[v5,10/17] string: Improve generic memchr
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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-opthr functions.
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 <rth@twiddle.net>
---
string/memchr.c | 168 +++++-------------
.../powerpc32/power4/multiarch/memchr-ppc32.c | 14 +-
.../powerpc64/multiarch/memchr-ppc64.c | 9 +-
3 files changed, 48 insertions(+), 143 deletions(-)
Comments
On Mon, Sep 19, 2022 at 1:05 PM Adhemerval Zanella via Libc-alpha
<libc-alpha@sourceware.org> 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-opthr functions.
>
> 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 <rth@twiddle.net>
> ---
> string/memchr.c | 168 +++++-------------
> .../powerpc32/power4/multiarch/memchr-ppc32.c | 14 +-
> .../powerpc64/multiarch/memchr-ppc64.c | 9 +-
> 3 files changed, 48 insertions(+), 143 deletions(-)
>
> diff --git a/string/memchr.c b/string/memchr.c
> index 422bcd0cd6..08d518b02d 100644
> --- a/string/memchr.c
> +++ b/string/memchr.c
> @@ -1,10 +1,6 @@
> -/* Copyright (C) 1991-2022 Free Software Foundation, Inc.
> +/* Scan memory for a character. Generic version
> + Copyright (C) 1991-2022 Free Software Foundation, Inc.
> This file is part of the GNU C Library.
> - Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
> - with help from Dan Sahlin (dan@sics.se) and
> - commentary by Jim Blandy (jimb@ai.mit.edu);
> - adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
> - and implemented by Roland McGrath (roland@ai.mit.edu).
>
> The GNU C Library is free software; you can redistribute it and/or
> modify it under the terms of the GNU Lesser General Public
> @@ -20,143 +16,65 @@
> License along with the GNU C Library; if not, see
> <https://www.gnu.org/licenses/>. */
>
> -#ifndef _LIBC
> -# include <config.h>
> -#endif
> -
> +#include <intprops.h>
> +#include <string-fza.h>
> +#include <string-fzb.h>
> +#include <string-fzi.h>
> +#include <string-maskoff.h>
> +#include <string-opthr.h>
> #include <string.h>
>
> -#include <stddef.h>
> +#undef memchr
>
> -#include <limits.h>
> -
> -#undef __memchr
> -#ifdef _LIBC
> -# undef memchr
> +#ifdef MEMCHR
> +# define __memchr MEMCHR
> #endif
>
> -#ifndef weak_alias
> -# define __memchr memchr
> -#endif
> -
> -#ifndef MEMCHR
> -# define MEMCHR __memchr
> -#endif
> +static inline const char *
> +sadd (uintptr_t x, uintptr_t y)
> +{
> + uintptr_t ret = INT_ADD_OVERFLOW (x, y) ? (uintptr_t)-1 : x + y;
> + return (const char *)ret;
> +}
>
> /* Search no more than N bytes of S for C. */
> void *
> -MEMCHR (void const *s, int c_in, size_t n)
> +__memchr (void const *s, int c_in, size_t n)
> {
> - /* On 32-bit hardware, choosing longword to be a 32-bit unsigned
> - long instead of a 64-bit uintmax_t tends to give better
> - performance. On 64-bit hardware, unsigned long is generally 64
> - bits already. Change this typedef to experiment with
> - performance. */
> - typedef unsigned long int longword;
> + if (__glibc_unlikely (n == 0))
> + return NULL;
>
> - const unsigned char *char_ptr;
> - const longword *longword_ptr;
> - longword repeated_one;
> - longword repeated_c;
> - unsigned char c;
> + uintptr_t s_int = (uintptr_t) s;
>
> - c = (unsigned char) c_in;
> + /* Set up a word, each of whose bytes is C. */
> + op_t repeated_c = repeat_bytes (c_in);
> + op_t before_mask = create_mask (s_int);
>
> - /* Handle the first few bytes by reading one byte at a time.
> - Do this until CHAR_PTR is aligned on a longword boundary. */
> - for (char_ptr = (const unsigned char *) s;
> - n > 0 && (size_t) char_ptr % sizeof (longword) != 0;
> - --n, ++char_ptr)
> - if (*char_ptr == c)
> - return (void *) char_ptr;
> + /* Compute the address of the last byte taking in consideration possible
> + overflow. */
> + const char *lbyte = sadd (s_int, n - 1);
Do you need this? The comparison in the loop is == so letting it
overflow should be fine no?
>
> - longword_ptr = (const longword *) char_ptr;
> + /* Compute the address of the word containing the last byte. */
> + const op_t *lword = word_containing (lbyte);
>
> - /* All these elucidatory comments refer to 4-byte longwords,
> - but the theory applies equally well to any size longwords. */
> + /* Read the first word, but munge it so that bytes before the array
> + will not match goal. */
> + const op_t *word_ptr = word_containing (s);
> + op_t word = (*word_ptr | before_mask) ^ (repeated_c & before_mask);
Likewise, prefer just shifting out the invalid comparisons on the first word.
>
> - /* Compute auxiliary longword values:
> - repeated_one is a value which has a 1 in every byte.
> - repeated_c has c in every byte. */
> - repeated_one = 0x01010101;
> - repeated_c = c | (c << 8);
> - repeated_c |= repeated_c << 16;
> - if (0xffffffffU < (longword) -1)
> + while (has_eq (word, repeated_c) == 0)
> {
> - repeated_one |= repeated_one << 31 << 1;
> - repeated_c |= repeated_c << 31 << 1;
> - if (8 < sizeof (longword))
> - {
> - size_t i;
> -
> - for (i = 64; i < sizeof (longword) * 8; i *= 2)
> - {
> - repeated_one |= repeated_one << i;
> - repeated_c |= repeated_c << i;
> - }
> - }
> + if (word_ptr == lword)
> + return NULL;
> + word = *++word_ptr;
> }
>
> - /* Instead of the traditional loop which tests each byte, 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 equal to c. We first use an xor
> - with repeated_c. This reduces the task to testing whether *any of the
> - four* bytes in longword1 is zero.
> -
> - We compute tmp =
> - ((longword1 - repeated_one) & ~longword1) & (repeated_one << 7).
> - That is, we perform the following operations:
> - 1. Subtract repeated_one.
> - 2. & ~longword1.
> - 3. & a mask consisting of 0x80 in every byte.
> - Consider what happens in each byte:
> - - If a byte of longword1 is zero, step 1 and 2 transform it into 0xff,
> - and step 3 transforms it into 0x80. A carry can also be propagated
> - to more significant bytes.
> - - If a byte of longword1 is nonzero, let its lowest 1 bit be at
> - position k (0 <= k <= 7); so the lowest k bits are 0. After step 1,
> - the byte ends in a single bit of value 0 and k bits of value 1.
> - After step 2, the result is just k bits of value 1: 2^k - 1. After
> - step 3, the result is 0. And no carry is produced.
> - So, if longword1 has only non-zero bytes, tmp is zero.
> - Whereas if longword1 has a zero byte, call j the position of the least
> - significant zero byte. Then the result has a zero at positions 0, ...,
> - j-1 and a 0x80 at position j. We cannot predict the result at the more
> - significant bytes (positions j+1..3), but it does not matter since we
> - already have a non-zero bit at position 8*j+7.
> -
> - So, the test whether any byte in longword1 is zero is equivalent to
> - testing whether tmp is nonzero. */
> -
> - while (n >= sizeof (longword))
> - {
> - longword longword1 = *longword_ptr ^ repeated_c;
> -
> - if ((((longword1 - repeated_one) & ~longword1)
> - & (repeated_one << 7)) != 0)
> - break;
> - longword_ptr++;
> - n -= sizeof (longword);
> - }
> -
> - char_ptr = (const unsigned char *) longword_ptr;
> -
> - /* At this point, we know that either n < sizeof (longword), or one of the
> - sizeof (longword) bytes starting at char_ptr is == c. On little-endian
> - machines, we could determine the first such byte without any further
> - memory accesses, just by looking at the tmp result from the last loop
> - iteration. But this does not work on big-endian machines. Choose code
> - that works in both cases. */
> -
> - for (; n > 0; --n, ++char_ptr)
> - {
> - if (*char_ptr == c)
> - return (void *) char_ptr;
> - }
> -
> - return NULL;
> + /* We found a match, but it might be in a byte past the end
> + of the array. */
> + char *ret = (char *) word_ptr + index_first_eq (word, repeated_c);
> + return (ret <= lbyte) ? ret : NULL;
> }
> -#ifdef weak_alias
> +#ifndef MEMCHR
> weak_alias (__memchr, memchr)
> -#endif
> libc_hidden_builtin_def (memchr)
> +#endif
> diff --git a/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c b/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c
> index fc69df54b3..02877d3c98 100644
> --- a/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c
> +++ b/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c
> @@ -18,17 +18,11 @@
>
> #include <string.h>
>
> -#define MEMCHR __memchr_ppc
> +extern __typeof (memchr) __memchr_ppc attribute_hidden;
>
> -#undef weak_alias
> -#define weak_alias(a, b)
> +#define MEMCHR __memchr_ppc
> +#include <string/memchr.c>
>
> #ifdef SHARED
> -# undef libc_hidden_builtin_def
> -# define libc_hidden_builtin_def(name) \
> - __hidden_ver1(__memchr_ppc, __GI_memchr, __memchr_ppc);
> +__hidden_ver1(__memchr_ppc, __GI_memchr, __memchr_ppc);
> #endif
> -
> -extern __typeof (memchr) __memchr_ppc attribute_hidden;
> -
> -#include <string/memchr.c>
> diff --git a/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c b/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c
> index 3c966f4403..15beca787b 100644
> --- a/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c
> +++ b/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c
> @@ -18,14 +18,7 @@
>
> #include <string.h>
>
> -#define MEMCHR __memchr_ppc
> -
> -#undef weak_alias
> -#define weak_alias(a, b)
> -
> -# undef libc_hidden_builtin_def
> -# define libc_hidden_builtin_def(name)
> -
> extern __typeof (memchr) __memchr_ppc attribute_hidden;
>
> +#define MEMCHR __memchr_ppc
> #include <string/memchr.c>
> --
> 2.34.1
>
On Mon, Sep 19, 2022 at 1:05 PM Adhemerval Zanella via Libc-alpha
<libc-alpha@sourceware.org> 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-opthr functions.
>
> 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 <rth@twiddle.net>
> ---
> string/memchr.c | 168 +++++-------------
> .../powerpc32/power4/multiarch/memchr-ppc32.c | 14 +-
> .../powerpc64/multiarch/memchr-ppc64.c | 9 +-
> 3 files changed, 48 insertions(+), 143 deletions(-)
>
> diff --git a/string/memchr.c b/string/memchr.c
> index 422bcd0cd6..08d518b02d 100644
> --- a/string/memchr.c
> +++ b/string/memchr.c
> @@ -1,10 +1,6 @@
> -/* Copyright (C) 1991-2022 Free Software Foundation, Inc.
> +/* Scan memory for a character. Generic version
> + Copyright (C) 1991-2022 Free Software Foundation, Inc.
> This file is part of the GNU C Library.
> - Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
> - with help from Dan Sahlin (dan@sics.se) and
> - commentary by Jim Blandy (jimb@ai.mit.edu);
> - adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
> - and implemented by Roland McGrath (roland@ai.mit.edu).
>
> The GNU C Library is free software; you can redistribute it and/or
> modify it under the terms of the GNU Lesser General Public
> @@ -20,143 +16,65 @@
> License along with the GNU C Library; if not, see
> <https://www.gnu.org/licenses/>. */
>
> -#ifndef _LIBC
> -# include <config.h>
> -#endif
> -
> +#include <intprops.h>
> +#include <string-fza.h>
> +#include <string-fzb.h>
> +#include <string-fzi.h>
> +#include <string-maskoff.h>
> +#include <string-opthr.h>
> #include <string.h>
>
> -#include <stddef.h>
> +#undef memchr
>
> -#include <limits.h>
> -
> -#undef __memchr
> -#ifdef _LIBC
> -# undef memchr
> +#ifdef MEMCHR
> +# define __memchr MEMCHR
> #endif
>
> -#ifndef weak_alias
> -# define __memchr memchr
> -#endif
> -
> -#ifndef MEMCHR
> -# define MEMCHR __memchr
> -#endif
> +static inline const char *
> +sadd (uintptr_t x, uintptr_t y)
> +{
> + uintptr_t ret = INT_ADD_OVERFLOW (x, y) ? (uintptr_t)-1 : x + y;
> + return (const char *)ret;
> +}
>
> /* Search no more than N bytes of S for C. */
> void *
> -MEMCHR (void const *s, int c_in, size_t n)
> +__memchr (void const *s, int c_in, size_t n)
> {
> - /* On 32-bit hardware, choosing longword to be a 32-bit unsigned
> - long instead of a 64-bit uintmax_t tends to give better
> - performance. On 64-bit hardware, unsigned long is generally 64
> - bits already. Change this typedef to experiment with
> - performance. */
> - typedef unsigned long int longword;
> + if (__glibc_unlikely (n == 0))
> + return NULL;
>
> - const unsigned char *char_ptr;
> - const longword *longword_ptr;
> - longword repeated_one;
> - longword repeated_c;
> - unsigned char c;
> + uintptr_t s_int = (uintptr_t) s;
>
> - c = (unsigned char) c_in;
> + /* Set up a word, each of whose bytes is C. */
> + op_t repeated_c = repeat_bytes (c_in);
> + op_t before_mask = create_mask (s_int);
>
> - /* Handle the first few bytes by reading one byte at a time.
> - Do this until CHAR_PTR is aligned on a longword boundary. */
> - for (char_ptr = (const unsigned char *) s;
> - n > 0 && (size_t) char_ptr % sizeof (longword) != 0;
> - --n, ++char_ptr)
> - if (*char_ptr == c)
> - return (void *) char_ptr;
> + /* Compute the address of the last byte taking in consideration possible
> + overflow. */
> + const char *lbyte = sadd (s_int, n - 1);
>
> - longword_ptr = (const longword *) char_ptr;
> + /* Compute the address of the word containing the last byte. */
> + const op_t *lword = word_containing (lbyte);
>
> - /* All these elucidatory comments refer to 4-byte longwords,
> - but the theory applies equally well to any size longwords. */
> + /* Read the first word, but munge it so that bytes before the array
> + will not match goal. */
> + const op_t *word_ptr = word_containing (s);
> + op_t word = (*word_ptr | before_mask) ^ (repeated_c & before_mask);
>
> - /* Compute auxiliary longword values:
> - repeated_one is a value which has a 1 in every byte.
> - repeated_c has c in every byte. */
> - repeated_one = 0x01010101;
> - repeated_c = c | (c << 8);
> - repeated_c |= repeated_c << 16;
> - if (0xffffffffU < (longword) -1)
> + while (has_eq (word, repeated_c) == 0)
> {
> - repeated_one |= repeated_one << 31 << 1;
> - repeated_c |= repeated_c << 31 << 1;
> - if (8 < sizeof (longword))
> - {
> - size_t i;
> -
> - for (i = 64; i < sizeof (longword) * 8; i *= 2)
> - {
> - repeated_one |= repeated_one << i;
> - repeated_c |= repeated_c << i;
> - }
> - }
> + if (word_ptr == lword)
> + return NULL;
Inuitively making lword, lword - 1 so that normal returns don't need the extra
null check would be faster.
> + word = *++word_ptr;
> }
>
> - /* Instead of the traditional loop which tests each byte, 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 equal to c. We first use an xor
> - with repeated_c. This reduces the task to testing whether *any of the
> - four* bytes in longword1 is zero.
> -
> - We compute tmp =
> - ((longword1 - repeated_one) & ~longword1) & (repeated_one << 7).
> - That is, we perform the following operations:
> - 1. Subtract repeated_one.
> - 2. & ~longword1.
> - 3. & a mask consisting of 0x80 in every byte.
> - Consider what happens in each byte:
> - - If a byte of longword1 is zero, step 1 and 2 transform it into 0xff,
> - and step 3 transforms it into 0x80. A carry can also be propagated
> - to more significant bytes.
> - - If a byte of longword1 is nonzero, let its lowest 1 bit be at
> - position k (0 <= k <= 7); so the lowest k bits are 0. After step 1,
> - the byte ends in a single bit of value 0 and k bits of value 1.
> - After step 2, the result is just k bits of value 1: 2^k - 1. After
> - step 3, the result is 0. And no carry is produced.
> - So, if longword1 has only non-zero bytes, tmp is zero.
> - Whereas if longword1 has a zero byte, call j the position of the least
> - significant zero byte. Then the result has a zero at positions 0, ...,
> - j-1 and a 0x80 at position j. We cannot predict the result at the more
> - significant bytes (positions j+1..3), but it does not matter since we
> - already have a non-zero bit at position 8*j+7.
> -
> - So, the test whether any byte in longword1 is zero is equivalent to
> - testing whether tmp is nonzero. */
> -
> - while (n >= sizeof (longword))
> - {
> - longword longword1 = *longword_ptr ^ repeated_c;
> -
> - if ((((longword1 - repeated_one) & ~longword1)
> - & (repeated_one << 7)) != 0)
> - break;
> - longword_ptr++;
> - n -= sizeof (longword);
> - }
> -
> - char_ptr = (const unsigned char *) longword_ptr;
> -
> - /* At this point, we know that either n < sizeof (longword), or one of the
> - sizeof (longword) bytes starting at char_ptr is == c. On little-endian
> - machines, we could determine the first such byte without any further
> - memory accesses, just by looking at the tmp result from the last loop
> - iteration. But this does not work on big-endian machines. Choose code
> - that works in both cases. */
> -
> - for (; n > 0; --n, ++char_ptr)
> - {
> - if (*char_ptr == c)
> - return (void *) char_ptr;
> - }
> -
> - return NULL;
> + /* We found a match, but it might be in a byte past the end
> + of the array. */
> + char *ret = (char *) word_ptr + index_first_eq (word, repeated_c);
> + return (ret <= lbyte) ? ret : NULL;
> }
> -#ifdef weak_alias
> +#ifndef MEMCHR
> weak_alias (__memchr, memchr)
> -#endif
> libc_hidden_builtin_def (memchr)
> +#endif
> diff --git a/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c b/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c
> index fc69df54b3..02877d3c98 100644
> --- a/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c
> +++ b/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c
> @@ -18,17 +18,11 @@
>
> #include <string.h>
>
> -#define MEMCHR __memchr_ppc
> +extern __typeof (memchr) __memchr_ppc attribute_hidden;
>
> -#undef weak_alias
> -#define weak_alias(a, b)
> +#define MEMCHR __memchr_ppc
> +#include <string/memchr.c>
>
> #ifdef SHARED
> -# undef libc_hidden_builtin_def
> -# define libc_hidden_builtin_def(name) \
> - __hidden_ver1(__memchr_ppc, __GI_memchr, __memchr_ppc);
> +__hidden_ver1(__memchr_ppc, __GI_memchr, __memchr_ppc);
> #endif
> -
> -extern __typeof (memchr) __memchr_ppc attribute_hidden;
> -
> -#include <string/memchr.c>
> diff --git a/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c b/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c
> index 3c966f4403..15beca787b 100644
> --- a/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c
> +++ b/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c
> @@ -18,14 +18,7 @@
>
> #include <string.h>
>
> -#define MEMCHR __memchr_ppc
> -
> -#undef weak_alias
> -#define weak_alias(a, b)
> -
> -# undef libc_hidden_builtin_def
> -# define libc_hidden_builtin_def(name)
> -
> extern __typeof (memchr) __memchr_ppc attribute_hidden;
>
> +#define MEMCHR __memchr_ppc
> #include <string/memchr.c>
> --
> 2.34.1
>
On 05/01/23 20:47, Noah Goldstein wrote:
> On Mon, Sep 19, 2022 at 1:05 PM Adhemerval Zanella via Libc-alpha
> <libc-alpha@sourceware.org> 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-opthr functions.
>>
>> 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 <rth@twiddle.net>
>> ---
>> string/memchr.c | 168 +++++-------------
>> .../powerpc32/power4/multiarch/memchr-ppc32.c | 14 +-
>> .../powerpc64/multiarch/memchr-ppc64.c | 9 +-
>> 3 files changed, 48 insertions(+), 143 deletions(-)
>>
>> diff --git a/string/memchr.c b/string/memchr.c
>> index 422bcd0cd6..08d518b02d 100644
>> --- a/string/memchr.c
>> +++ b/string/memchr.c
>> @@ -1,10 +1,6 @@
>> -/* Copyright (C) 1991-2022 Free Software Foundation, Inc.
>> +/* Scan memory for a character. Generic version
>> + Copyright (C) 1991-2022 Free Software Foundation, Inc.
>> This file is part of the GNU C Library.
>> - Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
>> - with help from Dan Sahlin (dan@sics.se) and
>> - commentary by Jim Blandy (jimb@ai.mit.edu);
>> - adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
>> - and implemented by Roland McGrath (roland@ai.mit.edu).
>>
>> The GNU C Library is free software; you can redistribute it and/or
>> modify it under the terms of the GNU Lesser General Public
>> @@ -20,143 +16,65 @@
>> License along with the GNU C Library; if not, see
>> <https://www.gnu.org/licenses/>. */
>>
>> -#ifndef _LIBC
>> -# include <config.h>
>> -#endif
>> -
>> +#include <intprops.h>
>> +#include <string-fza.h>
>> +#include <string-fzb.h>
>> +#include <string-fzi.h>
>> +#include <string-maskoff.h>
>> +#include <string-opthr.h>
>> #include <string.h>
>>
>> -#include <stddef.h>
>> +#undef memchr
>>
>> -#include <limits.h>
>> -
>> -#undef __memchr
>> -#ifdef _LIBC
>> -# undef memchr
>> +#ifdef MEMCHR
>> +# define __memchr MEMCHR
>> #endif
>>
>> -#ifndef weak_alias
>> -# define __memchr memchr
>> -#endif
>> -
>> -#ifndef MEMCHR
>> -# define MEMCHR __memchr
>> -#endif
>> +static inline const char *
>> +sadd (uintptr_t x, uintptr_t y)
>> +{
>> + uintptr_t ret = INT_ADD_OVERFLOW (x, y) ? (uintptr_t)-1 : x + y;
>> + return (const char *)ret;
>> +}
>>
>> /* Search no more than N bytes of S for C. */
>> void *
>> -MEMCHR (void const *s, int c_in, size_t n)
>> +__memchr (void const *s, int c_in, size_t n)
>> {
>> - /* On 32-bit hardware, choosing longword to be a 32-bit unsigned
>> - long instead of a 64-bit uintmax_t tends to give better
>> - performance. On 64-bit hardware, unsigned long is generally 64
>> - bits already. Change this typedef to experiment with
>> - performance. */
>> - typedef unsigned long int longword;
>> + if (__glibc_unlikely (n == 0))
>> + return NULL;
>>
>> - const unsigned char *char_ptr;
>> - const longword *longword_ptr;
>> - longword repeated_one;
>> - longword repeated_c;
>> - unsigned char c;
>> + uintptr_t s_int = (uintptr_t) s;
>>
>> - c = (unsigned char) c_in;
>> + /* Set up a word, each of whose bytes is C. */
>> + op_t repeated_c = repeat_bytes (c_in);
>> + op_t before_mask = create_mask (s_int);
>>
>> - /* Handle the first few bytes by reading one byte at a time.
>> - Do this until CHAR_PTR is aligned on a longword boundary. */
>> - for (char_ptr = (const unsigned char *) s;
>> - n > 0 && (size_t) char_ptr % sizeof (longword) != 0;
>> - --n, ++char_ptr)
>> - if (*char_ptr == c)
>> - return (void *) char_ptr;
>> + /* Compute the address of the last byte taking in consideration possible
>> + overflow. */
>> + const char *lbyte = sadd (s_int, n - 1);
>
> Do you need this? The comparison in the loop is == so letting it
> overflow should be fine no?
Do you mean the saturation add or the last lbyte check? For saturation add
I recall that it requires for memchr (..., SIZE_MAX), otherwise the last
byte/word would be incorrect (I fixed some assembly routines that triggered
this issue in the past).
>>
>> - longword_ptr = (const longword *) char_ptr;
>> + /* Compute the address of the word containing the last byte. */
>> + const op_t *lword = word_containing (lbyte);
>>
>> - /* All these elucidatory comments refer to 4-byte longwords,
>> - but the theory applies equally well to any size longwords. */
>> + /* Read the first word, but munge it so that bytes before the array
>> + will not match goal. */
>> + const op_t *word_ptr = word_containing (s);
>> + op_t word = (*word_ptr | before_mask) ^ (repeated_c & before_mask);
>
> Likewise, prefer just shifting out the invalid comparisons on the first word.
I will need to check why this is not really working, I think I suggest it
on previous iteration and I could not make it work for some reason.
>>
>> - /* Compute auxiliary longword values:
>> - repeated_one is a value which has a 1 in every byte.
>> - repeated_c has c in every byte. */
>> - repeated_one = 0x01010101;
>> - repeated_c = c | (c << 8);
>> - repeated_c |= repeated_c << 16;
>> - if (0xffffffffU < (longword) -1)
>> + while (has_eq (word, repeated_c) == 0)
>> {
>> - repeated_one |= repeated_one << 31 << 1;
>> - repeated_c |= repeated_c << 31 << 1;
>> - if (8 < sizeof (longword))
>> - {
>> - size_t i;
>> -
>> - for (i = 64; i < sizeof (longword) * 8; i *= 2)
>> - {
>> - repeated_one |= repeated_one << i;
>> - repeated_c |= repeated_c << i;
>> - }
>> - }
>> + if (word_ptr == lword)
>> + return NULL;
>> + word = *++word_ptr;
>> }
>>
>> - /* Instead of the traditional loop which tests each byte, 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 equal to c. We first use an xor
>> - with repeated_c. This reduces the task to testing whether *any of the
>> - four* bytes in longword1 is zero.
>> -
>> - We compute tmp =
>> - ((longword1 - repeated_one) & ~longword1) & (repeated_one << 7).
>> - That is, we perform the following operations:
>> - 1. Subtract repeated_one.
>> - 2. & ~longword1.
>> - 3. & a mask consisting of 0x80 in every byte.
>> - Consider what happens in each byte:
>> - - If a byte of longword1 is zero, step 1 and 2 transform it into 0xff,
>> - and step 3 transforms it into 0x80. A carry can also be propagated
>> - to more significant bytes.
>> - - If a byte of longword1 is nonzero, let its lowest 1 bit be at
>> - position k (0 <= k <= 7); so the lowest k bits are 0. After step 1,
>> - the byte ends in a single bit of value 0 and k bits of value 1.
>> - After step 2, the result is just k bits of value 1: 2^k - 1. After
>> - step 3, the result is 0. And no carry is produced.
>> - So, if longword1 has only non-zero bytes, tmp is zero.
>> - Whereas if longword1 has a zero byte, call j the position of the least
>> - significant zero byte. Then the result has a zero at positions 0, ...,
>> - j-1 and a 0x80 at position j. We cannot predict the result at the more
>> - significant bytes (positions j+1..3), but it does not matter since we
>> - already have a non-zero bit at position 8*j+7.
>> -
>> - So, the test whether any byte in longword1 is zero is equivalent to
>> - testing whether tmp is nonzero. */
>> -
>> - while (n >= sizeof (longword))
>> - {
>> - longword longword1 = *longword_ptr ^ repeated_c;
>> -
>> - if ((((longword1 - repeated_one) & ~longword1)
>> - & (repeated_one << 7)) != 0)
>> - break;
>> - longword_ptr++;
>> - n -= sizeof (longword);
>> - }
>> -
>> - char_ptr = (const unsigned char *) longword_ptr;
>> -
>> - /* At this point, we know that either n < sizeof (longword), or one of the
>> - sizeof (longword) bytes starting at char_ptr is == c. On little-endian
>> - machines, we could determine the first such byte without any further
>> - memory accesses, just by looking at the tmp result from the last loop
>> - iteration. But this does not work on big-endian machines. Choose code
>> - that works in both cases. */
>> -
>> - for (; n > 0; --n, ++char_ptr)
>> - {
>> - if (*char_ptr == c)
>> - return (void *) char_ptr;
>> - }
>> -
>> - return NULL;
>> + /* We found a match, but it might be in a byte past the end
>> + of the array. */
>> + char *ret = (char *) word_ptr + index_first_eq (word, repeated_c);
>> + return (ret <= lbyte) ? ret : NULL;
>> }
>> -#ifdef weak_alias
>> +#ifndef MEMCHR
>> weak_alias (__memchr, memchr)
>> -#endif
>> libc_hidden_builtin_def (memchr)
>> +#endif
>> diff --git a/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c b/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c
>> index fc69df54b3..02877d3c98 100644
>> --- a/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c
>> +++ b/sysdeps/powerpc/powerpc32/power4/multiarch/memchr-ppc32.c
>> @@ -18,17 +18,11 @@
>>
>> #include <string.h>
>>
>> -#define MEMCHR __memchr_ppc
>> +extern __typeof (memchr) __memchr_ppc attribute_hidden;
>>
>> -#undef weak_alias
>> -#define weak_alias(a, b)
>> +#define MEMCHR __memchr_ppc
>> +#include <string/memchr.c>
>>
>> #ifdef SHARED
>> -# undef libc_hidden_builtin_def
>> -# define libc_hidden_builtin_def(name) \
>> - __hidden_ver1(__memchr_ppc, __GI_memchr, __memchr_ppc);
>> +__hidden_ver1(__memchr_ppc, __GI_memchr, __memchr_ppc);
>> #endif
>> -
>> -extern __typeof (memchr) __memchr_ppc attribute_hidden;
>> -
>> -#include <string/memchr.c>
>> diff --git a/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c b/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c
>> index 3c966f4403..15beca787b 100644
>> --- a/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c
>> +++ b/sysdeps/powerpc/powerpc64/multiarch/memchr-ppc64.c
>> @@ -18,14 +18,7 @@
>>
>> #include <string.h>
>>
>> -#define MEMCHR __memchr_ppc
>> -
>> -#undef weak_alias
>> -#define weak_alias(a, b)
>> -
>> -# undef libc_hidden_builtin_def
>> -# define libc_hidden_builtin_def(name)
>> -
>> extern __typeof (memchr) __memchr_ppc attribute_hidden;
>>
>> +#define MEMCHR __memchr_ppc
>> #include <string/memchr.c>
>> --
>> 2.34.1
>>
On 05/01/23 20:49, Noah Goldstein wrote:
> On Mon, Sep 19, 2022 at 1:05 PM Adhemerval Zanella via Libc-alpha
> <libc-alpha@sourceware.org> 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-opthr functions.
>>
>> 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 <rth@twiddle.net>
>> ---
>> string/memchr.c | 168 +++++-------------
>> .../powerpc32/power4/multiarch/memchr-ppc32.c | 14 +-
>> .../powerpc64/multiarch/memchr-ppc64.c | 9 +-
>> 3 files changed, 48 insertions(+), 143 deletions(-)
>>
>> diff --git a/string/memchr.c b/string/memchr.c
>> index 422bcd0cd6..08d518b02d 100644
>> --- a/string/memchr.c
>> +++ b/string/memchr.c
>> @@ -1,10 +1,6 @@
>> -/* Copyright (C) 1991-2022 Free Software Foundation, Inc.
>> +/* Scan memory for a character. Generic version
>> + Copyright (C) 1991-2022 Free Software Foundation, Inc.
>> This file is part of the GNU C Library.
>> - Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
>> - with help from Dan Sahlin (dan@sics.se) and
>> - commentary by Jim Blandy (jimb@ai.mit.edu);
>> - adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
>> - and implemented by Roland McGrath (roland@ai.mit.edu).
>>
>> The GNU C Library is free software; you can redistribute it and/or
>> modify it under the terms of the GNU Lesser General Public
>> @@ -20,143 +16,65 @@
>> License along with the GNU C Library; if not, see
>> <https://www.gnu.org/licenses/>. */
>>
>> -#ifndef _LIBC
>> -# include <config.h>
>> -#endif
>> -
>> +#include <intprops.h>
>> +#include <string-fza.h>
>> +#include <string-fzb.h>
>> +#include <string-fzi.h>
>> +#include <string-maskoff.h>
>> +#include <string-opthr.h>
>> #include <string.h>
>>
>> -#include <stddef.h>
>> +#undef memchr
>>
>> -#include <limits.h>
>> -
>> -#undef __memchr
>> -#ifdef _LIBC
>> -# undef memchr
>> +#ifdef MEMCHR
>> +# define __memchr MEMCHR
>> #endif
>>
>> -#ifndef weak_alias
>> -# define __memchr memchr
>> -#endif
>> -
>> -#ifndef MEMCHR
>> -# define MEMCHR __memchr
>> -#endif
>> +static inline const char *
>> +sadd (uintptr_t x, uintptr_t y)
>> +{
>> + uintptr_t ret = INT_ADD_OVERFLOW (x, y) ? (uintptr_t)-1 : x + y;
>> + return (const char *)ret;
>> +}
>>
>> /* Search no more than N bytes of S for C. */
>> void *
>> -MEMCHR (void const *s, int c_in, size_t n)
>> +__memchr (void const *s, int c_in, size_t n)
>> {
>> - /* On 32-bit hardware, choosing longword to be a 32-bit unsigned
>> - long instead of a 64-bit uintmax_t tends to give better
>> - performance. On 64-bit hardware, unsigned long is generally 64
>> - bits already. Change this typedef to experiment with
>> - performance. */
>> - typedef unsigned long int longword;
>> + if (__glibc_unlikely (n == 0))
>> + return NULL;
>>
>> - const unsigned char *char_ptr;
>> - const longword *longword_ptr;
>> - longword repeated_one;
>> - longword repeated_c;
>> - unsigned char c;
>> + uintptr_t s_int = (uintptr_t) s;
>>
>> - c = (unsigned char) c_in;
>> + /* Set up a word, each of whose bytes is C. */
>> + op_t repeated_c = repeat_bytes (c_in);
>> + op_t before_mask = create_mask (s_int);
>>
>> - /* Handle the first few bytes by reading one byte at a time.
>> - Do this until CHAR_PTR is aligned on a longword boundary. */
>> - for (char_ptr = (const unsigned char *) s;
>> - n > 0 && (size_t) char_ptr % sizeof (longword) != 0;
>> - --n, ++char_ptr)
>> - if (*char_ptr == c)
>> - return (void *) char_ptr;
>> + /* Compute the address of the last byte taking in consideration possible
>> + overflow. */
>> + const char *lbyte = sadd (s_int, n - 1);
>>
>> - longword_ptr = (const longword *) char_ptr;
>> + /* Compute the address of the word containing the last byte. */
>> + const op_t *lword = word_containing (lbyte);
>>
>> - /* All these elucidatory comments refer to 4-byte longwords,
>> - but the theory applies equally well to any size longwords. */
>> + /* Read the first word, but munge it so that bytes before the array
>> + will not match goal. */
>> + const op_t *word_ptr = word_containing (s);
>> + op_t word = (*word_ptr | before_mask) ^ (repeated_c & before_mask);
>>
>> - /* Compute auxiliary longword values:
>> - repeated_one is a value which has a 1 in every byte.
>> - repeated_c has c in every byte. */
>> - repeated_one = 0x01010101;
>> - repeated_c = c | (c << 8);
>> - repeated_c |= repeated_c << 16;
>> - if (0xffffffffU < (longword) -1)
>> + while (has_eq (word, repeated_c) == 0)
>> {
>> - repeated_one |= repeated_one << 31 << 1;
>> - repeated_c |= repeated_c << 31 << 1;
>> - if (8 < sizeof (longword))
>> - {
>> - size_t i;
>> -
>> - for (i = 64; i < sizeof (longword) * 8; i *= 2)
>> - {
>> - repeated_one |= repeated_one << i;
>> - repeated_c |= repeated_c << i;
>> - }
>> - }
>> + if (word_ptr == lword)
>> + return NULL;
> Inuitively making lword, lword - 1 so that normal returns don't need the extra
> null check would be faster.
Hum, I did not follow; could you explain it with more details what you mean here?
On Mon, Jan 9, 2023 at 12:51 PM Adhemerval Zanella Netto
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 05/01/23 20:49, Noah Goldstein wrote:
> > On Mon, Sep 19, 2022 at 1:05 PM Adhemerval Zanella via Libc-alpha
> > <libc-alpha@sourceware.org> 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-opthr functions.
> >>
> >> 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 <rth@twiddle.net>
> >> ---
> >> string/memchr.c | 168 +++++-------------
> >> .../powerpc32/power4/multiarch/memchr-ppc32.c | 14 +-
> >> .../powerpc64/multiarch/memchr-ppc64.c | 9 +-
> >> 3 files changed, 48 insertions(+), 143 deletions(-)
> >>
> >> diff --git a/string/memchr.c b/string/memchr.c
> >> index 422bcd0cd6..08d518b02d 100644
> >> --- a/string/memchr.c
> >> +++ b/string/memchr.c
> >> @@ -1,10 +1,6 @@
> >> -/* Copyright (C) 1991-2022 Free Software Foundation, Inc.
> >> +/* Scan memory for a character. Generic version
> >> + Copyright (C) 1991-2022 Free Software Foundation, Inc.
> >> This file is part of the GNU C Library.
> >> - Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
> >> - with help from Dan Sahlin (dan@sics.se) and
> >> - commentary by Jim Blandy (jimb@ai.mit.edu);
> >> - adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
> >> - and implemented by Roland McGrath (roland@ai.mit.edu).
> >>
> >> The GNU C Library is free software; you can redistribute it and/or
> >> modify it under the terms of the GNU Lesser General Public
> >> @@ -20,143 +16,65 @@
> >> License along with the GNU C Library; if not, see
> >> <https://www.gnu.org/licenses/>. */
> >>
> >> -#ifndef _LIBC
> >> -# include <config.h>
> >> -#endif
> >> -
> >> +#include <intprops.h>
> >> +#include <string-fza.h>
> >> +#include <string-fzb.h>
> >> +#include <string-fzi.h>
> >> +#include <string-maskoff.h>
> >> +#include <string-opthr.h>
> >> #include <string.h>
> >>
> >> -#include <stddef.h>
> >> +#undef memchr
> >>
> >> -#include <limits.h>
> >> -
> >> -#undef __memchr
> >> -#ifdef _LIBC
> >> -# undef memchr
> >> +#ifdef MEMCHR
> >> +# define __memchr MEMCHR
> >> #endif
> >>
> >> -#ifndef weak_alias
> >> -# define __memchr memchr
> >> -#endif
> >> -
> >> -#ifndef MEMCHR
> >> -# define MEMCHR __memchr
> >> -#endif
> >> +static inline const char *
> >> +sadd (uintptr_t x, uintptr_t y)
> >> +{
> >> + uintptr_t ret = INT_ADD_OVERFLOW (x, y) ? (uintptr_t)-1 : x + y;
> >> + return (const char *)ret;
> >> +}
> >>
> >> /* Search no more than N bytes of S for C. */
> >> void *
> >> -MEMCHR (void const *s, int c_in, size_t n)
> >> +__memchr (void const *s, int c_in, size_t n)
> >> {
> >> - /* On 32-bit hardware, choosing longword to be a 32-bit unsigned
> >> - long instead of a 64-bit uintmax_t tends to give better
> >> - performance. On 64-bit hardware, unsigned long is generally 64
> >> - bits already. Change this typedef to experiment with
> >> - performance. */
> >> - typedef unsigned long int longword;
> >> + if (__glibc_unlikely (n == 0))
> >> + return NULL;
> >>
> >> - const unsigned char *char_ptr;
> >> - const longword *longword_ptr;
> >> - longword repeated_one;
> >> - longword repeated_c;
> >> - unsigned char c;
> >> + uintptr_t s_int = (uintptr_t) s;
> >>
> >> - c = (unsigned char) c_in;
> >> + /* Set up a word, each of whose bytes is C. */
> >> + op_t repeated_c = repeat_bytes (c_in);
> >> + op_t before_mask = create_mask (s_int);
> >>
> >> - /* Handle the first few bytes by reading one byte at a time.
> >> - Do this until CHAR_PTR is aligned on a longword boundary. */
> >> - for (char_ptr = (const unsigned char *) s;
> >> - n > 0 && (size_t) char_ptr % sizeof (longword) != 0;
> >> - --n, ++char_ptr)
> >> - if (*char_ptr == c)
> >> - return (void *) char_ptr;
> >> + /* Compute the address of the last byte taking in consideration possible
> >> + overflow. */
> >> + const char *lbyte = sadd (s_int, n - 1);
> >>
> >> - longword_ptr = (const longword *) char_ptr;
> >> + /* Compute the address of the word containing the last byte. */
> >> + const op_t *lword = word_containing (lbyte);
> >>
> >> - /* All these elucidatory comments refer to 4-byte longwords,
> >> - but the theory applies equally well to any size longwords. */
> >> + /* Read the first word, but munge it so that bytes before the array
> >> + will not match goal. */
> >> + const op_t *word_ptr = word_containing (s);
> >> + op_t word = (*word_ptr | before_mask) ^ (repeated_c & before_mask);
> >>
> >> - /* Compute auxiliary longword values:
> >> - repeated_one is a value which has a 1 in every byte.
> >> - repeated_c has c in every byte. */
> >> - repeated_one = 0x01010101;
> >> - repeated_c = c | (c << 8);
> >> - repeated_c |= repeated_c << 16;
> >> - if (0xffffffffU < (longword) -1)
> >> + while (has_eq (word, repeated_c) == 0)
> >> {
> >> - repeated_one |= repeated_one << 31 << 1;
> >> - repeated_c |= repeated_c << 31 << 1;
> >> - if (8 < sizeof (longword))
> >> - {
> >> - size_t i;
> >> -
> >> - for (i = 64; i < sizeof (longword) * 8; i *= 2)
> >> - {
> >> - repeated_one |= repeated_one << i;
> >> - repeated_c |= repeated_c << i;
> >> - }
> >> - }
> >> + if (word_ptr == lword)
> >> + return NULL;
> > Inuitively making lword, lword - 1 so that normal returns don't need the extra
> > null check would be faster.
>
> Hum, I did not follow; could you explain it with more details what you mean here?
I was thinking something like:
```
op_t word = *word_ptr;
op_t mask = find_eq_low (word, repeated_c)
>> (CHAR_BIT * (s_int % sizeof (uintptr_t)));
if (mask)
{
char *ret = (char *) s + index_first_ (mask);
return (ret <= lbyte) ? ret : NULL;
}
if (word_ptr == lword)
return NULL;
word = *++word_ptr;
while (word_ptr != lword)
{
if (has_eq (word, repeated_c))
return (char *) word_ptr + index_first_eq (word, repeated_c);
word = *++word_ptr;
}
if (has_eq (word, repeated_c))
{
/* We found a match, but it might be in a byte past the end
of the array. */
char *ret = (char *) word_ptr + index_first_eq (word, repeated_c);
if (ret <= lbyte)
return ret;
}
return NULL;
```
The idea is until the last byte you don't need the extra bounds check (tested
on test-memchr.c on little-endian).
On 09/01/23 18:26, Noah Goldstein wrote:
> On Mon, Jan 9, 2023 at 12:51 PM Adhemerval Zanella Netto
> <adhemerval.zanella@linaro.org> wrote:
>>
>>
>>
>> On 05/01/23 20:49, Noah Goldstein wrote:
>>> On Mon, Sep 19, 2022 at 1:05 PM Adhemerval Zanella via Libc-alpha
>>> <libc-alpha@sourceware.org> 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-opthr functions.
>>>>
>>>> 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 <rth@twiddle.net>
>>>> ---
>>>> string/memchr.c | 168 +++++-------------
>>>> .../powerpc32/power4/multiarch/memchr-ppc32.c | 14 +-
>>>> .../powerpc64/multiarch/memchr-ppc64.c | 9 +-
>>>> 3 files changed, 48 insertions(+), 143 deletions(-)
>>>>
>>>> diff --git a/string/memchr.c b/string/memchr.c
>>>> index 422bcd0cd6..08d518b02d 100644
>>>> --- a/string/memchr.c
>>>> +++ b/string/memchr.c
>>>> @@ -1,10 +1,6 @@
>>>> -/* Copyright (C) 1991-2022 Free Software Foundation, Inc.
>>>> +/* Scan memory for a character. Generic version
>>>> + Copyright (C) 1991-2022 Free Software Foundation, Inc.
>>>> This file is part of the GNU C Library.
>>>> - Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
>>>> - with help from Dan Sahlin (dan@sics.se) and
>>>> - commentary by Jim Blandy (jimb@ai.mit.edu);
>>>> - adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
>>>> - and implemented by Roland McGrath (roland@ai.mit.edu).
>>>>
>>>> The GNU C Library is free software; you can redistribute it and/or
>>>> modify it under the terms of the GNU Lesser General Public
>>>> @@ -20,143 +16,65 @@
>>>> License along with the GNU C Library; if not, see
>>>> <https://www.gnu.org/licenses/>. */
>>>>
>>>> -#ifndef _LIBC
>>>> -# include <config.h>
>>>> -#endif
>>>> -
>>>> +#include <intprops.h>
>>>> +#include <string-fza.h>
>>>> +#include <string-fzb.h>
>>>> +#include <string-fzi.h>
>>>> +#include <string-maskoff.h>
>>>> +#include <string-opthr.h>
>>>> #include <string.h>
>>>>
>>>> -#include <stddef.h>
>>>> +#undef memchr
>>>>
>>>> -#include <limits.h>
>>>> -
>>>> -#undef __memchr
>>>> -#ifdef _LIBC
>>>> -# undef memchr
>>>> +#ifdef MEMCHR
>>>> +# define __memchr MEMCHR
>>>> #endif
>>>>
>>>> -#ifndef weak_alias
>>>> -# define __memchr memchr
>>>> -#endif
>>>> -
>>>> -#ifndef MEMCHR
>>>> -# define MEMCHR __memchr
>>>> -#endif
>>>> +static inline const char *
>>>> +sadd (uintptr_t x, uintptr_t y)
>>>> +{
>>>> + uintptr_t ret = INT_ADD_OVERFLOW (x, y) ? (uintptr_t)-1 : x + y;
>>>> + return (const char *)ret;
>>>> +}
>>>>
>>>> /* Search no more than N bytes of S for C. */
>>>> void *
>>>> -MEMCHR (void const *s, int c_in, size_t n)
>>>> +__memchr (void const *s, int c_in, size_t n)
>>>> {
>>>> - /* On 32-bit hardware, choosing longword to be a 32-bit unsigned
>>>> - long instead of a 64-bit uintmax_t tends to give better
>>>> - performance. On 64-bit hardware, unsigned long is generally 64
>>>> - bits already. Change this typedef to experiment with
>>>> - performance. */
>>>> - typedef unsigned long int longword;
>>>> + if (__glibc_unlikely (n == 0))
>>>> + return NULL;
>>>>
>>>> - const unsigned char *char_ptr;
>>>> - const longword *longword_ptr;
>>>> - longword repeated_one;
>>>> - longword repeated_c;
>>>> - unsigned char c;
>>>> + uintptr_t s_int = (uintptr_t) s;
>>>>
>>>> - c = (unsigned char) c_in;
>>>> + /* Set up a word, each of whose bytes is C. */
>>>> + op_t repeated_c = repeat_bytes (c_in);
>>>> + op_t before_mask = create_mask (s_int);
>>>>
>>>> - /* Handle the first few bytes by reading one byte at a time.
>>>> - Do this until CHAR_PTR is aligned on a longword boundary. */
>>>> - for (char_ptr = (const unsigned char *) s;
>>>> - n > 0 && (size_t) char_ptr % sizeof (longword) != 0;
>>>> - --n, ++char_ptr)
>>>> - if (*char_ptr == c)
>>>> - return (void *) char_ptr;
>>>> + /* Compute the address of the last byte taking in consideration possible
>>>> + overflow. */
>>>> + const char *lbyte = sadd (s_int, n - 1);
>>>>
>>>> - longword_ptr = (const longword *) char_ptr;
>>>> + /* Compute the address of the word containing the last byte. */
>>>> + const op_t *lword = word_containing (lbyte);
>>>>
>>>> - /* All these elucidatory comments refer to 4-byte longwords,
>>>> - but the theory applies equally well to any size longwords. */
>>>> + /* Read the first word, but munge it so that bytes before the array
>>>> + will not match goal. */
>>>> + const op_t *word_ptr = word_containing (s);
>>>> + op_t word = (*word_ptr | before_mask) ^ (repeated_c & before_mask);
>>>>
>>>> - /* Compute auxiliary longword values:
>>>> - repeated_one is a value which has a 1 in every byte.
>>>> - repeated_c has c in every byte. */
>>>> - repeated_one = 0x01010101;
>>>> - repeated_c = c | (c << 8);
>>>> - repeated_c |= repeated_c << 16;
>>>> - if (0xffffffffU < (longword) -1)
>>>> + while (has_eq (word, repeated_c) == 0)
>>>> {
>>>> - repeated_one |= repeated_one << 31 << 1;
>>>> - repeated_c |= repeated_c << 31 << 1;
>>>> - if (8 < sizeof (longword))
>>>> - {
>>>> - size_t i;
>>>> -
>>>> - for (i = 64; i < sizeof (longword) * 8; i *= 2)
>>>> - {
>>>> - repeated_one |= repeated_one << i;
>>>> - repeated_c |= repeated_c << i;
>>>> - }
>>>> - }
>>>> + if (word_ptr == lword)
>>>> + return NULL;
>>> Inuitively making lword, lword - 1 so that normal returns don't need the extra
>>> null check would be faster.
>>
>> Hum, I did not follow; could you explain it with more details what you mean here?
>
> I was thinking something like:
>
> ```
> op_t word = *word_ptr;
> op_t mask = find_eq_low (word, repeated_c)
> >> (CHAR_BIT * (s_int % sizeof (uintptr_t)));
> if (mask)
> {
> char *ret = (char *) s + index_first_ (mask);
> return (ret <= lbyte) ? ret : NULL;
> }
> if (word_ptr == lword)
> return NULL;
>
> word = *++word_ptr;
> while (word_ptr != lword)
> {
> if (has_eq (word, repeated_c))
> return (char *) word_ptr + index_first_eq (word, repeated_c);
> word = *++word_ptr;
> }
>
> if (has_eq (word, repeated_c))
> {
>
> /* We found a match, but it might be in a byte past the end
> of the array. */
> char *ret = (char *) word_ptr + index_first_eq (word, repeated_c);
> if (ret <= lbyte)
> return ret;
> }
> return NULL;
> ```
>
> The idea is until the last byte you don't need the extra bounds check (tested
> on test-memchr.c on little-endian).
Alright, this works. I will update the path.
@@ -1,10 +1,6 @@
-/* Copyright (C) 1991-2022 Free Software Foundation, Inc.
+/* Scan memory for a character. Generic version
+ Copyright (C) 1991-2022 Free Software Foundation, Inc.
This file is part of the GNU C Library.
- Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
- with help from Dan Sahlin (dan@sics.se) and
- commentary by Jim Blandy (jimb@ai.mit.edu);
- adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
- and implemented by Roland McGrath (roland@ai.mit.edu).
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
@@ -20,143 +16,65 @@
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
-#ifndef _LIBC
-# include <config.h>
-#endif
-
+#include <intprops.h>
+#include <string-fza.h>
+#include <string-fzb.h>
+#include <string-fzi.h>
+#include <string-maskoff.h>
+#include <string-opthr.h>
#include <string.h>
-#include <stddef.h>
+#undef memchr
-#include <limits.h>
-
-#undef __memchr
-#ifdef _LIBC
-# undef memchr
+#ifdef MEMCHR
+# define __memchr MEMCHR
#endif
-#ifndef weak_alias
-# define __memchr memchr
-#endif
-
-#ifndef MEMCHR
-# define MEMCHR __memchr
-#endif
+static inline const char *
+sadd (uintptr_t x, uintptr_t y)
+{
+ uintptr_t ret = INT_ADD_OVERFLOW (x, y) ? (uintptr_t)-1 : x + y;
+ return (const char *)ret;
+}
/* Search no more than N bytes of S for C. */
void *
-MEMCHR (void const *s, int c_in, size_t n)
+__memchr (void const *s, int c_in, size_t n)
{
- /* On 32-bit hardware, choosing longword to be a 32-bit unsigned
- long instead of a 64-bit uintmax_t tends to give better
- performance. On 64-bit hardware, unsigned long is generally 64
- bits already. Change this typedef to experiment with
- performance. */
- typedef unsigned long int longword;
+ if (__glibc_unlikely (n == 0))
+ return NULL;
- const unsigned char *char_ptr;
- const longword *longword_ptr;
- longword repeated_one;
- longword repeated_c;
- unsigned char c;
+ uintptr_t s_int = (uintptr_t) s;
- c = (unsigned char) c_in;
+ /* Set up a word, each of whose bytes is C. */
+ op_t repeated_c = repeat_bytes (c_in);
+ op_t before_mask = create_mask (s_int);
- /* Handle the first few bytes by reading one byte at a time.
- Do this until CHAR_PTR is aligned on a longword boundary. */
- for (char_ptr = (const unsigned char *) s;
- n > 0 && (size_t) char_ptr % sizeof (longword) != 0;
- --n, ++char_ptr)
- if (*char_ptr == c)
- return (void *) char_ptr;
+ /* Compute the address of the last byte taking in consideration possible
+ overflow. */
+ const char *lbyte = sadd (s_int, n - 1);
- longword_ptr = (const longword *) char_ptr;
+ /* Compute the address of the word containing the last byte. */
+ const op_t *lword = word_containing (lbyte);
- /* All these elucidatory comments refer to 4-byte longwords,
- but the theory applies equally well to any size longwords. */
+ /* Read the first word, but munge it so that bytes before the array
+ will not match goal. */
+ const op_t *word_ptr = word_containing (s);
+ op_t word = (*word_ptr | before_mask) ^ (repeated_c & before_mask);
- /* Compute auxiliary longword values:
- repeated_one is a value which has a 1 in every byte.
- repeated_c has c in every byte. */
- repeated_one = 0x01010101;
- repeated_c = c | (c << 8);
- repeated_c |= repeated_c << 16;
- if (0xffffffffU < (longword) -1)
+ while (has_eq (word, repeated_c) == 0)
{
- repeated_one |= repeated_one << 31 << 1;
- repeated_c |= repeated_c << 31 << 1;
- if (8 < sizeof (longword))
- {
- size_t i;
-
- for (i = 64; i < sizeof (longword) * 8; i *= 2)
- {
- repeated_one |= repeated_one << i;
- repeated_c |= repeated_c << i;
- }
- }
+ if (word_ptr == lword)
+ return NULL;
+ word = *++word_ptr;
}
- /* Instead of the traditional loop which tests each byte, 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 equal to c. We first use an xor
- with repeated_c. This reduces the task to testing whether *any of the
- four* bytes in longword1 is zero.
-
- We compute tmp =
- ((longword1 - repeated_one) & ~longword1) & (repeated_one << 7).
- That is, we perform the following operations:
- 1. Subtract repeated_one.
- 2. & ~longword1.
- 3. & a mask consisting of 0x80 in every byte.
- Consider what happens in each byte:
- - If a byte of longword1 is zero, step 1 and 2 transform it into 0xff,
- and step 3 transforms it into 0x80. A carry can also be propagated
- to more significant bytes.
- - If a byte of longword1 is nonzero, let its lowest 1 bit be at
- position k (0 <= k <= 7); so the lowest k bits are 0. After step 1,
- the byte ends in a single bit of value 0 and k bits of value 1.
- After step 2, the result is just k bits of value 1: 2^k - 1. After
- step 3, the result is 0. And no carry is produced.
- So, if longword1 has only non-zero bytes, tmp is zero.
- Whereas if longword1 has a zero byte, call j the position of the least
- significant zero byte. Then the result has a zero at positions 0, ...,
- j-1 and a 0x80 at position j. We cannot predict the result at the more
- significant bytes (positions j+1..3), but it does not matter since we
- already have a non-zero bit at position 8*j+7.
-
- So, the test whether any byte in longword1 is zero is equivalent to
- testing whether tmp is nonzero. */
-
- while (n >= sizeof (longword))
- {
- longword longword1 = *longword_ptr ^ repeated_c;
-
- if ((((longword1 - repeated_one) & ~longword1)
- & (repeated_one << 7)) != 0)
- break;
- longword_ptr++;
- n -= sizeof (longword);
- }
-
- char_ptr = (const unsigned char *) longword_ptr;
-
- /* At this point, we know that either n < sizeof (longword), or one of the
- sizeof (longword) bytes starting at char_ptr is == c. On little-endian
- machines, we could determine the first such byte without any further
- memory accesses, just by looking at the tmp result from the last loop
- iteration. But this does not work on big-endian machines. Choose code
- that works in both cases. */
-
- for (; n > 0; --n, ++char_ptr)
- {
- if (*char_ptr == c)
- return (void *) char_ptr;
- }
-
- return NULL;
+ /* We found a match, but it might be in a byte past the end
+ of the array. */
+ char *ret = (char *) word_ptr + index_first_eq (word, repeated_c);
+ return (ret <= lbyte) ? ret : NULL;
}
-#ifdef weak_alias
+#ifndef MEMCHR
weak_alias (__memchr, memchr)
-#endif
libc_hidden_builtin_def (memchr)
+#endif
@@ -18,17 +18,11 @@
#include <string.h>
-#define MEMCHR __memchr_ppc
+extern __typeof (memchr) __memchr_ppc attribute_hidden;
-#undef weak_alias
-#define weak_alias(a, b)
+#define MEMCHR __memchr_ppc
+#include <string/memchr.c>
#ifdef SHARED
-# undef libc_hidden_builtin_def
-# define libc_hidden_builtin_def(name) \
- __hidden_ver1(__memchr_ppc, __GI_memchr, __memchr_ppc);
+__hidden_ver1(__memchr_ppc, __GI_memchr, __memchr_ppc);
#endif
-
-extern __typeof (memchr) __memchr_ppc attribute_hidden;
-
-#include <string/memchr.c>
@@ -18,14 +18,7 @@
#include <string.h>
-#define MEMCHR __memchr_ppc
-
-#undef weak_alias
-#define weak_alias(a, b)
-
-# undef libc_hidden_builtin_def
-# define libc_hidden_builtin_def(name)
-
extern __typeof (memchr) __memchr_ppc attribute_hidden;
+#define MEMCHR __memchr_ppc
#include <string/memchr.c>