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([2804:1b3:a7c0:a93a:9d68:9321:503b:c278]) by smtp.gmail.com with ESMTPSA id bb9-20020a056820160900b0049be9c3c15dsm4586749oob.33.2023.01.09.11.39.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 09 Jan 2023 11:39:38 -0800 (PST) Message-ID: <643eb66c-57c8-8551-1ba5-e1ff83f00b29@linaro.org> Date: Mon, 9 Jan 2023 16:39:36 -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 07/17] string: Improve generic strchr Content-Language: en-US To: Noah Goldstein Cc: libc-alpha@sourceware.org, Richard Henderson References: <20220919195920.956393-1-adhemerval.zanella@linaro.org> <20220919195920.956393-8-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=-12.2 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,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 20:19, Noah Goldstein wrote: > On Thu, Jan 5, 2023 at 3:09 PM Noah Goldstein wrote: >> >> On Mon, Sep 19, 2022 at 1:01 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-extbyte function. >>> >>> 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/strchr.c | 172 +++++++--------------------------------- >>> sysdeps/s390/strchr-c.c | 11 +-- >>> 2 files changed, 34 insertions(+), 149 deletions(-) >>> >>> diff --git a/string/strchr.c b/string/strchr.c >>> index bfd0c4e4bc..6bbee7f79d 100644 >>> --- a/string/strchr.c >>> +++ b/string/strchr.c >>> @@ -22,164 +22,48 @@ >>> >>> #include >>> #include >>> +#include >>> +#include >>> +#include >>> +#include >>> +#include >>> +#include >>> >>> #undef strchr >>> +#undef index >>> >>> -#ifndef STRCHR >>> -# define STRCHR strchr >>> +#ifdef STRCHR >>> +# define strchr STRCHR >>> #endif >>> >>> /* Find the first occurrence of C in S. */ >>> char * >>> -STRCHR (const char *s, int c_in) >>> +strchr (const char *s, 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) >>> - return (void *) char_ptr; >>> - else if (*char_ptr == '\0') >>> - return NULL; >>> - >>> - /* 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. */ >>> + unsigned char c = (unsigned char) c_in; >>> + 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) s; >>> + const op_t *word_ptr = word_containing (s); >>> >>> - 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)); >>> >>> - So it ignores everything except 128's, when they're aligned >>> - properly. >>> + while (! has_zero_eq (word, repeated_c)) >>> + word = *++word_ptr; >>> >>> - 3) But wait! Aren't we looking for C as well as 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++; >>> - >>> - /* 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 >>> - >>> - /* That caught zeroes. Now test for C. */ >>> - || ((((longword ^ charmask) + magic_bits) ^ ~(longword ^ charmask)) >>> - & ~magic_bits) != 0) >>> - { >>> - /* Which of the bytes was C or zero? >>> - If none of them were, it was a misfire; continue the search. */ >>> - >>> - const unsigned char *cp = (const unsigned char *) (longword_ptr - 1); >>> - >>> - if (*cp == c) >>> - return (char *) cp; >>> - else if (*cp == '\0') >>> - return NULL; >>> - if (*++cp == c) >>> - return (char *) cp; >>> - else if (*cp == '\0') >>> - return NULL; >>> - if (*++cp == c) >>> - return (char *) cp; >>> - else if (*cp == '\0') >>> - return NULL; >>> - if (*++cp == c) >>> - return (char *) cp; >>> - else if (*cp == '\0') >>> - return NULL; >>> - if (sizeof (longword) > 4) >>> - { >>> - if (*++cp == c) >>> - return (char *) cp; >>> - else if (*cp == '\0') >>> - return NULL; >>> - if (*++cp == c) >>> - return (char *) cp; >>> - else if (*cp == '\0') >>> - return NULL; >>> - if (*++cp == c) >>> - return (char *) cp; >>> - else if (*cp == '\0') >>> - return NULL; >>> - if (*++cp == c) >>> - return (char *) cp; >>> - else if (*cp == '\0') >>> - return NULL; >>> - } >>> - } >>> - } >>> + op_t found = index_first_zero_eq (word, repeated_c); >>> >>> + if (extractbyte (word, found) == c) >>> + return (char *) (word_ptr) + found; >>> return NULL; >>> } >>> - >>> -#ifdef weak_alias >>> -# undef index >>> +#ifndef STRCHR >>> weak_alias (strchr, index) >>> -#endif >>> libc_hidden_builtin_def (strchr) >>> +#endif >>> diff --git a/sysdeps/s390/strchr-c.c b/sysdeps/s390/strchr-c.c >>> index 4ac3a62fba..a5a1781b1c 100644 >>> --- a/sysdeps/s390/strchr-c.c >>> +++ b/sysdeps/s390/strchr-c.c >>> @@ -21,13 +21,14 @@ >>> #if HAVE_STRCHR_C >>> # if HAVE_STRCHR_IFUNC >>> # define STRCHR STRCHR_C >>> -# undef weak_alias >>> +# endif >>> + >>> +# include >>> + >>> +# if HAVE_STRCHR_IFUNC >>> # if defined SHARED && IS_IN (libc) >>> -# undef libc_hidden_builtin_def >>> -# define libc_hidden_builtin_def(name) \ >>> - __hidden_ver1 (__strchr_c, __GI_strchr, __strchr_c); >>> +__hidden_ver1 (__strchr_c, __GI_strchr, __strchr_c); >>> # endif >>> # endif >>> >>> -# include >>> #endif >>> -- >>> 2.34.1 >>> >> >> Can this just be implemented as: >> >> char * r = strchrnul(p, c); >> return *r ? r : NULL; > Thats wrong, should be: `return (*r == c) ? r : NULL;` >> >> then only have strchrnul impl to worry about? Yes, although I think strchr is a more used symbol than strchrnul. However, we can optimize it later by adding a __strchrnul_inline and expand it on both strchr and strchrnul. I will change to use strchrnul as you suggested.