1 /* Copyright (c) 2013, Linaro Limited
4 Redistribution and use in source and binary forms, with or without
5 modification, are permitted provided that the following conditions are met:
6 * Redistributions of source code must retain the above copyright
7 notice, this list of conditions and the following disclaimer.
8 * Redistributions in binary form must reproduce the above copyright
9 notice, this list of conditions and the following disclaimer in the
10 documentation and/or other materials provided with the distribution.
11 * Neither the name of the Linaro nor the
12 names of its contributors may be used to endorse or promote products
13 derived from this software without specific prior written permission.
15 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
16 "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
17 LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
18 A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
19 HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
20 SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
21 LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
22 DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
23 THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
24 (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
25 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */
32 .macro def_fn f p2align=0
40 #define REP8_01 0x0101010101010101
41 #define REP8_7f 0x7f7f7f7f7f7f7f7f
42 #define REP8_80 0x8080808080808080
44 /* Parameters and result. */
50 /* Internal variables. */
70 nop /* Pad so that the loop below fits a cache line. */
75 mov zeroones, #REP8_01
80 /* Calculate the number of full and partial words -1. */
81 sub limit_wd, limit, #1 /* limit != 0, so no underflow. */
82 lsr limit_wd, limit_wd, #3 /* Convert to Dwords. */
84 /* NUL detection works on the principle that (X - 1) & (~X) & 0x80
85 (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
86 can be done in parallel across the entire word. */
87 /* Start of performance-critical section -- one 64B cache line. */
92 subs limit_wd, limit_wd, #1
93 sub tmp1, data1, zeroones
94 orr tmp2, data1, #REP8_7f
95 eor diff, data1, data2 /* Non-zero if differences found. */
96 csinv endloop, diff, xzr, pl /* Last Dword or differences. */
97 bics has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */
98 ccmp endloop, #0, #0, eq
100 /* End of performance-critical section -- one 64B cache line. */
102 /* Not reached the limit, must have found the end or a diff. */
103 tbz limit_wd, #63, .Lnot_limit
105 /* Limit % 8 == 0 => all bytes significant. */
106 ands limit, limit, #7
109 lsl limit, limit, #3 /* Bits -> bytes. */
112 lsr mask, mask, limit
114 lsl mask, mask, limit
116 bic data1, data1, mask
117 bic data2, data2, mask
119 /* Make sure that the NUL byte is marked in the syndrome. */
120 orr has_nul, has_nul, mask
123 orr syndrome, diff, has_nul
125 #ifndef __AARCH64EB__
126 rev syndrome, syndrome
128 /* The MS-non-zero bit of the syndrome marks either the first bit
129 that is different, or the top bit of the first zero byte.
130 Shifting left now will bring the critical information into the
134 lsl data1, data1, pos
135 lsl data2, data2, pos
136 /* But we need to zero-extend (char is unsigned) the value and then
137 perform a signed 32-bit subtraction. */
138 lsr data1, data1, #56
139 sub result, data1, data2, lsr #56
142 /* For big-endian we cannot use the trick with the syndrome value
143 as carry-propagation can corrupt the upper bits if the trailing
144 bytes in the string contain 0x01. */
145 /* However, if there is no NUL byte in the dword, we can generate
146 the result directly. We can't just subtract the bytes as the
147 MSB might be significant. */
151 cneg result, result, lo
154 /* Re-compute the NUL-byte detection, using a byte-reversed value. */
156 sub tmp1, tmp3, zeroones
157 orr tmp2, tmp3, #REP8_7f
158 bic has_nul, tmp1, tmp2
160 orr syndrome, diff, has_nul
162 /* The MS-non-zero bit of the syndrome marks either the first bit
163 that is different, or the top bit of the first zero byte.
164 Shifting left now will bring the critical information into the
166 lsl data1, data1, pos
167 lsl data2, data2, pos
168 /* But we need to zero-extend (char is unsigned) the value and then
169 perform a signed 32-bit subtraction. */
170 lsr data1, data1, #56
171 sub result, data1, data2, lsr #56
176 /* Sources are mutually aligned, but are not currently at an
177 alignment boundary. Round down the addresses and then mask off
178 the bytes that precede the start point.
179 We also need to adjust the limit calculations, but without
180 overflowing if the limit is near ULONG_MAX. */
183 ldr data1, [src1], #8
184 neg tmp3, tmp1, lsl #3 /* 64 - bits(bytes beyond align). */
185 ldr data2, [src2], #8
187 sub limit_wd, limit, #1 /* limit != 0, so no underflow. */
189 /* Big-endian. Early bytes are at MSB. */
190 lsl tmp2, tmp2, tmp3 /* Shift (tmp1 & 63). */
192 /* Little-endian. Early bytes are at LSB. */
193 lsr tmp2, tmp2, tmp3 /* Shift (tmp1 & 63). */
195 and tmp3, limit_wd, #7
196 lsr limit_wd, limit_wd, #3
197 /* Adjust the limit. Only low 3 bits used, so overflow irrelevant. */
198 add limit, limit, tmp1
200 orr data1, data1, tmp2
201 orr data2, data2, tmp2
202 add limit_wd, limit_wd, tmp3, lsr #3
213 /* Perhaps we can do better than this. */
214 ldrb data1w, [src1], #1
215 ldrb data2w, [src2], #1
216 subs limit, limit, #1
217 ccmp data1w, #1, #0, cs /* NZCV = 0b0000. */
218 ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */
220 sub result, data1, data2
222 .size strncmp, . - strncmp