[04/17] Add string vectorized find and detection functions

This patch adds generic string find and detection meant to be used in
generic vectorized string implementation.  The idea is to decompose the
basic string operation so each architecture can reimplement if it
provides any specialized hardware instruction.

The 'string-fza.h' provides zero byte detection functions (find_zero_low,
find_zero_all, find_eq_low, find_eq_all, find_zero_eq_low, find_zero_eq_all,
find_zero_ne_low, and find_zero_ne_all).  They are used on both functions
provided by 'string-fzb.h' and 'string-fzi'.

The 'string-fzb.h' provides boolean zero byte detection with the
functions:

  - has_zero: determine if any byte within a word is zero.
  - has_eq: determine byte equality between two words.
  - has_zero_eq: determine if any byte within a word is zero along with
    byte equality between two words.

The 'string-fzi.h' provides zero byte detection along with its positions:

  - index_first_zero: return index of first zero byte within a word.
  - index_first_eq: return index of first byte different between two words.
  - index_first_zero_eq: return index of first zero byte within a word or
    first byte different between two words.
  - index_first_zero_ne: return index of first zero byte within a word or
    first byte equal between two words.
  - index_last_zero: return index of last zero byte within a word.
  - index_last_eq: return index of last byte different between two words.

Also, to avoid libcalls in the '__builtin_c{t,l}z{l}' calls (which may
add performance degradation), inline implementation based on De Bruijn
sequences are added (enabled by a configure check).

Co-authored-by: Richard Henderson <rth@twiddle.net>
---
 config.h.in                      |   8 ++
 configure                        |  54 ++++++++
 configure.ac                     |  34 +++++
 sysdeps/generic/string-extbyte.h |  37 ++++++
 sysdeps/generic/string-fza.h     | 106 ++++++++++++++++
 sysdeps/generic/string-fzb.h     |  49 ++++++++
 sysdeps/generic/string-fzi.h     | 208 +++++++++++++++++++++++++++++++
 7 files changed, 496 insertions(+)
 create mode 100644 sysdeps/generic/string-extbyte.h
 create mode 100644 sysdeps/generic/string-fza.h
 create mode 100644 sysdeps/generic/string-fzb.h
 create mode 100644 sysdeps/generic/string-fzi.h

On Fri, Sep 2, 2022 at 1:42 PM Adhemerval Zanella via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> This patch adds generic string find and detection meant to be used in
> generic vectorized string implementation.  The idea is to decompose the
> basic string operation so each architecture can reimplement if it
> provides any specialized hardware instruction.
>
> The 'string-fza.h' provides zero byte detection functions (find_zero_low,
> find_zero_all, find_eq_low, find_eq_all, find_zero_eq_low, find_zero_eq_all,
> find_zero_ne_low, and find_zero_ne_all).  They are used on both functions
> provided by 'string-fzb.h' and 'string-fzi'.
>
> The 'string-fzb.h' provides boolean zero byte detection with the
> functions:
>
>   - has_zero: determine if any byte within a word is zero.
>   - has_eq: determine byte equality between two words.
>   - has_zero_eq: determine if any byte within a word is zero along with
>     byte equality between two words.
>
> The 'string-fzi.h' provides zero byte detection along with its positions:
>
>   - index_first_zero: return index of first zero byte within a word.
>   - index_first_eq: return index of first byte different between two words.
>   - index_first_zero_eq: return index of first zero byte within a word or
>     first byte different between two words.
>   - index_first_zero_ne: return index of first zero byte within a word or
>     first byte equal between two words.
>   - index_last_zero: return index of last zero byte within a word.
>   - index_last_eq: return index of last byte different between two words.
>
> Also, to avoid libcalls in the '__builtin_c{t,l}z{l}' calls (which may
> add performance degradation), inline implementation based on De Bruijn
> sequences are added (enabled by a configure check).
>
> Co-authored-by: Richard Henderson <rth@twiddle.net>
> ---
>  config.h.in                      |   8 ++
>  configure                        |  54 ++++++++
>  configure.ac                     |  34 +++++
>  sysdeps/generic/string-extbyte.h |  37 ++++++
>  sysdeps/generic/string-fza.h     | 106 ++++++++++++++++
>  sysdeps/generic/string-fzb.h     |  49 ++++++++
>  sysdeps/generic/string-fzi.h     | 208 +++++++++++++++++++++++++++++++
>  7 files changed, 496 insertions(+)
>  create mode 100644 sysdeps/generic/string-extbyte.h
>  create mode 100644 sysdeps/generic/string-fza.h
>  create mode 100644 sysdeps/generic/string-fzb.h
>  create mode 100644 sysdeps/generic/string-fzi.h
>
> diff --git a/config.h.in b/config.h.in
> index 43d32518ab..8f0540d85a 100644
> --- a/config.h.in
> +++ b/config.h.in
> @@ -202,6 +202,14 @@
>  /* An integer used to scale the timeout of test programs.  */
>  #define TIMEOUTFACTOR 1
>
> +/* If compiler supports __builtin_ctz{l} without any external dependencies
> +   (libgcc for instance).  */
> +#define HAVE_BUILTIN_CTZ 0
> +
> +/* If compiler supports __builtin_clz{l} without any external dependencies
> +   (libgcc for instance).  */
> +#define HAVE_BUILTIN_CLZ 0
> +
>  /*
>   */
>
> diff --git a/configure b/configure
> index ff2c406b3b..99cdec1bef 100755
> --- a/configure
> +++ b/configure
> @@ -6726,6 +6726,60 @@ if test $libc_cv_builtin_trap = yes; then
>
>  fi
>
> +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for __builtin_ctz{l} with no external dependencies" >&5
> +$as_echo_n "checking for __builtin_ctz{l} with no external dependencies... " >&6; }
> +if ${libc_cv_builtin_ctz+:} false; then :
> +  $as_echo_n "(cached) " >&6
> +else
> +  libc_cv_builtin_ctz=yes
> +echo 'int foo (unsigned long x) { return __builtin_ctz (x); }' > conftest.c
> +if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS -S conftest.c -o conftest.s 1>&5'
> +  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
> +  (eval $ac_try) 2>&5
> +  ac_status=$?
> +  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
> +  test $ac_status = 0; }; }; then
> +  if grep '__ctz[s,d]i2' conftest.s > /dev/null; then
> +    libc_cv_builtin_ctz=no
> +  fi
> +fi
> +rm -f conftest.c conftest.s
> +
> +fi
> +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_builtin_ctz" >&5
> +$as_echo "$libc_cv_builtin_ctz" >&6; }
> +if test x$libc_cv_builtin_ctz = xyes; then
> +  $as_echo "#define HAVE_BUILTIN_CTZ 1" >>confdefs.h
> +
> +fi
> +
> +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for __builtin_clz{l} with no external dependencies" >&5
> +$as_echo_n "checking for __builtin_clz{l} with no external dependencies... " >&6; }
> +if ${libc_cv_builtin_clz+:} false; then :
> +  $as_echo_n "(cached) " >&6
> +else
> +  libc_cv_builtin_clz=yes
> +echo 'int foo (unsigned long x) { return __builtin_clz (x); }' > conftest.c
> +if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS -S conftest.c -o conftest.s 1>&5'
> +  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
> +  (eval $ac_try) 2>&5
> +  ac_status=$?
> +  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
> +  test $ac_status = 0; }; }; then
> +  if grep '__clz[s,d]i2' conftest.s > /dev/null; then
> +    libc_cv_builtin_clz=no
> +  fi
> +fi
> +rm -f conftest.c conftest.s
> +
> +fi
> +{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_builtin_clz" >&5
> +$as_echo "$libc_cv_builtin_clz" >&6; }
> +if test x$libc_cv_builtin_clz = xyes; then
> +  $as_echo "#define HAVE_BUILTIN_CLZ 1" >>confdefs.h
> +
> +fi
> +
>  ac_ext=cpp
>  ac_cpp='$CXXCPP $CPPFLAGS'
>  ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
> diff --git a/configure.ac b/configure.ac
> index eb5bc6a131..66e02a0566 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -1618,6 +1618,40 @@ if test $libc_cv_builtin_trap = yes; then
>    AC_DEFINE([HAVE_BUILTIN_TRAP])
>  fi
>
> +AC_CACHE_CHECK(for __builtin_ctz{l} with no external dependencies,
> +              libc_cv_builtin_ctz, [dnl
> +libc_cv_builtin_ctz=yes
> +echo 'int foo (unsigned long x) { return __builtin_ctz (x); }' > conftest.c
> +if AC_TRY_COMMAND(${CC-cc} $CFLAGS $CPPFLAGS -S conftest.c -o conftest.s 1>&AS_MESSAGE_LOG_FD); then
> +changequote(,)dnl
> +  if grep '__ctz[s,d]i2' conftest.s > /dev/null; then
> +    libc_cv_builtin_ctz=no
> +  fi
> +changequote([,])dnl
> +fi
> +rm -f conftest.c conftest.s
> +])
> +if test x$libc_cv_builtin_ctz = xyes; then
> +  AC_DEFINE(HAVE_BUILTIN_CTZ)
> +fi
> +
> +AC_CACHE_CHECK(for __builtin_clz{l} with no external dependencies,
> +              libc_cv_builtin_clz, [dnl
> +libc_cv_builtin_clz=yes
> +echo 'int foo (unsigned long x) { return __builtin_clz (x); }' > conftest.c
> +if AC_TRY_COMMAND(${CC-cc} $CFLAGS $CPPFLAGS -S conftest.c -o conftest.s 1>&AS_MESSAGE_LOG_FD); then
> +changequote(,)dnl
> +  if grep '__clz[s,d]i2' conftest.s > /dev/null; then
> +    libc_cv_builtin_clz=no
> +  fi
> +changequote([,])dnl
> +fi
> +rm -f conftest.c conftest.s
> +])
> +if test x$libc_cv_builtin_clz = xyes; then
> +  AC_DEFINE(HAVE_BUILTIN_CLZ)
> +fi
> +
>  dnl C++ feature tests.
>  AC_LANG_PUSH([C++])
>
> diff --git a/sysdeps/generic/string-extbyte.h b/sysdeps/generic/string-extbyte.h
> new file mode 100644
> index 0000000000..c8fecd259f
> --- /dev/null
> +++ b/sysdeps/generic/string-extbyte.h
> @@ -0,0 +1,37 @@
> +/* Extract by from memory word.  Generic C version.
> +   Copyright (C) 2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef _STRING_EXTBYTE_H
> +#define _STRING_EXTBYTE_H 1
> +
> +#include <limits.h>
> +#include <endian.h>
> +#include <string-optype.h>
> +
> +/* Extract the byte at index IDX from word X, with index 0 being the
> +   least significant byte.  */
> +static inline unsigned char
> +extractbyte (op_t x, unsigned int idx)
> +{
> +  if (__BYTE_ORDER == __LITTLE_ENDIAN)
> +    return x >> (idx * CHAR_BIT);
> +  else
> +    return x >> (sizeof (x) - 1 - idx) * CHAR_BIT;
> +}
> +
> +#endif /* _STRING_EXTBYTE_H */
> diff --git a/sysdeps/generic/string-fza.h b/sysdeps/generic/string-fza.h
> new file mode 100644
> index 0000000000..8470437824
> --- /dev/null
> +++ b/sysdeps/generic/string-fza.h
> @@ -0,0 +1,106 @@
> +/* Basic zero byte detection.  Generic C version.
> +   Copyright (C) 2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef _STRING_FZA_H
> +#define _STRING_FZA_H 1
> +
> +#include <limits.h>
> +#include <string-optype.h>
> +#include <string-maskoff.h>
> +
> +/* This function returns non-zero if any byte in X is zero.
> +   More specifically, at least one bit set within the least significant
> +   byte that was zero; other bytes within the word are indeterminate.  */
> +static inline op_t
> +find_zero_low (op_t x)
> +{
> +  /* This expression comes from
> +       https://graphics.stanford.edu/~seander/bithacks.html#ZeroInWord
> +     Subtracting 1 sets 0x80 in a byte that was 0; anding ~x clears
> +     0x80 in a byte that was >= 128; anding 0x80 isolates that test bit.  */
> +  op_t lsb = repeat_bytes (0x01);
> +  op_t msb = repeat_bytes (0x80);
> +  return (x - lsb) & ~x & msb;
> +}
> +
> +/* This function returns at least one bit set within every byte of X that
> +   is zero.  The result is exact in that, unlike find_zero_low, all bytes
> +   are determinate.  This is usually used for finding the index of the
> +   most significant byte that was zero.  */
> +static inline op_t
> +find_zero_all (op_t x)
> +{
> +  /* For each byte, find not-zero by
> +     (0) And 0x7f so that we cannot carry between bytes,
> +     (1) Add 0x7f so that non-zero carries into 0x80,
> +     (2) Or in the original byte (which might have had 0x80 set).
> +     Then invert and mask such that 0x80 is set iff that byte was zero.  */
> +  op_t m = ((op_t)-1 / 0xff) * 0x7f;
> +  return ~(((x & m) + m) | x | m);
> +}
> +
> +/* With similar caveats, identify bytes that are equal between X1 and X2.  */
> +static inline op_t
> +find_eq_low (op_t x1, op_t x2)
> +{
> +  return find_zero_low (x1 ^ x2);
> +}
> +
> +static inline op_t
> +find_eq_all (op_t x1, op_t x2)
> +{
> +  return find_zero_all (x1 ^ x2);
> +}
> +
> +/* With similar caveats, identify zero bytes in X1 and bytes that are
> +   equal between in X1 and X2.  */
> +static inline op_t
> +find_zero_eq_low (op_t x1, op_t x2)
> +{
> +  return find_zero_low (x1) | find_zero_low (x1 ^ x2);
> +}
> +
> +static inline op_t
> +find_zero_eq_all (op_t x1, op_t x2)
> +{
> +  return find_zero_all (x1) | find_zero_all (x1 ^ x2);
> +}
> +
> +/* With similar caveats, identify zero bytes in X1 and bytes that are
> +   not equal between in X1 and X2.  */
> +static inline op_t
> +find_zero_ne_low (op_t x1, op_t x2)
> +{
> +  op_t m = ((op_t)-1 / 0xff) * 0x7f;

Can this use repeat_bytes?
> +  op_t eq = x1 ^ x2;
> +  op_t nz1 = (x1 + m) | x1;    /* msb set if byte not zero.  */
> +  op_t ne2 = (eq + m) | eq;    /* msb set if byte not equal.  */
> +  return (ne2 | ~nz1) & ~m;    /* msb set if x1 zero or x2 not equal.  */
> +}
> +
> +static inline op_t
> +find_zero_ne_all (op_t x1, op_t x2)
> +{
> +  op_t m = ((op_t)-1 / 0xff) * 0x7f;

Likewise. Elsewhere too.
> +  op_t eq = x1 ^ x2;
> +  op_t nz1 = ((x1 & m) + m) | x1;
> +  op_t ne2 = ((eq & m) + m) | eq;
> +  return (ne2 | ~nz1) & ~m;
> +}
> +
> +#endif /* _STRING_FZA_H */
> diff --git a/sysdeps/generic/string-fzb.h b/sysdeps/generic/string-fzb.h
> new file mode 100644
> index 0000000000..f1c0ae0922
> --- /dev/null
> +++ b/sysdeps/generic/string-fzb.h
> @@ -0,0 +1,49 @@
> +/* Zero byte detection, boolean.  Generic C version.
> +   Copyright (C) 2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef _STRING_FZB_H
> +#define _STRING_FZB_H 1
> +
> +#include <endian.h>
> +#include <string-fza.h>
> +
> +/* Determine if any byte within X is zero.  This is a pure boolean test.  */
> +
> +static inline _Bool
> +has_zero (op_t x)
> +{
> +  return find_zero_low (x) != 0;
> +}
> +
> +/* Likewise, but for byte equality between X1 and X2.  */
> +
> +static inline _Bool
> +has_eq (op_t x1, op_t x2)
> +{
> +  return find_eq_low (x1, x2) != 0;
> +}
> +
> +/* Likewise, but for zeros in X1 and equal bytes between X1 and X2.  */
> +
> +static inline _Bool
> +has_zero_eq (op_t x1, op_t x2)
> +{
> +  return find_zero_eq_low (x1, x2);
> +}
> +
> +#endif /* _STRING_FZB_H */
> diff --git a/sysdeps/generic/string-fzi.h b/sysdeps/generic/string-fzi.h
> new file mode 100644
> index 0000000000..8c6b6dc3d8
> --- /dev/null
> +++ b/sysdeps/generic/string-fzi.h
> @@ -0,0 +1,208 @@
> +/* Zero byte detection; indexes.  Generic C version.
> +   Copyright (C) 2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef _STRING_FZI_H
> +#define _STRING_FZI_H 1
> +
> +#include <limits.h>
> +#include <endian.h>
> +#include <string-fza.h>
> +
> +/* An improved bitscan routine, multiplying the De Bruijn sequence with a
> +   0-1 mask separated by the least significant one bit of a scanned integer
> +   or bitboard [1].
> +
> +   [1] https://chessprogramming.wikispaces.com/Kim+Walisch  */
> +static inline unsigned int
> +index_access (const op_t i)
> +{
> +  static const char index[] =
> +  {
> +# if __WORDSIZE == 64
> +     0, 47,  1, 56, 48, 27,  2, 60,
> +    57, 49, 41, 37, 28, 16,  3, 61,
> +    54, 58, 35, 52, 50, 42, 21, 44,
> +    38, 32, 29, 23, 17, 11,  4, 62,
> +    46, 55, 26, 59, 40, 36, 15, 53,
> +    34, 51, 20, 43, 31, 22, 10, 45,
> +    25, 39, 14, 33, 19, 30,  9, 24,
> +    13, 18,  8, 12,  7,  6,  5, 63
> +# else
> +     0,  9,  1, 10, 13, 21,  2, 29,
> +    11, 14, 16, 18, 22, 25,  3, 30,
> +     8, 12, 20, 28, 15, 17, 24,  7,
> +    19, 27, 23,  6, 26,  5,  4, 31
> +# endif
> +  };
> +  return index[i];
> +}
> +
> +/* For architectures which only provides __builtin_clz{l} (HAVE_BUILTIN_CLZ)
> +   and/or __builtin_ctz{l} (HAVE_BUILTIN_CTZ) which uses external libcalls
> +   (for intance __c{l,t}z{s,d}i2 from libgcc) the following wrapper provides
> +   inline implementation for both count leading zeros and count trailing
> +   zeros using branchless computation.  As for builtin, if x is 0 the
> +   result is undefined.*/
> +static inline unsigned int
> +__ctz (op_t x)
> +{
> +#if !HAVE_BUILTIN_CTZ
> +  op_t i;
> +# if __WORDSIZE == 64
> +  i = (x ^ (x - 1)) * 0x03F79D71B4CB0A89ull >> 58;
> +# else
> +  i = (x ^ (x - 1)) * 0x07C4ACDDU >> 27;
> +# endif
> +  return index_access (i);
> +#else
> +  if (sizeof (op_t) == sizeof (long int))
> +    return __builtin_ctzl (x);
> +  else
> +    return __builtin_ctzll (x);
> +#endif
> +};
> +
> +static inline unsigned int
> +__clz (op_t x)
> +{
> +#if !HAVE_BUILTIN_CLZ
> +  unsigned r;
> +  op_t i;
> +
> +  x |= x >> 1;
> +  x |= x >> 2;
> +  x |= x >> 4;
> +  x |= x >> 8;
> +  x |= x >> 16;
> +# if __WORDSIZE == 64
> +  x |= x >> 32;
> +  i = x * 0x03F79D71B4CB0A89ull >> 58;
> +# else
> +  i = x * 0x07C4ACDDU >> 27;
> +# endif
> +  r = index_access (i);
> +  return r ^ (sizeof (op_t) * CHAR_BIT - 1);
> +#else
> +  if (sizeof (op_t) == sizeof (long int))
> +    return __builtin_clzl (x);
> +  else
> +    return __builtin_clzll (x);
> +#endif
> +}
> +
> +/* A subroutine for the index_zero functions.  Given a test word C, return
> +   the (memory order) index of the first byte (in memory order) that is
> +   non-zero.  */
> +static inline unsigned int
> +index_first_ (op_t c)
> +{
> +  _Static_assert (sizeof (op_t) == sizeof (long int)
> +                 || sizeof (op_t) == sizeof (long long int),
> +                 "Unhandled word size");
> +
> +  unsigned r;
> +  if (__BYTE_ORDER == __LITTLE_ENDIAN)
> +    r = __ctz (c);
> +  else
> +    r = __clz (c);
> +  return r / CHAR_BIT;
> +}
> +
> +/* Similarly, but return the (memory order) index of the last byte that is
> +   non-zero.  */
> +static inline unsigned int
> +index_last_ (op_t c)
> +{
> +  _Static_assert (sizeof (op_t) == sizeof (long int)
> +                 || sizeof (op_t) == sizeof (long long int),
> +                 "Unhandled word size");
> +
> +  unsigned r;
> +  if (__BYTE_ORDER == __LITTLE_ENDIAN)
> +    r = __clz (c);
> +  else
> +    r = __ctz (c);
> +  return sizeof (op_t) - 1 - (r / CHAR_BIT);
> +}
> +
> +/* Given a word X that is known to contain a zero byte, return the
> +   index of the first such within the word in memory order.  */
> +
> +static inline unsigned int
> +index_first_zero (op_t x)
> +{
> +  if (__BYTE_ORDER == __LITTLE_ENDIAN)
> +    x = find_zero_low (x);
> +  else
> +    x = find_zero_all (x);
> +  return index_first_ (x);
> +}
> +
> +/* Similarly, but perform the search for byte equality between X1 and X2.  */
> +static inline unsigned int
> +index_first_eq (op_t x1, op_t x2)
> +{
> +  if (__BYTE_ORDER == __LITTLE_ENDIAN)
> +    x1 = find_eq_low (x1, x2);
> +  else
> +    x1 = find_eq_all (x1, x2);
> +  return index_first_ (x1);
> +}
> +
> +/* Similarly, but perform the search for zero within X1 or equality between
> +   X1 and X2.  */
> +static inline unsigned int
> +index_first_zero_eq (op_t x1, op_t x2)
> +{
> +  if (__BYTE_ORDER == __LITTLE_ENDIAN)
> +    x1 = find_zero_eq_low (x1, x2);
> +  else
> +    x1 = find_zero_eq_all (x1, x2);
> +  return index_first_ (x1);
> +}
> +
> +/* Similarly, but perform the search for zero within X1 or
> +   inequality between X1 and X2.  */
> +static inline unsigned int
> +index_first_zero_ne (op_t x1, op_t x2)
> +{
> +  if (__BYTE_ORDER == __LITTLE_ENDIAN)
> +    x1 = find_zero_ne_low (x1, x2);
> +  else
> +    x1 = find_zero_ne_all (x1, x2);
> +  return index_first_ (x1);
> +}
> +
> +/* Similarly, but search for the last zero within X.  */
> +static inline unsigned int
> +index_last_zero (op_t x)
> +{
> +  if (__BYTE_ORDER == __LITTLE_ENDIAN)
> +    x = find_zero_all (x);
> +  else
> +    x = find_zero_low (x);
> +  return index_last_ (x);
> +}
> +
> +static inline unsigned int
> +index_last_eq (op_t x1, op_t x2)
> +{
> +  return index_last_zero (x1 ^ x2);
> +}
> +
> +#endif /* STRING_FZI_H */
> --
> 2.34.1
>

On 03/09/22 00:20, Noah Goldstein wrote:
>> +
>> +/* With similar caveats, identify zero bytes in X1 and bytes that are
>> +   not equal between in X1 and X2.  */
>> +static inline op_t
>> +find_zero_ne_low (op_t x1, op_t x2)
>> +{
>> +  op_t m = ((op_t)-1 / 0xff) * 0x7f;
> 
> Can this use repeat_bytes?

We can, I will update it.

>> +  op_t eq = x1 ^ x2;
>> +  op_t nz1 = (x1 + m) | x1;    /* msb set if byte not zero.  */
>> +  op_t ne2 = (eq + m) | eq;    /* msb set if byte not equal.  */
>> +  return (ne2 | ~nz1) & ~m;    /* msb set if x1 zero or x2 not equal.  */
>> +}
>> +
>> +static inline op_t
>> +find_zero_ne_all (op_t x1, op_t x2)
>> +{
>> +  op_t m = ((op_t)-1 / 0xff) * 0x7f;
> 
> Likewise. Elsewhere too.

Ack.