[PATCH v11 09/29] string: Improve generic strncmp
Noah Goldstein
goldstein.w.n@gmail.com
Wed Feb 1 19:42:41 GMT 2023
On Wed, Feb 1, 2023 at 11:04 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
> It follows the strategy:
>
> - Align the first input to word boundary using byte operations.
>
> - If second input is also word aligned, read a word per time, check
> for null (using has_zero), and check final words using byte
> operation.
>
> - If second input is not word aligned, loop by aligning the source,
> and merge the result of two reads. Similar to aligned case, check
> for null with has_zero, and check final words using byte operation.
>
> Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc64-linux-gnu,
> and powerpc-linux-gnu by removing the arch-specific assembly
> implementation and disabling multi-arch (it covers both LE and BE
> for 64 and 32 bits).
> ---
> string/strcmp-impl.h | 41 ++++++++++++++
> string/strcmp.c | 23 ++------
> string/strncmp.c | 130 +++++++++++++++++++++++++++++++------------
> 3 files changed, 138 insertions(+), 56 deletions(-)
> create mode 100644 string/strcmp-impl.h
>
> diff --git a/string/strcmp-impl.h b/string/strcmp-impl.h
> new file mode 100644
> index 0000000000..618240368a
> --- /dev/null
> +++ b/string/strcmp-impl.h
> @@ -0,0 +1,41 @@
> +/* Common definition for string compare implementations.
> + Copyright (C) 2023 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
> + <https://www.gnu.org/licenses/>. */
> +
> +#ifndef _STRCMP_IMPL_H
> +#define _STRCMP_IMPL_H
> +
> +#include <stddef.h>
> +#include <string-misc.h>
> +
> +static inline int
> +final_cmp (const op_t w1, const op_t w2, size_t n)
> +{
> + /* It can not use index_first_zero_ne because it must not compare past the
> + final '\0' is present (and final_cmp is called before has_zero check).
> + */
> + for (size_t i = 0; i < n; i++)
> + {
> + unsigned char c1 = extractbyte (w1, i);
> + unsigned char c2 = extractbyte (w2, i);
> + if (c1 == '\0' || c1 != c2)
> + return c1 - c2;
> + }
> + return 0;
> +}
> +
> +#endif
> diff --git a/string/strcmp.c b/string/strcmp.c
> index 42e24242b6..8e7b3310db 100644
> --- a/string/strcmp.c
> +++ b/string/strcmp.c
> @@ -16,6 +16,7 @@
> <https://www.gnu.org/licenses/>. */
>
> #include <stdint.h>
> +#include <strcmp-impl.h>
> #include <string-fzb.h>
> #include <string-fzi.h>
> #include <string.h>
> @@ -25,22 +26,6 @@
> # define strcmp STRCMP
> #endif
>
> -static inline int
> -final_cmp (const op_t w1, const op_t w2)
> -{
> - /* It can not use index_first_zero_ne because it must not compare past the
> - final '\0' is present (and final_cmp is called before has_zero check).
> - */
> - for (size_t i = 0; i < sizeof (op_t); i++)
> - {
> - unsigned char c1 = extractbyte (w1, i);
> - unsigned char c2 = extractbyte (w2, i);
> - if (c1 == '\0' || c1 != c2)
> - return c1 - c2;
> - }
> - return 0;
> -}
> -
> /* Aligned loop: if a difference is found, exit to compare the bytes. Else
> if a zero is found we have equal strings. */
> static inline int
> @@ -56,7 +41,7 @@ strcmp_aligned_loop (const op_t *x1, const op_t *x2, op_t w1)
> w2 = *x2++;
> }
>
> - return final_cmp (w1, w2);
> + return final_cmp (w1, w2, sizeof (op_t));
> }
>
> /* Unaligned loop: align the first partial of P2, with 0xff for the rest of
> @@ -83,7 +68,7 @@ strcmp_unaligned_loop (const op_t *x1, const op_t *x2, op_t w1, uintptr_t ofs)
> w2b = *x2++;
> w2 = MERGE (w2a, sh_1, w2b, sh_2);
> if (w1 != w2)
> - return final_cmp (w1, w2);
> + return final_cmp (w1, w2, sizeof (op_t));
> if (has_zero (w2b))
> break;
> w1 = *x1++;
> @@ -100,7 +85,7 @@ strcmp_unaligned_loop (const op_t *x1, const op_t *x2, op_t w1, uintptr_t ofs)
> w2 = MERGE (w2b, sh_1, 0, sh_2);
> }
>
> - return final_cmp (w1, w2);
> + return final_cmp (w1, w2, sizeof (op_t));
> }
>
> /* Compare S1 and S2, returning less than, equal to or
> diff --git a/string/strncmp.c b/string/strncmp.c
> index fd7cee09b6..3e6040df09 100644
> --- a/string/strncmp.c
> +++ b/string/strncmp.c
> @@ -15,7 +15,13 @@
> License along with the GNU C Library; if not, see
> <https://www.gnu.org/licenses/>. */
>
> +#include <stdint.h>
> +#include <strcmp-impl.h>
> +#include <string-fza.h>
> +#include <string-fzb.h>
> +#include <string-fzi.h>
> #include <string.h>
> +#include <sys/param.h>
> #include <memcopy.h>
>
> #undef strncmp
> @@ -24,51 +30,101 @@
> #define STRNCMP strncmp
> #endif
>
> -/* Compare no more than N characters of S1 and S2,
> - returning less than, equal to or greater than zero
> - if S1 is lexicographically less than, equal to or
> - greater than S2. */
> -int
> -STRNCMP (const char *s1, const char *s2, size_t n)
> +/* Aligned loop: if a difference is found, exit to compare the bytes. Else
> + if a zero is found we have equal strings. */
> +static inline int
> +strncmp_aligned_loop (const op_t *x1, const op_t *x2, op_t w1, size_t n)
> {
> - unsigned char c1 = '\0';
> - unsigned char c2 = '\0';
> + op_t w2 = *x2++;
>
> - if (n >= 4)
> + while (w1 == w2)
> {
> - size_t n4 = n >> 2;
> - do
> - {
> - c1 = (unsigned char) *s1++;
> - c2 = (unsigned char) *s2++;
> - if (c1 == '\0' || c1 != c2)
> - return c1 - c2;
> - c1 = (unsigned char) *s1++;
> - c2 = (unsigned char) *s2++;
> - if (c1 == '\0' || c1 != c2)
> - return c1 - c2;
> - c1 = (unsigned char) *s1++;
> - c2 = (unsigned char) *s2++;
> - if (c1 == '\0' || c1 != c2)
> - return c1 - c2;
> - c1 = (unsigned char) *s1++;
> - c2 = (unsigned char) *s2++;
> - if (c1 == '\0' || c1 != c2)
> - return c1 - c2;
> - } while (--n4 > 0);
> - n &= 3;
> + if (n <= sizeof (op_t))
> + break;
> + n -= sizeof (op_t);
> +
> + if (has_zero (w1))
> + return 0;
> + w1 = *x1++;
> + w2 = *x2++;
> }
>
> - while (n > 0)
> + return final_cmp (w1, w2, n);
> +}
> +
> +/* Unaligned loop: align the first partial of P2, with 0xff for the rest of
> + the bytes so that we can also apply the has_zero test to see if we have
> + already reached EOS. If we have, then we can simply fall through to the
> + final comparison. */
> +static inline int
> +strncmp_unaligned_loop (const op_t *x1, const op_t *x2, op_t w1, uintptr_t ofs,
> + size_t n)
> +{
> + op_t w2a = *x2++;
> + uintptr_t sh_1 = ofs * CHAR_BIT;
> + uintptr_t sh_2 = sizeof(op_t) * CHAR_BIT - sh_1;
> +
> + op_t w2 = MERGE (w2a, sh_1, (op_t)-1, sh_2);
> + if (!has_zero (w2) && n > (sizeof (op_t) - ofs))
> {
> - c1 = (unsigned char) *s1++;
> - c2 = (unsigned char) *s2++;
> - if (c1 == '\0' || c1 != c2)
> - return c1 - c2;
> - n--;
> + op_t w2b;
> +
> + /* Unaligned loop. The invariant is that W2B, which is "ahead" of W1,
> + does not contain end-of-string. Therefore it is safe (and necessary)
> + to read another word from each while we do not have a difference. */
> + while (1)
> + {
> + w2b = *x2++;
> + w2 = MERGE (w2a, sh_1, w2b, sh_2);
> + if (n <= sizeof (op_t) || w1 != w2)
> + return final_cmp (w1, w2, n);
> + n -= sizeof(op_t);
> + if (has_zero (w2b) || n <= (sizeof (op_t) - ofs))
> + break;
> + w1 = *x1++;
> + w2a = w2b;
> + }
> +
> + /* Zero found in the second partial of P2. If we had EOS in the aligned
> + word, we have equality. */
> + if (has_zero (w1))
> + return 0;
> +
> + /* Load the final word of P1 and align the final partial of P2. */
> + w1 = *x1++;
> + w2 = MERGE (w2b, sh_1, 0, sh_2);
> }
>
> - return c1 - c2;
> + return final_cmp (w1, w2, n);
> }
>
> +/* Compare no more than N characters of S1 and S2,
> + returning less than, equal to or greater than zero
> + if S1 is lexicographically less than, equal to or
> + greater than S2. */
> +int
> +STRNCMP (const char *p1, const char *p2, size_t n)
> +{
> + /* Handle the unaligned bytes of p1 first. */
> + uintptr_t a = MIN (-(uintptr_t)p1 % sizeof(op_t), n);
> + int diff = 0;
> + for (int i = 0; i < a; ++i)
> + {
> + unsigned char c1 = *p1++;
> + unsigned char c2 = *p2++;
> + diff = c1 - c2;
> + if (c1 == '\0' || diff != 0)
> + return diff;
> + }
> + if (a == n)
> + return diff;
NIT: return 0 IMO
> +
> + /* P1 is now aligned to op_t. P2 may or may not be. */
> + const op_t *x1 = (const op_t *) p1;
> + op_t w1 = *x1++;
> + uintptr_t ofs = (uintptr_t) p2 % sizeof(op_t);
> + return ofs == 0
> + ? strncmp_aligned_loop (x1, (const op_t *) p2, w1, n - a)
> + : strncmp_unaligned_loop (x1, (const op_t *) (p2 - ofs), w1, ofs, n - a);
> +}
> libc_hidden_builtin_def (STRNCMP)
> --
> 2.34.1
>
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