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([2804:1b3:a7c0:8177:87f4:ac6d:3153:10b3]) by smtp.gmail.com with ESMTPSA id i28-20020a056a00005c00b006e37bedbcbdsm4369802pfk.115.2024.02.19.12.45.10 for (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 19 Feb 2024 12:45:10 -0800 (PST) From: Adhemerval Zanella To: libc-alpha@sourceware.org Subject: [PATCH 3/3] wcsmbs: Ensure wcstr worst-case linear execution time (BZ 23865) Date: Mon, 19 Feb 2024 17:45:02 -0300 Message-Id: <20240219204502.3095343-4-adhemerval.zanella@linaro.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20240219204502.3095343-1-adhemerval.zanella@linaro.org> References: <20240219204502.3095343-1-adhemerval.zanella@linaro.org> MIME-Version: 1.0 X-Spam-Status: No, score=-12.5 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, KAM_SHORT, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP, T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.30 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: libc-alpha-bounces+patch=linaro.org@sourceware.org It uses the same two-way algorithm used on strstr, strcasestr, and memmem. Different than strstr, neither the "shift table" optimization nor the self-adapting filtering check is used because it would result in a too-large shift table (and it also simplifies the implementation bit). Checked on x86_64-linux-gnu and aarch64-linux-gnu. --- wcsmbs/wcs-two-way.h | 312 +++++++++++++++++++++++++++++++++++++++++++ wcsmbs/wcsstr.c | 104 +++++---------- 2 files changed, 344 insertions(+), 72 deletions(-) create mode 100644 wcsmbs/wcs-two-way.h diff --git a/wcsmbs/wcs-two-way.h b/wcsmbs/wcs-two-way.h new file mode 100644 index 0000000000..2dcee7fc1a --- /dev/null +++ b/wcsmbs/wcs-two-way.h @@ -0,0 +1,312 @@ +/* Byte-wise substring search, using the Two-Way algorithm. + Copyright (C) 2024 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 + . */ + +/* Before including this file, you need to include (and + before that, if not part of libc), and define: + AVAILABLE(h, h_l, j, n_l) + A macro that returns nonzero if there are + at least N_L characters left starting at H[J]. + H is 'wchar_t *', H_L, J, and N_L are 'size_t'; + H_L is an lvalue. For NUL-terminated searches, + H_L can be modified each iteration to avoid + having to compute the end of H up front. + + For case-insensitivity, you may optionally define: + CMP_FUNC(p1, p2, l) A macro that returns 0 iff the first L + characters of P1 and P2 are equal. + CANON_ELEMENT(c) A macro that canonicalizes an element right after + it has been fetched from one of the two strings. + The argument is an 'wchar_t'; the result must + be an 'wchar_t' as well. +*/ + +#include +#include +#include /* Defines MAX. */ + +/* We use the Two-Way string matching algorithm, which guarantees + linear complexity with constant space. + + See http://www-igm.univ-mlv.fr/~lecroq/string/node26.html#SECTION00260 + and http://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm +*/ + +#ifndef CANON_ELEMENT +# define CANON_ELEMENT(c) c +#endif +#ifndef CMP_FUNC +# define CMP_FUNC __wmemcmp +#endif + +/* Perform a critical factorization of NEEDLE, of length NEEDLE_LEN. + Return the index of the first character in the right half, and set + *PERIOD to the global period of the right half. + + The global period of a string is the smallest index (possibly its + length) at which all remaining bytes in the string are repetitions + of the prefix (the last repetition may be a subset of the prefix). + + When NEEDLE is factored into two halves, a local period is the + length of the smallest word that shares a suffix with the left half + and shares a prefix with the right half. All factorizations of a + non-empty NEEDLE have a local period of at least 1 and no greater + than NEEDLE_LEN. + + A critical factorization has the property that the local period + equals the global period. All strings have at least one critical + factorization with the left half smaller than the global period. + + Given an ordered alphabet, a critical factorization can be computed + in linear time, with 2 * NEEDLE_LEN comparisons, by computing the + larger of two ordered maximal suffixes. The ordered maximal + suffixes are determined by lexicographic comparison of + periodicity. */ +static size_t +critical_factorization (const wchar_t *needle, size_t needle_len, + size_t *period) +{ + /* Index of last character of left half, or SIZE_MAX. */ + size_t max_suffix, max_suffix_rev; + size_t j; /* Index into NEEDLE for current candidate suffix. */ + size_t k; /* Offset into current period. */ + size_t p; /* Intermediate period. */ + wchar_t a, b; /* Current comparison bytes. */ + + /* Special case NEEDLE_LEN of 1 or 2 (all callers already filtered + out 0-length needles. */ + if (needle_len < 3) + { + *period = 1; + return needle_len - 1; + } + + /* Invariants: + 0 <= j < NEEDLE_LEN - 1 + -1 <= max_suffix{,_rev} < j (treating SIZE_MAX as if it were signed) + min(max_suffix, max_suffix_rev) < global period of NEEDLE + 1 <= p <= global period of NEEDLE + p == global period of the substring NEEDLE[max_suffix{,_rev}+1...j] + 1 <= k <= p + */ + + /* Perform lexicographic search. */ + max_suffix = SIZE_MAX; + j = 0; + k = p = 1; + while (j + k < needle_len) + { + a = CANON_ELEMENT (needle[j + k]); + b = CANON_ELEMENT (needle[max_suffix + k]); + if (a < b) + { + /* Suffix is smaller, period is entire prefix so far. */ + j += k; + k = 1; + p = j - max_suffix; + } + else if (a == b) + { + /* Advance through repetition of the current period. */ + if (k != p) + ++k; + else + { + j += p; + k = 1; + } + } + else /* b < a */ + { + /* Suffix is larger, start over from current location. */ + max_suffix = j++; + k = p = 1; + } + } + *period = p; + + /* Perform reverse lexicographic search. */ + max_suffix_rev = SIZE_MAX; + j = 0; + k = p = 1; + while (j + k < needle_len) + { + a = CANON_ELEMENT (needle[j + k]); + b = CANON_ELEMENT (needle[max_suffix_rev + k]); + if (b < a) + { + /* Suffix is smaller, period is entire prefix so far. */ + j += k; + k = 1; + p = j - max_suffix_rev; + } + else if (a == b) + { + /* Advance through repetition of the current period. */ + if (k != p) + ++k; + else + { + j += p; + k = 1; + } + } + else /* a < b */ + { + /* Suffix is larger, start over from current location. */ + max_suffix_rev = j++; + k = p = 1; + } + } + + /* Choose the shorted suffix. Return the first character of the right + half, rather than the last character of the left half. */ + if (max_suffix_rev + 1 < max_suffix + 1) + return max_suffix + 1; + *period = p; + return max_suffix_rev + 1; +} + +/* Return the first location of non-empty NEEDLE within HAYSTACK, or + NULL. HAYSTACK_LEN is the minimum known length of HAYSTACK. + + If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at + most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching. + If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 * + HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching. */ +static inline wchar_t * +two_way_short_needle (const wchar_t *haystack, size_t haystack_len, + const wchar_t *needle, size_t needle_len) +{ + size_t i; /* Index into current character of NEEDLE. */ + size_t j; /* Index into current window of HAYSTACK. */ + size_t period; /* The period of the right half of needle. */ + size_t suffix; /* The index of the right half of needle. */ + + /* Factor the needle into two halves, such that the left half is + smaller than the global period, and the right half is + periodic (with a period as large as NEEDLE_LEN - suffix). */ + suffix = critical_factorization (needle, needle_len, &period); + + /* Perform the search. Each iteration compares the right half + first. */ + if (CMP_FUNC (needle, needle + period, suffix) == 0) + { + /* Entire needle is periodic; a mismatch can only advance by the + period, so use memory to avoid rescanning known occurrences + of the period. */ + size_t memory = 0; + j = 0; + while (AVAILABLE (haystack, haystack_len, j, needle_len)) + { + const wchar_t *pneedle; + const wchar_t *phaystack; + + /* Scan for matches in right half. */ + i = MAX (suffix, memory); + pneedle = &needle[i]; + phaystack = &haystack[i + j]; + while (i < needle_len && (CANON_ELEMENT (*pneedle++) + == CANON_ELEMENT (*phaystack++))) + ++i; + if (needle_len <= i) + { + /* Scan for matches in left half. */ + i = suffix - 1; + pneedle = &needle[i]; + phaystack = &haystack[i + j]; + while (memory < i + 1 && (CANON_ELEMENT (*pneedle--) + == CANON_ELEMENT (*phaystack--))) + --i; + if (i + 1 < memory + 1) + return (wchar_t *) (haystack + j); + /* No match, so remember how many repetitions of period + on the right half were scanned. */ + j += period; + memory = needle_len - period; + } + else + { + j += i - suffix + 1; + memory = 0; + } + } + } + else + { + const wchar_t *phaystack; + /* The comparison always starts from needle[suffix], so cache it + and use an optimized first-character loop. */ + wchar_t needle_suffix = CANON_ELEMENT (needle[suffix]); + + /* The two halves of needle are distinct; no extra memory is + required, and any mismatch results in a maximal shift. */ + period = MAX (suffix, needle_len - suffix) + 1; + j = 0; + while (AVAILABLE (haystack, haystack_len, j, needle_len)) + { + wchar_t haystack_char; + const wchar_t *pneedle; + + phaystack = &haystack[suffix + j]; + + while (needle_suffix + != (haystack_char = CANON_ELEMENT (*phaystack++))) + { + ++j; + if (!AVAILABLE (haystack, haystack_len, j, needle_len)) + goto ret0; + } + + /* Scan for matches in right half. */ + i = suffix + 1; + pneedle = &needle[i]; + while (i < needle_len) + { + if (CANON_ELEMENT (*pneedle++) + != (haystack_char = CANON_ELEMENT (*phaystack++))) + break; + ++i; + } + if (needle_len <= i) + { + /* Scan for matches in left half. */ + i = suffix - 1; + pneedle = &needle[i]; + phaystack = &haystack[i + j]; + while (i != SIZE_MAX) + { + if (CANON_ELEMENT (*pneedle--) + != (haystack_char = CANON_ELEMENT (*phaystack--))) + break; + --i; + } + if (i == SIZE_MAX) + return (wchar_t *) (haystack + j); + j += period; + } + else + j += i - suffix + 1; + } + } +ret0: __attribute__ ((unused)) + return NULL; +} + +#undef AVAILABLE +#undef CANON_ELEMENT +#undef CMP_FUNC diff --git a/wcsmbs/wcsstr.c b/wcsmbs/wcsstr.c index 78f1cc9ce0..7e791a5356 100644 --- a/wcsmbs/wcsstr.c +++ b/wcsmbs/wcsstr.c @@ -1,4 +1,5 @@ -/* Copyright (C) 1995-2024 Free Software Foundation, Inc. +/* Locate a substring in a wide-character string. + Copyright (C) 1995-2024 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 @@ -15,82 +16,41 @@ License along with the GNU C Library; if not, see . */ -/* - * The original strstr() file contains the following comment: - * - * My personal strstr() implementation that beats most other algorithms. - * Until someone tells me otherwise, I assume that this is the - * fastest implementation of strstr() in C. - * I deliberately chose not to comment it. You should have at least - * as much fun trying to understand it, as I had to write it :-). - * - * Stephen R. van den Berg, berg@pool.informatik.rwth-aachen.de */ - #include +#include + +#define AVAILABLE(h, h_l, j, n_l) \ + (((j) + (n_l) <= (h_l)) \ + || ((h_l) += __wcsnlen ((void*)((h) + (h_l)), (n_l) + 128), \ + (j) + (n_l) <= (h_l))) +#include "wcs-two-way.h" + +/* Hash character pairs so a small shift table can be used. All bits of + p[0] are included, but not all bits from p[-1]. So if two equal hashes + match on p[-1], p[0] matches too. Hash collisions are harmless and result + in smaller shifts. */ +#define hash2(p) (((size_t)(p)[0] - ((size_t)(p)[-1] << 3)) % sizeof (shift)) wchar_t * wcsstr (const wchar_t *haystack, const wchar_t *needle) { - wchar_t b, c; - - if ((b = *needle) != L'\0') - { - haystack--; /* possible ANSI violation */ - do - if ((c = *++haystack) == L'\0') - goto ret0; - while (c != b); - - if (!(c = *++needle)) - goto foundneedle; - ++needle; - goto jin; - - for (;;) - { - wchar_t a; - const wchar_t *rhaystack, *rneedle; - - do - { - if (!(a = *++haystack)) - goto ret0; - if (a == b) - break; - if ((a = *++haystack) == L'\0') - goto ret0; -shloop: ; - } - while (a != b); - -jin: if (!(a = *++haystack)) - goto ret0; - - if (a != c) - goto shloop; - - if (*(rhaystack = haystack-- + 1) == (a = *(rneedle = needle))) - do - { - if (a == L'\0') - goto foundneedle; - if (*++rhaystack != (a = *++needle)) - break; - if (a == L'\0') - goto foundneedle; - } - while (*++rhaystack == (a = *++needle)); - - needle = rneedle; /* took the register-poor approach */ - - if (a == L'\0') - break; - } - } -foundneedle: - return (wchar_t*) haystack; -ret0: - return NULL; + const wchar_t *hs = (const wchar_t *) haystack; + const wchar_t *ne = (const wchar_t *) needle; + + /* Ensure haystack length is at least as long as needle length. + Since a match may occur early on in a huge haystack, use strnlen + and read ahead a few cachelines for improved performance. */ + size_t ne_len = __wcslen (ne); + size_t hs_len = __wcsnlen (hs, ne_len | 128); + if (hs_len < ne_len) + return NULL; + + /* Check whether we have a match. This improves performance since we + avoid initialization overheads. */ + if (__wmemcmp (hs, ne, ne_len) == 0) + return (wchar_t *) hs; + + return two_way_short_needle (hs, hs_len, ne, ne_len); } /* This alias is for backward compatibility with drafts of the ISO C standard. Unfortunately the Unix(TM) standard requires this name. */