[v5,07/17] string: Improve generic strchr
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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-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 <rth@twiddle.net>
---
string/strchr.c | 172 +++++++---------------------------------
sysdeps/s390/strchr-c.c | 11 +--
2 files changed, 34 insertions(+), 149 deletions(-)
Comments
On Mon, Sep 19, 2022 at 1:01 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-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 <rth@twiddle.net>
> ---
> 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 <string.h>
> #include <stdlib.h>
> +#include <stdint.h>
> +#include <string-fza.h>
> +#include <string-fzb.h>
> +#include <string-fzi.h>
> +#include <string-extbyte.h>
> +#include <string-maskoff.h>
>
> #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 <string/strchr.c>
> +
> +# 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 <string/strchr.c>
> #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?
On Thu, Jan 5, 2023 at 3:09 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Mon, Sep 19, 2022 at 1:01 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-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 <rth@twiddle.net>
> > ---
> > 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 <string.h>
> > #include <stdlib.h>
> > +#include <stdint.h>
> > +#include <string-fza.h>
> > +#include <string-fzb.h>
> > +#include <string-fzi.h>
> > +#include <string-extbyte.h>
> > +#include <string-maskoff.h>
> >
> > #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 <string/strchr.c>
> > +
> > +# 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 <string/strchr.c>
> > #endif
> > --
> > 2.34.1
> >
>
> Can this just be implemented as:
>
> char * r = strchrnul(p, c);
> return *r ? r : NULL;
Thats wrong, should be: `return (*r == c) ? r : NULL;`
>
> then only have strchrnul impl to worry about?
On 05/01/23 20:19, Noah Goldstein wrote:
> On Thu, Jan 5, 2023 at 3:09 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>>
>> On Mon, Sep 19, 2022 at 1:01 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-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 <rth@twiddle.net>
>>> ---
>>> 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 <string.h>
>>> #include <stdlib.h>
>>> +#include <stdint.h>
>>> +#include <string-fza.h>
>>> +#include <string-fzb.h>
>>> +#include <string-fzi.h>
>>> +#include <string-extbyte.h>
>>> +#include <string-maskoff.h>
>>>
>>> #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 <string/strchr.c>
>>> +
>>> +# 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 <string/strchr.c>
>>> #endif
>>> --
>>> 2.34.1
>>>
>>
>> Can this just be implemented as:
>>
>> char * r = strchrnul(p, c);
>> return *r ? r : NULL;
> Thats wrong, should be: `return (*r == c) ? r : NULL;`
>>
>> then only have strchrnul impl to worry about?
Yes, although I think strchr is a more used symbol than strchrnul. However,
we can optimize it later by adding a __strchrnul_inline and expand it
on both strchr and strchrnul. I will change to use strchrnul as you suggested.
@@ -22,164 +22,48 @@
#include <string.h>
#include <stdlib.h>
+#include <stdint.h>
+#include <string-fza.h>
+#include <string-fzb.h>
+#include <string-fzi.h>
+#include <string-extbyte.h>
+#include <string-maskoff.h>
#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
@@ -21,13 +21,14 @@
#if HAVE_STRCHR_C
# if HAVE_STRCHR_IFUNC
# define STRCHR STRCHR_C
-# undef weak_alias
+# endif
+
+# include <string/strchr.c>
+
+# 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 <string/strchr.c>
#endif