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([2804:1b3:a7c0:a93a:a504:f3f6:dd7b:801]) by smtp.gmail.com with ESMTPSA id kw18-20020a056870ac1200b0014c8b5d54b2sm7990274oab.20.2023.01.11.12.46.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 Jan 2023 12:46:26 -0800 (PST) To: libc-alpha@sourceware.org, Noah Goldstein , Richard Henderson Cc: Adhemerval Zanella Netto Subject: [PATCH v7 11/17] string: Improve generic memrchr Date: Wed, 11 Jan 2023 17:45:52 -0300 Message-Id: <20230111204558.2402155-12-adhemerval.zanella@linaro.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20230111204558.2402155-1-adhemerval.zanella@linaro.org> References: <20230111204558.2402155-1-adhemerval.zanella@linaro.org> MIME-Version: 1.0 X-Spam-Status: No, score=-13.0 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 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: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-Patchwork-Original-From: Adhemerval Zanella via Libc-alpha From: Adhemerval Zanella Reply-To: Adhemerval Zanella Errors-To: libc-alpha-bounces+patch=linaro.org@sourceware.org Sender: "Libc-alpha" From: Adhemerval Zanella Netto New algorithm have the following key differences: - Use string-fz{b,i} functions. Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc-linux-gnu, and powerpc64-linux-gnu by removing the arch-specific assembly implementation and disabling multi-arch (it covers both LE and BE for 64 and 32 bits). Co-authored-by: Richard Henderson --- string/memrchr.c | 190 ++++++++--------------------------------------- 1 file changed, 33 insertions(+), 157 deletions(-) diff --git a/string/memrchr.c b/string/memrchr.c index 18b20ff76a..54647d47c2 100644 --- a/string/memrchr.c +++ b/string/memrchr.c @@ -1,11 +1,6 @@ /* memrchr -- find the last occurrence of a byte in a memory block Copyright (C) 1991-2023 Free Software Foundation, Inc. This file is part of the GNU C Library. - Based on strlen implementation by Torbjorn Granlund (tege@sics.se), - with help from Dan Sahlin (dan@sics.se) and - commentary by Jim Blandy (jimb@ai.mit.edu); - adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu), - and implemented by Roland McGrath (roland@ai.mit.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 @@ -21,177 +16,58 @@ License along with the GNU C Library; if not, see . */ -#include - -#ifdef HAVE_CONFIG_H -# include -#endif - -#if defined _LIBC -# include -# include -#endif - -#if defined HAVE_LIMITS_H || defined _LIBC -# include -#endif - -#define LONG_MAX_32_BITS 2147483647 - -#ifndef LONG_MAX -# define LONG_MAX LONG_MAX_32_BITS -#endif - -#include +#include +#include +#include +#include +#include #undef __memrchr #undef memrchr -#ifndef weak_alias -# define __memrchr memrchr +#ifdef MEMRCHR +# define __memrchr MEMRCHR #endif -/* Search no more than N bytes of S for C. */ void * -#ifndef MEMRCHR -__memrchr -#else -MEMRCHR -#endif - (const void *s, int c_in, size_t n) +__memrchr (const void *s, int c_in, size_t n) { - const unsigned char *char_ptr; - const unsigned long int *longword_ptr; - unsigned long int longword, magic_bits, charmask; - unsigned char c; - - c = (unsigned char) c_in; - /* Handle the last few characters by reading one character at a time. - Do this until CHAR_PTR is aligned on a longword boundary. */ - for (char_ptr = (const unsigned char *) s + n; - n > 0 && ((unsigned long int) char_ptr - & (sizeof (longword) - 1)) != 0; - --n) - if (*--char_ptr == c) + Do this until CHAR_PTR is aligned on a word boundary, or + the entirety of small inputs. */ + const unsigned char *char_ptr = (const unsigned char *) (s + n); + size_t align = (uintptr_t) char_ptr % sizeof (op_t); + if (n < OP_T_THRES || align > n) + align = n; + + for (size_t i = 0; i < align; ++i) + if (*--char_ptr == c_in) return (void *) char_ptr; - /* All these elucidatory comments refer to 4-byte longwords, - but the theory applies equally well to 8-byte longwords. */ + const op_t *word_ptr = (const op_t *) char_ptr; + n -= align; + if (__glibc_unlikely (n == 0)) + return NULL; - longword_ptr = (const unsigned long int *) char_ptr; + /* Compute the address of the word containing the initial byte. */ + const op_t *lword = word_containing (s); - /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits - the "holes." Note that there is a hole just to the left of - each byte, with an extra at the end: - - bits: 01111110 11111110 11111110 11111111 - bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD - - The 1-bits make sure that carries propagate to the next 0-bit. - The 0-bits provide holes for carries to fall into. */ - magic_bits = -1; - magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1; - - /* Set up a longword, each of whose bytes is C. */ - charmask = c | (c << 8); - charmask |= charmask << 16; -#if LONG_MAX > LONG_MAX_32_BITS - charmask |= charmask << 32; -#endif + /* Set up a word, each of whose bytes is C. */ + op_t repeated_c = repeat_bytes (c_in); - /* Instead of the traditional loop which tests each character, - we will test a longword at a time. The tricky part is testing - if *any of the four* bytes in the longword in question are zero. */ - while (n >= sizeof (longword)) + while (word_ptr != lword) { - /* We tentatively exit the loop if adding MAGIC_BITS to - LONGWORD fails to change any of the hole bits of LONGWORD. - - 1) Is this safe? Will it catch all the zero bytes? - Suppose there is a byte with all zeros. Any carry bits - propagating from its left will fall into the hole at its - least significant bit and stop. Since there will be no - carry from its most significant bit, the LSB of the - byte to the left will be unchanged, and the zero will be - detected. - - 2) Is this worthwhile? Will it ignore everything except - zero bytes? Suppose every byte of LONGWORD has a bit set - somewhere. There will be a carry into bit 8. If bit 8 - is set, this will carry into bit 16. If bit 8 is clear, - one of bits 9-15 must be set, so there will be a carry - into bit 16. Similarly, there will be a carry into bit - 24. If one of bits 24-30 is set, there will be a carry - into bit 31, so all of the hole bits will be changed. - - The one misfire occurs when bits 24-30 are clear and bit - 31 is set; in this case, the hole at bit 31 is not - changed. If we had access to the processor carry flag, - we could close this loophole by putting the fourth hole - at bit 32! - - So it ignores everything except 128's, when they're aligned - properly. - - 3) But wait! Aren't we looking for C, not zero? - Good point. So what we do is XOR LONGWORD with a longword, - each of whose bytes is C. This turns each byte that is C - into a zero. */ - - longword = *--longword_ptr ^ charmask; - - /* Add MAGIC_BITS to LONGWORD. */ - if ((((longword + magic_bits) - - /* Set those bits that were unchanged by the addition. */ - ^ ~longword) - - /* Look at only the hole bits. If any of the hole bits - are unchanged, most likely one of the bytes was a - zero. */ - & ~magic_bits) != 0) + op_t word = *--word_ptr; + if (has_eq (word, repeated_c)) { - /* Which of the bytes was C? If none of them were, it was - a misfire; continue the search. */ - - const unsigned char *cp = (const unsigned char *) longword_ptr; - -#if LONG_MAX > 2147483647 - if (cp[7] == c) - return (void *) &cp[7]; - if (cp[6] == c) - return (void *) &cp[6]; - if (cp[5] == c) - return (void *) &cp[5]; - if (cp[4] == c) - return (void *) &cp[4]; -#endif - if (cp[3] == c) - return (void *) &cp[3]; - if (cp[2] == c) - return (void *) &cp[2]; - if (cp[1] == c) - return (void *) &cp[1]; - if (cp[0] == c) - return (void *) cp; + /* We found a match, but it might be in a byte past the start + of the array. */ + char *ret = (char *) word_ptr + index_last_eq (word, repeated_c); + return ret >= (char *) s ? ret : NULL; } - - n -= sizeof (longword); } - - char_ptr = (const unsigned char *) longword_ptr; - - while (n-- > 0) - { - if (*--char_ptr == c) - return (void *) char_ptr; - } - - return 0; + return NULL; } #ifndef MEMRCHR -# ifdef weak_alias weak_alias (__memrchr, memrchr) -# endif #endif