1/* Copyright (c) 2012, Linaro Limited 2 All rights reserved. 3 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. 14 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. 26*/ 27 28/* Assumptions: 29 * 30 * ARMv8-a, AArch64 31 */ 32 33#include <private/bionic_asm.h> 34 35#define REP8_01 0x0101010101010101 36#define REP8_7f 0x7f7f7f7f7f7f7f7f 37#define REP8_80 0x8080808080808080 38 39/* Parameters and result. */ 40#define src1 x0 41#define src2 x1 42#define result x0 43 44/* Internal variables. */ 45#define data1 x2 46#define data1w w2 47#define data2 x3 48#define data2w w3 49#define has_nul x4 50#define diff x5 51#define syndrome x6 52#define tmp1 x7 53#define tmp2 x8 54#define tmp3 x9 55#define zeroones x10 56#define pos x11 57 58 /* Start of performance-critical section -- one 64B cache line. */ 59ENTRY(strcmp) 60 eor tmp1, src1, src2 61 mov zeroones, #REP8_01 62 tst tmp1, #7 63 b.ne .Lmisaligned8 64 ands tmp1, src1, #7 65 b.ne .Lmutual_align 66 /* NUL detection works on the principle that (X - 1) & (~X) & 0x80 67 (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and 68 can be done in parallel across the entire word. */ 69.Lloop_aligned: 70 ldr data1, [src1], #8 71 ldr data2, [src2], #8 72.Lstart_realigned: 73 sub tmp1, data1, zeroones 74 orr tmp2, data1, #REP8_7f 75 eor diff, data1, data2 /* Non-zero if differences found. */ 76 bic has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */ 77 orr syndrome, diff, has_nul 78 cbz syndrome, .Lloop_aligned 79 /* End of performance-critical section -- one 64B cache line. */ 80 81#ifndef __AARCH64EB__ 82 rev syndrome, syndrome 83 rev data1, data1 84 /* The MS-non-zero bit of the syndrome marks either the first bit 85 that is different, or the top bit of the first zero byte. 86 Shifting left now will bring the critical information into the 87 top bits. */ 88 clz pos, syndrome 89 rev data2, data2 90 lsl data1, data1, pos 91 lsl data2, data2, pos 92 /* But we need to zero-extend (char is unsigned) the value and then 93 perform a signed 32-bit subtraction. */ 94 lsr data1, data1, #56 95 sub result, data1, data2, lsr #56 96 ret 97#else 98 /* For big-endian we cannot use the trick with the syndrome value 99 as carry-propagation can corrupt the upper bits if the trailing 100 bytes in the string contain 0x01. */ 101 /* However, if there is no NUL byte in the dword, we can generate 102 the result directly. We can't just subtract the bytes as the 103 MSB might be significant. */ 104 cbnz has_nul, 1f 105 cmp data1, data2 106 cset result, ne 107 cneg result, result, lo 108 ret 1091: 110 /* Re-compute the NUL-byte detection, using a byte-reversed value. */ 111 rev tmp3, data1 112 sub tmp1, tmp3, zeroones 113 orr tmp2, tmp3, #REP8_7f 114 bic has_nul, tmp1, tmp2 115 rev has_nul, has_nul 116 orr syndrome, diff, has_nul 117 clz pos, syndrome 118 /* The MS-non-zero bit of the syndrome marks either the first bit 119 that is different, or the top bit of the first zero byte. 120 Shifting left now will bring the critical information into the 121 top bits. */ 122 lsl data1, data1, pos 123 lsl data2, data2, pos 124 /* But we need to zero-extend (char is unsigned) the value and then 125 perform a signed 32-bit subtraction. */ 126 lsr data1, data1, #56 127 sub result, data1, data2, lsr #56 128 ret 129#endif 130 131.Lmutual_align: 132 /* Sources are mutually aligned, but are not currently at an 133 alignment boundary. Round down the addresses and then mask off 134 the bytes that preceed the start point. */ 135 bic src1, src1, #7 136 bic src2, src2, #7 137 lsl tmp1, tmp1, #3 /* Bytes beyond alignment -> bits. */ 138 ldr data1, [src1], #8 139 neg tmp1, tmp1 /* Bits to alignment -64. */ 140 ldr data2, [src2], #8 141 mov tmp2, #~0 142#ifdef __AARCH64EB__ 143 /* Big-endian. Early bytes are at MSB. */ 144 lsl tmp2, tmp2, tmp1 /* Shift (tmp1 & 63). */ 145#else 146 /* Little-endian. Early bytes are at LSB. */ 147 lsr tmp2, tmp2, tmp1 /* Shift (tmp1 & 63). */ 148#endif 149 orr data1, data1, tmp2 150 orr data2, data2, tmp2 151 b .Lstart_realigned 152 153.Lmisaligned8: 154 /* We can do better than this. */ 155 ldrb data1w, [src1], #1 156 ldrb data2w, [src2], #1 157 cmp data1w, #1 158 ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */ 159 b.eq .Lmisaligned8 160 sub result, data1, data2 161 ret 162END(strcmp) 163