1/* Copyright (C) 1991-2024 Free Software Foundation, Inc.
2 This file is part of the GNU C Library.
3
4 The GNU C Library is free software; you can redistribute it and/or
5 modify it under the terms of the GNU Lesser General Public
6 License as published by the Free Software Foundation; either
7 version 2.1 of the License, or (at your option) any later version.
8
9 The GNU C Library is distributed in the hope that it will be useful,
10 but WITHOUT ANY WARRANTY; without even the implied warranty of
11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
12 Lesser General Public License for more details.
13
14 You should have received a copy of the GNU Lesser General Public
15 License along with the GNU C Library; if not, see
16 <https://www.gnu.org/licenses/>. */
17
18#include <stdint.h>
19#include <string-fzb.h>
20#include <string-fzc.h>
21#include <string-fzi.h>
22#include <string.h>
23#include <sys/param.h>
24#include <memcopy.h>
25
26#undef strncmp
27
28#ifndef STRNCMP
29#define STRNCMP strncmp
30#endif
31
32static inline int
33final_cmp (const op_t w1, const op_t w2, size_t n)
34{
35 unsigned int idx = index_first_zero_ne (x1: w1, x2: w2);
36 if (n <= idx)
37 return 0;
38 return extractbyte (x: w1, idx) - extractbyte (x: w2, idx);
39}
40
41/* Aligned loop: if a difference is found, exit to compare the bytes. Else
42 if a zero is found we have equal strings. */
43static inline int
44strncmp_aligned_loop (const op_t *x1, const op_t *x2, op_t w1, size_t n)
45{
46 op_t w2 = *x2++;
47
48 while (w1 == w2)
49 {
50 if (n <= sizeof (op_t))
51 break;
52 n -= sizeof (op_t);
53
54 if (has_zero (x: w1))
55 return 0;
56 w1 = *x1++;
57 w2 = *x2++;
58 }
59
60 return final_cmp (w1, w2, n);
61}
62
63/* Unaligned loop: align the first partial of P2, with 0xff for the rest of
64 the bytes so that we can also apply the has_zero test to see if we have
65 already reached EOS. If we have, then we can simply fall through to the
66 final comparison. */
67static inline int
68strncmp_unaligned_loop (const op_t *x1, const op_t *x2, op_t w1, uintptr_t ofs,
69 size_t n)
70{
71 op_t w2a = *x2++;
72 uintptr_t sh_1 = ofs * CHAR_BIT;
73 uintptr_t sh_2 = sizeof(op_t) * CHAR_BIT - sh_1;
74
75 op_t w2 = MERGE (w2a, sh_1, (op_t)-1, sh_2);
76 if (!has_zero (x: w2) && n > (sizeof (op_t) - ofs))
77 {
78 op_t w2b;
79
80 /* Unaligned loop. The invariant is that W2B, which is "ahead" of W1,
81 does not contain end-of-string. Therefore it is safe (and necessary)
82 to read another word from each while we do not have a difference. */
83 while (1)
84 {
85 w2b = *x2++;
86 w2 = MERGE (w2a, sh_1, w2b, sh_2);
87 if (n <= sizeof (op_t) || w1 != w2)
88 return final_cmp (w1, w2, n);
89 n -= sizeof(op_t);
90 if (has_zero (x: w2b) || n <= (sizeof (op_t) - ofs))
91 break;
92 w1 = *x1++;
93 w2a = w2b;
94 }
95
96 /* Zero found in the second partial of P2. If we had EOS in the aligned
97 word, we have equality. */
98 if (has_zero (x: w1))
99 return 0;
100
101 /* Load the final word of P1 and align the final partial of P2. */
102 w1 = *x1++;
103 w2 = MERGE (w2b, sh_1, 0, sh_2);
104 }
105
106 return final_cmp (w1, w2, n);
107}
108
109/* Compare no more than N characters of S1 and S2,
110 returning less than, equal to or greater than zero
111 if S1 is lexicographically less than, equal to or
112 greater than S2. */
113int
114STRNCMP (const char *p1, const char *p2, size_t n)
115{
116 /* Handle the unaligned bytes of p1 first. */
117 uintptr_t a = MIN (-(uintptr_t)p1 % sizeof(op_t), n);
118 int diff = 0;
119 for (int i = 0; i < a; ++i)
120 {
121 unsigned char c1 = *p1++;
122 unsigned char c2 = *p2++;
123 diff = c1 - c2;
124 if (c1 == '\0' || diff != 0)
125 return diff;
126 }
127 if (a == n)
128 return 0;
129
130 /* P1 is now aligned to op_t. P2 may or may not be. */
131 const op_t *x1 = (const op_t *) p1;
132 op_t w1 = *x1++;
133 uintptr_t ofs = (uintptr_t) p2 % sizeof(op_t);
134 return ofs == 0
135 ? strncmp_aligned_loop (x1, x2: (const op_t *) p2, w1, n: n - a)
136 : strncmp_unaligned_loop (x1, x2: (const op_t *) (p2 - ofs), w1, ofs, n: n - a);
137}
138libc_hidden_builtin_def (STRNCMP)
139

source code of glibc/string/strncmp.c