From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ej1-x62e.google.com (mail-ej1-x62e.google.com [IPv6:2a00:1450:4864:20::62e]) by sourceware.org (Postfix) with ESMTPS id B0DB53858D28 for ; Thu, 5 Jan 2023 23:51:25 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org B0DB53858D28 Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=gmail.com Received: by mail-ej1-x62e.google.com with SMTP id qk9so252092ejc.3 for ; Thu, 05 Jan 2023 15:51:25 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date:message-id:reply-to; bh=i9T6YVdWpEmgH6WUWINX4kjl4nGsfpmjCljh5MJPgkY=; b=j7DrLEqQjCGfwh1D7hOvL2cAVjoJXCbzhqFZ2ShoXDgrpecPbCCs7uuZI5AjkYiMML QGntamwLXScmuBWzhfqlo+bBAlECgBKmObZlW77OX1CDCRTX1QA0ZXJe15I0WhX1H4HR KZASVLUeHkmEjRsxwiCwrdQV45BPCPRgg6YKDM7f4FYHQgezDheAOUPNgS7YRrqPL/bT nE5/Rg7PYT6ex3stK1BrVDZlf5MtHIGrh9b4l6QXBcktOUHWk1OTxCRcFbK0ZTYsdgMo YnlmnhXIZbXCpaGbFwaM+3UqFW0UKoIZPvILNbOFJUdcfQ0KbqIqBL4yD/jjQTB7Fr9O WzCg== X-Google-DKIM-Signature: v=1; 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charset="UTF-8" X-Spam-Status: No, score=-9.2 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM,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 List-Id: On Mon, Sep 19, 2022 at 1:02 PM Adhemerval Zanella via Libc-alpha wrote: > > 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 | 189 ++++++++--------------------------------------- > 1 file changed, 32 insertions(+), 157 deletions(-) > > diff --git a/string/memrchr.c b/string/memrchr.c > index 8eb6829e45..5491689c66 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) 1991-2022 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 +16,57 @@ > 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) again I would make lword, lword - 1, and move the out-of-bounds return check to the return outside of the loop. > { > - /* 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 > -- > 2.34.1 >