From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ej1-x62a.google.com (mail-ej1-x62a.google.com [IPv6:2a00:1450:4864:20::62a]) by sourceware.org (Postfix) with ESMTPS id 1DD243858D33 for ; Thu, 5 Jan 2023 23:10:01 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 1DD243858D33 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-x62a.google.com with SMTP id u19so43400ejm.8 for ; Thu, 05 Jan 2023 15:10:01 -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=PxbAjj0YwQX2MdezROH3wfJuna+puyE3OCwJotmuEjo=; b=ViLIyF3JFg+8oyxVyIjkGe0EG03kcGFVhtuOyBP363neluiu+CDI41C9lCpkZINVs2 5bYjN2+wPlUi7TAr/Bvv7E1DYArvMUne8jmKw6pnMV1fH3XRhWjEjbTK61gtDGLbJCVX 9PGD9wgMDk7rCCOE6GKAKfffUzyh60cLShtur+1tKtUvVixiatendlzjDJacV3R2tZHk VZQFy7mcLK3fQE/PflBMzhqR+HrPEdeWoLHGv7SN344TAoL7u58585qhsUZvbnUU7Bx9 boxtguOePTgscGZlvW2rilUxsdRAPSzNlQBw35iGwddzpfWbtvMrIWD8RaD7GVC4I+em rSVg== 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=PxbAjj0YwQX2MdezROH3wfJuna+puyE3OCwJotmuEjo=; b=zd38INi9Ga23EwvdLoVc7zhHqF3P1E4d9SLN/tGCHauKJMMuGPXijSoAE+2XY/IuwE XVc9D67j4UDMQZQtOpfEal5bB5OIyjsFHOUPRAT2UvEQ0ZL1EN0dR11nghYJVwqfQUW9 T+t5+6EVpSEsmiGFLKWOxDMjzrLy+3kZvwR3uYbFhpmbIzE0UbHv+IN1lvVUklfd28mL MB9UOScWyUkTAY6/K5Eg7NPMk4mryhFJcPU123I6ehB/LbdozZx+BT+Yi9aQAEoqG8hB lP/2VBN7fzTFkVvuFC1w+gPwSeloG7vhRzY0EQNEqAQCi+I9Y3OlzECHxy9oW+/pc/7S yX0Q== X-Gm-Message-State: AFqh2koRfnbRA2HW8oqakZ4TZPCAGJgCbnHHNMCsxOkgbbsO4hT6EIZA O8q07quFq2CfL+Po7QqADBq3IXRXEPXCUtzBlio= X-Google-Smtp-Source: AMrXdXuEjQOWJ9VjZ/rAtFdX1ynL/jsTlr7dW9R/cRLKncWCNuS4yq1GZVH+8zBdPCAvNobAauAliKYE2wDCQ5K9NZ0= X-Received: by 2002:a17:906:b4b:b0:7c3:6be3:279e with SMTP id v11-20020a1709060b4b00b007c36be3279emr6515853ejg.526.1672960199861; Thu, 05 Jan 2023 15:09:59 -0800 (PST) MIME-Version: 1.0 References: <20220919195920.956393-1-adhemerval.zanella@linaro.org> <20220919195920.956393-8-adhemerval.zanella@linaro.org> In-Reply-To: <20220919195920.956393-8-adhemerval.zanella@linaro.org> From: Noah Goldstein Date: Thu, 5 Jan 2023 15:09:48 -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=-9.8 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 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; then only have strchrnul impl to worry about?