettime(CLOCK_UPTIME, &t0); + qsort_bench(i); + clock_gettime(CLOCK_UPTIME, &t1); + tus = t1.tv_sec * 1000000 + t1.tv_nsec / 1000; + tus -= t0.tv_sec * 1000000 + t0.tv_nsec / 1000; + if (debugging) { + fprintf(stderr, "N = 2^%d in %ju.%06jus\n", + i, tus / 1000000, tus % 1000000); + } + /* stop once an individual run exceeds our limit */ + if (tus / 1000000 >= MAXRUNSECS) + break; + } +} + +ATF_TP_ADD_TCS(tp) +{ + debugging = !getenv("__RUNNING_INSIDE_ATF_RUN") && + isatty(STDERR_FILENO); + ATF_TP_ADD_TC(tp, qsort_bench); + return (atf_no_error()); +} |