From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-yb1-xb2f.google.com (mail-yb1-xb2f.google.com [IPv6:2607:f8b0:4864:20::b2f]) by sourceware.org (Postfix) with ESMTPS id 69DCE3858D37 for ; Wed, 1 Feb 2023 19:44:16 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 69DCE3858D37 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-yb1-xb2f.google.com with SMTP id x139so1068831ybe.13 for ; Wed, 01 Feb 2023 11:44:16 -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=5txbAkBfWp+62tF1dr7eea4xQnNSpNJJWOSIiQX0MFE=; b=EF3R6djWmg5AHm/SaKuT1z8mkih8voPB/6dy87I46Imq/wV4JG1w/WioodOU5yTZzg WztHr+huakQh31PXa1tXVaeBIWnkP6ZOb3qLElWLDGRDgvLhLirGAkhR4voL2hTthye9 pEUC/0t8O0+HgSmr5xbamcXwhqMtG1P0Pc9wunL8bw5Ppwl129a8Apsew5fym19osDiG 104urV9eS90rvXQqPSiflqLKh8nrj1dL6Ha+JMISVNRylWjmD2cZ5vdfmsOsj/4xSBty EbikYfjAXapR9zBez6vKgXJvUChlCltwW4s1TYsUeJvbw2N3NmuXC/sqlzPwZNjikYf3 DO7Q== 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=5txbAkBfWp+62tF1dr7eea4xQnNSpNJJWOSIiQX0MFE=; b=dmU+BxTBYZcNCuecHgRItcDqo2N2dbd3iJbDsrw4ep8F/7RrAE9yfL8PkJq0Pijbu3 pY5siPDZQVYfctK9us/wsfJTAjAaAgvTPg7dNM4wKmGBRwqG2PDYHODIa2OeikkQd07e aHwpDBEO3IimK5BpgyyG+tLsrb1352p42FvMdeU+orIeV8Fm+m9bj8WRzBLxZ92bgZXR ZPX7PCl52Dqb7JB1QUlhKc5gPHUP6beF3BqQ2L+84o75RWWqMsRjJzfjRUwMjWVwB/IS eOhRQV3ZLdeCwTILZkp9Glh1VyJhBolq7bkEji2JIbUABWB6+SpGCmN8tDFM0SU8ltYP KUDA== X-Gm-Message-State: AO0yUKUEPBSQEVrDyvPwuoVgKc4+CiO2JMb72kGdwW+JDm4tOpQKCu0t LF1U/O2BzOvh1mWr3JduIbIoIZD0y5wngiFxxYE= X-Google-Smtp-Source: AK7set+jb3LaJkhtJQglA+nPCEfIrFDnAvuRJLm2F30G1n/IGSp/IWxy0Dwb5QKg0A3Vqfyyo62FDw5AidDY69QrK28= X-Received: by 2002:a25:d38b:0:b0:7c7:ecbd:18f9 with SMTP id e133-20020a25d38b000000b007c7ecbd18f9mr522289ybf.69.1675280655657; Wed, 01 Feb 2023 11:44:15 -0800 (PST) MIME-Version: 1.0 References: <20230201170406.303978-1-adhemerval.zanella@linaro.org> <20230201170406.303978-7-adhemerval.zanella@linaro.org> In-Reply-To: <20230201170406.303978-7-adhemerval.zanella@linaro.org> From: Noah Goldstein Date: Wed, 1 Feb 2023 13:44:03 -0600 Message-ID: Subject: Re: [PATCH v11 06/29] string: Improve generic strchr To: Adhemerval Zanella Cc: libc-alpha@sourceware.org, Richard Henderson , Jeff Law , Xi Ruoyao Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-8.7 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 Wed, Feb 1, 2023 at 11:04 AM Adhemerval Zanella wrote: > > New algorithm now calls strchrnul. > > 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). > > Reviewed-by: Richard Henderson > --- > string/strchr.c | 159 ++-------------------------------------- > sysdeps/s390/strchr-c.c | 11 +-- > 2 files changed, 14 insertions(+), 156 deletions(-) > > diff --git a/string/strchr.c b/string/strchr.c > index 1572b8b42e..30c3eb10f2 100644 > --- a/string/strchr.c > +++ b/string/strchr.c > @@ -21,165 +21,22 @@ > . */ > > #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. > - > - 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. > - > - 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! > - > - So it ignores everything except 128's, when they're aligned > - properly. > - > - 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; > - } > - } > - } > - > - return NULL; > + char *r = __strchrnul (s, c_in); > + return (*(unsigned char *)r == (unsigned char)c_in) ? r : 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 c00f2cceea..90822ae0f4 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 > NIT: Imo this patch should come after the strchrnul one.