From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ej1-x636.google.com (mail-ej1-x636.google.com [IPv6:2a00:1450:4864:20::636]) by sourceware.org (Postfix) with ESMTPS id D41323858D28 for ; Thu, 5 Jan 2023 23:19:22 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org D41323858D28 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-x636.google.com with SMTP id gh17so103966ejb.6 for ; Thu, 05 Jan 2023 15:19:22 -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=Tm8ERWDaOyhOFGYsBy9BRs48gDDOZ2I1XUw8vsC/wQ0=; b=SKeXFsRlfcWoFBz9iAPmr7ulga8Ww6Apa/n8CkrhFQlBaLHKtkVQgQz7O+bAQuLUVq YCvh4ZQThPRvI2+VYYB4T8MEFPlZXwC9W4anDFv4Mkvv6BB93143jAiCSKVbHgBOxRKR r9PZ6j3U1/qS7LZ+OEsn+WWB6fU7+94HLYYXxlWimF3aBiQ+NaIkcaGXJkD3ZlQxVibj 3oBZ24AEZlWePhy3h2/Zwj/yGYrv1VQaoWXTXNEUsWIH/S6Lpmk65m2Vh66SQDtmcfj5 rq1o8d87Z0CfMDozUj8EDXRTd5Cra3s5CpN2BX+2LPqrdUvxYSNgmnLbZW93BAyhvcAc kaFw== 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=Tm8ERWDaOyhOFGYsBy9BRs48gDDOZ2I1XUw8vsC/wQ0=; b=VfaWMTvab9HBzoauvLpV/YWzlWdnmB9tzIarbPZaTd6zoD6F0SzdMvF/sV+Bu6p4lN WhntAXZNUDXBM5jXnrv7DMM/EDDSTmoXn2upW4RTaHjl/Wvoc036SVD4WVrLAA/UwnTD Hd+lrubv4k5eb43dDdpq/eNvegByUX4EhQIAQJ9IQxutxGMlWhJLdQH8erp1FMUwfh1D Gh6bXvbDQ+RSL2legpeTqVUT8sXYfUmM4ONJ2vStzBt1mCP/4XzORGPhehJg1uCz8+rb NxE6xGBWCRDGS6W+81mR6A2cG1aVEBz6uTM7groMsJXEFDSGkGkslTxigXVpDh584s9i MQhw== X-Gm-Message-State: AFqh2kp0sQ1w8C3lHTRzTCCNvFywc2Fc8rBs4N/42fYZvLdGUkrhCYlI dozEvYs8Q70vgLD6Mu3x7CW4bMC0DSNDxf3q6UY= X-Google-Smtp-Source: AMrXdXuFZ1E+s5F7e6QENtKfZAnQyKRi68Gv/rXsZbVx4PW6PMvqno/n6yoSe2PzLBAYORvVYuxorQfPW0kySi35gk4= X-Received: by 2002:a17:906:2bce:b0:84c:c4a4:bd4 with SMTP id n14-20020a1709062bce00b0084cc4a40bd4mr919010ejg.645.1672960761619; Thu, 05 Jan 2023 15:19:21 -0800 (PST) MIME-Version: 1.0 References: <20220919195920.956393-1-adhemerval.zanella@linaro.org> <20220919195920.956393-8-adhemerval.zanella@linaro.org> In-Reply-To: From: Noah Goldstein Date: Thu, 5 Jan 2023 15:19:10 -0800 Message-ID: Subject: Re: [PATCH v5 07/17] string: Improve generic strchr To: Adhemerval Zanella Cc: libc-alpha@sourceware.org, Richard Henderson Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-9.4 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM,GIT_PATCH_0,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 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?