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([2804:1b3:a7c0:a93a:8d00:c4d9:6d86:9f2b]) by smtp.gmail.com with ESMTPSA id f13-20020a4ae60d000000b00492f9f46aa4sm5984586oot.36.2023.01.10.10.16.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 10 Jan 2023 10:16:13 -0800 (PST) Message-ID: <47954f91-76a2-bb45-2db8-caa36f81132b@linaro.org> Date: Tue, 10 Jan 2023 15:16:10 -0300 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.6.1 Subject: Re: [PATCH v5 08/17] string: Improve generic strchrnul Content-Language: en-US To: Noah Goldstein Cc: Richard Henderson , libc-alpha@sourceware.org References: <20220919195920.956393-1-adhemerval.zanella@linaro.org> <20220919195920.956393-9-adhemerval.zanella@linaro.org> From: Adhemerval Zanella Netto Organization: Linaro In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-12.7 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,KAM_SHORT,NICE_REPLY_A,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 10/01/23 14:17, Noah Goldstein wrote: > 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. Yeap, this is what I have done.