From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ej1-x62a.google.com (mail-ej1-x62a.google.com [IPv6:2a00:1450:4864:20::62a]) by sourceware.org (Postfix) with ESMTPS id 1C4D43858D33 for ; Tue, 10 Jan 2023 17:18:10 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 1C4D43858D33 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-x62a.google.com with SMTP id ss4so23208760ejb.11 for ; Tue, 10 Jan 2023 09:18:10 -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=/k3pYw5CGOuPM91vHV6l4cr7dQ5Zj18q/AjHLb2Hv9U=; b=JBsho/u0N5wsS4BGYeX38HE6MZwrFOYqQhGnJvCJ/3G1BNiRktzhqxCBUkI0AfrREU 5+Xg7CVFwQs5s9/Pj3GGsjAw8mExKEBJKpdoZAjoKXeorARl3dOlsmqCU3ubYzPlTmb6 sOiqVxWMmhH7DWSCDQL5kb5ZIR3TJhQAG/M4lnebFJ4kREFmJJ6xBfgYpb44VGXUpgGZ 4/9y6WvA1YDyVsAnyGghsYDufVVj85bwrvmLdEg7EC/ag8rtJ1trcSD4IHYQS2djhycr STVMfMcgw+JgooDhw3gGQY+oRKwA08eE1Fl8T8F6zT16rGi0BvIa57Fdwytq/MP+2er+ 6gyw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=/k3pYw5CGOuPM91vHV6l4cr7dQ5Zj18q/AjHLb2Hv9U=; b=KORkcujOM3tJcN2zxnmieiPWco2i5ObTyKww6gSVgjFl6mkFfBPGcJI0PIPEX7Uu7i xewMC8FGDcivL89pFP1G9P2AF6Ca342K9xq+O26kfJSF7sebTRYFCzuM6XLVTr3eQP1N 6fG+nHomCbhYOpQkRyQyatBRsRAfT+q8JXsvN2zL+B2ihRMY+o+1lfmdWv9nB6rVyqlI QOXNtrpEAvE9Wt07eFf1uyNeDX7I8eSctURgFqxZR+fdyL3ejmqrsXj1ToG97X/EuWvU bjIa0FcJNYv107+5Rt9OgWw/mh68ZXT3uXN5yBd9m8bA5jt77XdSh9R0KSTzmFJGyHZf ra3w== X-Gm-Message-State: AFqh2kq5ajcXGOyTX7hw/J3YJvJfuD+TpkCxWyMtnaYwwatlMdSFOM49 BERvfaHiTeKvUlQFFkAnouYMXwvAbhRZRcVQzfk= X-Google-Smtp-Source: AMrXdXuk5dMN/yrmprm+dTcevSNGu7XC9pIXTsMkq2yYA6OVG3LhBlrMOlpB1Q6f3/lGx1QHRPWkmi30R/pY479xMwU= X-Received: by 2002:a17:906:281b:b0:7c1:98f:be57 with SMTP id r27-20020a170906281b00b007c1098fbe57mr3944809ejc.97.1673371088812; Tue, 10 Jan 2023 09:18:08 -0800 (PST) MIME-Version: 1.0 References: <20220919195920.956393-1-adhemerval.zanella@linaro.org> <20220919195920.956393-9-adhemerval.zanella@linaro.org> In-Reply-To: From: Noah Goldstein Date: Tue, 10 Jan 2023 09:17:58 -0800 Message-ID: Subject: Re: [PATCH v5 08/17] string: Improve generic strchrnul To: Adhemerval Zanella Netto Cc: Richard Henderson , libc-alpha@sourceware.org Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-9.8 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 Tue, Jan 10, 2023 at 6:18 AM Adhemerval Zanella Netto wrote: > > > > On 09/01/23 20:33, Richard Henderson wrote: > > On 1/9/23 12:35, Adhemerval Zanella Netto wrote: > >> > >> > >> On 05/01/23 20:17, Noah Goldstein wrote: > >>> On Mon, Sep 19, 2022 at 1:04 PM Adhemerval Zanella via Libc-alpha > >>> wrote: > >>>> > >>>> New algorithm have the following key differences: > >>>> > >>>> - Reads first word unaligned and use string-maskoff function to > >>>> remove unwanted data. This strategy follow arch-specific > >>>> optimization used on aarch64 and powerpc. > >>>> > >>>> - Use string-fz{b,i} functions. > >>>> > >>>> Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc64-linux-gnu, > >>>> and powerpc-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/strchrnul.c | 156 +++--------------- > >>>> .../power4/multiarch/strchrnul-ppc32.c | 4 - > >>>> sysdeps/s390/strchrnul-c.c | 2 - > >>>> 3 files changed, 24 insertions(+), 138 deletions(-) > >>>> > >>>> diff --git a/string/strchrnul.c b/string/strchrnul.c > >>>> index 0cc1fc6bb0..67defa3dab 100644 > >>>> --- a/string/strchrnul.c > >>>> +++ b/string/strchrnul.c > >>>> @@ -1,10 +1,5 @@ > >>>> /* 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 > >>>> - bug fix and commentary by Jim Blandy (jimb@ai.mit.edu); > >>>> - adaptation to strchr 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 > >>>> @@ -21,146 +16,43 @@ > >>>> . */ > >>>> > >>>> #include > >>>> -#include > >>>> #include > >>>> +#include > >>>> +#include > >>>> +#include > >>>> +#include > >>>> +#include > >>>> > >>>> #undef __strchrnul > >>>> #undef strchrnul > >>>> > >>>> -#ifndef STRCHRNUL > >>>> -# define STRCHRNUL __strchrnul > >>>> +#ifdef STRCHRNUL > >>>> +# define __strchrnul STRCHRNUL > >>>> #endif > >>>> > >>>> /* Find the first occurrence of C in S or the final NUL byte. */ > >>>> char * > >>>> -STRCHRNUL (const char *s, int c_in) > >>>> +__strchrnul (const char *str, int c_in) > >>>> { > >>>> - 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 first 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; > >>>> - ((unsigned long int) char_ptr & (sizeof (longword) - 1)) != 0; > >>>> - ++char_ptr) > >>>> - if (*char_ptr == c || *char_ptr == '\0') > >>>> - return (void *) char_ptr; > >>>> - > >>>> - /* All these elucidatory comments refer to 4-byte longwords, > >>>> - but the theory applies equally well to 8-byte longwords. */ > >>>> - > >>>> - longword_ptr = (unsigned long int *) char_ptr; > >>>> - > >>>> - /* 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 (sizeof (longword) > 4) > >>>> - /* Do the shift in two steps to avoid a warning if long has 32 bits. */ > >>>> - charmask |= (charmask << 16) << 16; > >>>> - if (sizeof (longword) > 8) > >>>> - abort (); > >>>> - > >>>> - /* 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. */ > >>>> - for (;;) > >>>> - { > >>>> - /* 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. > >>>> + /* Set up a word, each of whose bytes is C. */ > >>>> + op_t repeated_c = repeat_bytes (c_in); > >>>> > >>>> - 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. > >>>> + /* Align the input address to op_t. */ > >>>> + uintptr_t s_int = (uintptr_t) str; > >>>> + const op_t *word_ptr = word_containing (str); > >>>> > >>>> - 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! > >>>> + /* Read the first aligned word, but force bytes before the string to > >>>> + match neither zero nor goal (we make sure the high bit of each byte > >>>> + is 1, and the low 7 bits are all the opposite of the goal byte). */ > >>>> + op_t bmask = create_mask (s_int); > >>>> + op_t word = (*word_ptr | bmask) ^ (repeated_c & highbit_mask (bmask)); > >>> > >>> Think much clearer (and probably better codegen) is: > >>> find_zero_eq_low/all(word, repeated) >> (s_int * CHAR_BIT) > >> > >> It does not seem to work, at least not replacing the two lines with: > >> > >> op_t word = find_zero_eq_all/low (*word_ptr, repeated_c) >> (s_int * CHAR_BIT); > > > > Oh, two fine points: > > > > (1) big-endian would want shifting left, > > (2) alpha would want shifting by bits not bytes, > > because the cmpbge insn produces an 8-bit mask. > > > > so you'd need to hide this shift in the headers like create_mask(). > > Alright, the following works: > > > static __always_inline op_t > check_mask (op_t word, uintptr_t s_int) > { > if (__BYTE_ORDER == __LITTLE_ENDIAN) > return word >> (CHAR_BIT * (s_int % sizeof (s_int))); > else > return word << (CHAR_BIT * (s_int % sizeof (s_int))); > } Imo put this in with "[PATCH v5 03/17] Add string-maskoff.h generic header" think may also be needed for memchr. > > char * > __strchrnul (const char *str, int c_in) > { > op_t repeated_c = repeat_bytes (c_in); > > uintptr_t s_int = (uintptr_t) str; > const op_t *word_ptr = word_containing (str); > > op_t word = *word_ptr; > > op_t mask = check_mask (find_zero_eq_all (word, repeated_c), s_int); > if (mask != 0) > return (char *) str + index_first_(mask); > > do > word = *++word_ptr; > while (! has_zero_eq (word, repeated_c)); > > op_t found = index_first_zero_eq (word, repeated_c); > return (char *) word_ptr + found; > } > > > I had to use find_zero_eq_all to avoid uninitialized bytes, that triggered > some regression on tests that use strchr (for instance test-strpbrk). > > I will update the patch based on this version.