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([2804:1b3:a7c0:a93a:8d00:c4d9:6d86:9f2b]) by smtp.gmail.com with ESMTPSA id bl6-20020a056830370600b00670461b8be4sm5013511otb.33.2023.01.09.12.50.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 09 Jan 2023 12:50:19 -0800 (PST) Message-ID: <1e1e5e75-0bd7-be05-a89c-d94e290c4d38@linaro.org> Date: Mon, 9 Jan 2023 17:50:17 -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 10/17] string: Improve generic memchr Content-Language: en-US To: Noah Goldstein Cc: libc-alpha@sourceware.org, Richard Henderson References: <20220919195920.956393-1-adhemerval.zanella@linaro.org> <20220919195920.956393-11-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=-12.9 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 05/01/23 20:47, Noah Goldstein wrote: > On Mon, Sep 19, 2022 at 1:05 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} and string-opthr 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/memchr.c | 168 +++++------------- >> .../powerpc32/power4/multiarch/memchr-ppc32.c | 14 +- >> .../powerpc64/multiarch/memchr-ppc64.c | 9 +- >> 3 files changed, 48 insertions(+), 143 deletions(-) >> >> diff --git a/string/memchr.c b/string/memchr.c >> index 422bcd0cd6..08d518b02d 100644 >> --- a/string/memchr.c >> +++ b/string/memchr.c >> @@ -1,10 +1,6 @@ >> -/* Copyright (C) 1991-2022 Free Software Foundation, Inc. >> +/* Scan memory for a character. Generic version >> + 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 >> @@ -20,143 +16,65 @@ >> License along with the GNU C Library; if not, see >> . */ >> >> -#ifndef _LIBC >> -# include >> -#endif >> - >> +#include >> +#include >> +#include >> +#include >> +#include >> +#include >> #include >> >> -#include >> +#undef memchr >> >> -#include >> - >> -#undef __memchr >> -#ifdef _LIBC >> -# undef memchr >> +#ifdef MEMCHR >> +# define __memchr MEMCHR >> #endif >> >> -#ifndef weak_alias >> -# define __memchr memchr >> -#endif >> - >> -#ifndef MEMCHR >> -# define MEMCHR __memchr >> -#endif >> +static inline const char * >> +sadd (uintptr_t x, uintptr_t y) >> +{ >> + uintptr_t ret = INT_ADD_OVERFLOW (x, y) ? (uintptr_t)-1 : x + y; >> + return (const char *)ret; >> +} >> >> /* Search no more than N bytes of S for C. */ >> void * >> -MEMCHR (void const *s, int c_in, size_t n) >> +__memchr (void const *s, int c_in, size_t n) >> { >> - /* On 32-bit hardware, choosing longword to be a 32-bit unsigned >> - long instead of a 64-bit uintmax_t tends to give better >> - performance. On 64-bit hardware, unsigned long is generally 64 >> - bits already. Change this typedef to experiment with >> - performance. */ >> - typedef unsigned long int longword; >> + if (__glibc_unlikely (n == 0)) >> + return NULL; >> >> - const unsigned char *char_ptr; >> - const longword *longword_ptr; >> - longword repeated_one; >> - longword repeated_c; >> - unsigned char c; >> + uintptr_t s_int = (uintptr_t) s; >> >> - c = (unsigned char) c_in; >> + /* Set up a word, each of whose bytes is C. */ >> + op_t repeated_c = repeat_bytes (c_in); >> + op_t before_mask = create_mask (s_int); >> >> - /* Handle the first few bytes by reading one byte at a time. >> - Do this until CHAR_PTR is aligned on a longword boundary. */ >> - for (char_ptr = (const unsigned char *) s; >> - n > 0 && (size_t) char_ptr % sizeof (longword) != 0; >> - --n, ++char_ptr) >> - if (*char_ptr == c) >> - return (void *) char_ptr; >> + /* Compute the address of the last byte taking in consideration possible >> + overflow. */ >> + const char *lbyte = sadd (s_int, n - 1); > > Do you need this? The comparison in the loop is == so letting it > overflow should be fine no? Do you mean the saturation add or the last lbyte check? For saturation add I recall that it requires for memchr (..., SIZE_MAX), otherwise the last byte/word would be incorrect (I fixed some assembly routines that triggered this issue in the past). >> >> - longword_ptr = (const longword *) char_ptr; >> + /* Compute the address of the word containing the last byte. */ >> + const op_t *lword = word_containing (lbyte); >> >> - /* All these elucidatory comments refer to 4-byte longwords, >> - but the theory applies equally well to any size longwords. */ >> + /* Read the first word, but munge it so that bytes before the array >> + will not match goal. */ >> + const op_t *word_ptr = word_containing (s); >> + op_t word = (*word_ptr | before_mask) ^ (repeated_c & before_mask); > > Likewise, prefer just shifting out the invalid comparisons on the first word. I will need to check why this is not really working, I think I suggest it on previous iteration and I could not make it work for some reason. >> >> - /* Compute auxiliary longword values: >> - repeated_one is a value which has a 1 in every byte. >> - repeated_c has c in every byte. */ >> - repeated_one = 0x01010101; >> - repeated_c = c | (c << 8); >> - repeated_c |= repeated_c << 16; >> - if (0xffffffffU < (longword) -1) >> + while (has_eq (word, repeated_c) == 0) >> { >> - repeated_one |= repeated_one << 31 << 1; >> - repeated_c |= repeated_c << 31 << 1; >> - if (8 < sizeof (longword)) >> - { >> - size_t i; >> - >> - for (i = 64; i < sizeof (longword) * 8; i *= 2) >> - { >> - repeated_one |= repeated_one << i; >> - repeated_c |= repeated_c << i; >> - } >> - } >> + if (word_ptr == lword) >> + return NULL; >> + word = *++word_ptr; >> } >> >> - /* Instead of the traditional loop which tests each byte, 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 equal to c. We first use an xor >> - with repeated_c. This reduces the task to testing whether *any of the >> - four* bytes in longword1 is zero. >> - >> - We compute tmp = >> - ((longword1 - repeated_one) & ~longword1) & (repeated_one << 7). >> - That is, we perform the following operations: >> - 1. Subtract repeated_one. >> - 2. & ~longword1. >> - 3. & a mask consisting of 0x80 in every byte. >> - Consider what happens in each byte: >> - - If a byte of longword1 is zero, step 1 and 2 transform it into 0xff, >> - and step 3 transforms it into 0x80. A carry can also be propagated >> - to more significant bytes. >> - - If a byte of longword1 is nonzero, let its lowest 1 bit be at >> - position k (0 <= k <= 7); so the lowest k bits are 0. After step 1, >> - the byte ends in a single bit of value 0 and k bits of value 1. >> - After step 2, the result is just k bits of value 1: 2^k - 1. After >> - step 3, the result is 0. And no carry is produced. >> - So, if longword1 has only non-zero bytes, tmp is zero. >> - Whereas if longword1 has a zero byte, call j the position of the least >> - significant zero byte. Then the result has a zero at positions 0, ..., >> - j-1 and a 0x80 at position j. We cannot predict the result at the more >> - significant bytes (positions j+1..3), but it does not matter since we >> - already have a non-zero bit at position 8*j+7. >> - >> - So, the test whether any byte in longword1 is zero is equivalent to >> - testing whether tmp is nonzero. */ >> - >> - while (n >= sizeof (longword)) >> - { >> - longword longword1 = *longword_ptr ^ repeated_c; >> - >> - if ((((longword1 - repeated_one) & ~longword1) >> - & (repeated_one << 7)) != 0) >> - break; >> - longword_ptr++; >> - n -= sizeof (longword); >> - } >> - >> - char_ptr = (const unsigned char *) longword_ptr; >> - >> - /* At this point, we know that either n < sizeof (longword), or one of the >> - sizeof (longword) bytes starting at char_ptr is == c. On little-endian >> - machines, we could determine the first such byte without any further >> - memory accesses, just by looking at the tmp result from the last loop >> - iteration. But this does not work on big-endian machines. Choose code >> - that works in both cases. */ >> - >> - for (; n > 0; --n, ++char_ptr) >> - { >> - if (*char_ptr == c) >> - return (void *) char_ptr; >> - } >> - >> - return NULL; >> + /* We found a match, but it might be in a byte past the end >> + of the array. */ >> + char *ret = (char *) word_ptr + index_first_eq (word, repeated_c); >> + return (ret <= lbyte) ? ret : NULL; >> } >> -#ifdef weak_alias >> +#ifndef MEMCHR >> weak_alias (__memchr, memchr) >> -#endif >> libc_hidden_builtin_def (memchr) >> +#endif >> diff --git a/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c b/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c >> index fc69df54b3..02877d3c98 100644 >> --- a/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c >> +++ b/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c >> @@ -18,17 +18,11 @@ >> >> #include >> >> -#define MEMCHR __memchr_ppc >> +extern __typeof (memchr) __memchr_ppc attribute_hidden; >> >> -#undef weak_alias >> -#define weak_alias(a, b) >> +#define MEMCHR __memchr_ppc >> +#include >> >> #ifdef SHARED >> -# undef libc_hidden_builtin_def >> -# define libc_hidden_builtin_def(name) \ >> - __hidden_ver1(__memchr_ppc, __GI_memchr, __memchr_ppc); >> +__hidden_ver1(__memchr_ppc, __GI_memchr, __memchr_ppc); >> #endif >> - >> -extern __typeof (memchr) __memchr_ppc attribute_hidden; >> - >> -#include >> diff --git a/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c b/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c >> index 3c966f4403..15beca787b 100644 >> --- a/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c >> +++ b/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c >> @@ -18,14 +18,7 @@ >> >> #include >> >> -#define MEMCHR __memchr_ppc >> - >> -#undef weak_alias >> -#define weak_alias(a, b) >> - >> -# undef libc_hidden_builtin_def >> -# define libc_hidden_builtin_def(name) >> - >> extern __typeof (memchr) __memchr_ppc attribute_hidden; >> >> +#define MEMCHR __memchr_ppc >> #include >> -- >> 2.34.1 >>