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30cf54bf30
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
merging 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).
Co-authored-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>
126 lines
3.5 KiB
C
126 lines
3.5 KiB
C
/* Copyright (C) 1991-2023 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include <stdint.h>
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#include <string-fzb.h>
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#include <string-fzc.h>
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#include <string-fzi.h>
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#include <string.h>
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#include <memcopy.h>
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#ifdef STRCMP
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# define strcmp STRCMP
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#endif
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static inline int
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final_cmp (const op_t w1, const op_t w2)
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{
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unsigned int idx = index_first_zero_ne (w1, w2);
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return extractbyte (w1, idx) - extractbyte (w2, idx);
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}
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/* Aligned loop: if a difference is found, exit to compare the bytes. Else
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if a zero is found we have equal strings. */
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static inline int
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strcmp_aligned_loop (const op_t *x1, const op_t *x2, op_t w1)
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{
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op_t w2 = *x2++;
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while (w1 == w2)
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{
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if (has_zero (w1))
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return 0;
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w1 = *x1++;
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w2 = *x2++;
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}
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return final_cmp (w1, w2);
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}
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/* Unaligned loop: align the first partial of P2, with 0xff for the rest of
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the bytes so that we can also apply the has_zero test to see if we have
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already reached EOS. If we have, then we can simply fall through to the
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final comparison. */
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static inline int
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strcmp_unaligned_loop (const op_t *x1, const op_t *x2, op_t w1, uintptr_t ofs)
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{
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op_t w2a = *x2++;
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uintptr_t sh_1 = ofs * CHAR_BIT;
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uintptr_t sh_2 = sizeof(op_t) * CHAR_BIT - sh_1;
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op_t w2 = MERGE (w2a, sh_1, (op_t)-1, sh_2);
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if (!has_zero (w2))
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{
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op_t w2b;
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/* Unaligned loop. The invariant is that W2B, which is "ahead" of W1,
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does not contain end-of-string. Therefore it is safe (and necessary)
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to read another word from each while we do not have a difference. */
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while (1)
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{
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w2b = *x2++;
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w2 = MERGE (w2a, sh_1, w2b, sh_2);
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if (w1 != w2)
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return final_cmp (w1, w2);
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if (has_zero (w2b))
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break;
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w1 = *x1++;
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w2a = w2b;
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}
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/* Zero found in the second partial of P2. If we had EOS in the aligned
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word, we have equality. */
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if (has_zero (w1))
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return 0;
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/* Load the final word of P1 and align the final partial of P2. */
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w1 = *x1++;
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w2 = MERGE (w2b, sh_1, 0, sh_2);
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}
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return final_cmp (w1, w2);
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}
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/* Compare S1 and S2, returning less than, equal to or
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greater than zero if S1 is lexicographically less than,
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equal to or greater than S2. */
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int
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strcmp (const char *p1, const char *p2)
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{
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/* Handle the unaligned bytes of p1 first. */
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uintptr_t n = -(uintptr_t)p1 % sizeof(op_t);
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for (int i = 0; i < n; ++i)
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{
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unsigned char c1 = *p1++;
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unsigned char c2 = *p2++;
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int diff = c1 - c2;
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if (c1 == '\0' || diff != 0)
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return diff;
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}
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/* P1 is now aligned to op_t. P2 may or may not be. */
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const op_t *x1 = (const op_t *) p1;
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op_t w1 = *x1++;
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uintptr_t ofs = (uintptr_t) p2 % sizeof(op_t);
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return ofs == 0
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? strcmp_aligned_loop (x1, (const op_t *)p2, w1)
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: strcmp_unaligned_loop (x1, (const op_t *)(p2 - ofs), w1, ofs);
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}
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#ifndef STRCMP
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libc_hidden_builtin_def (strcmp)
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#endif
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