| 1 | /* Compare two memory blocks for differences in the first COUNT bytes. |
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| 2 | Copyright (C) 1995-2022 Free Software Foundation, Inc. |
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| 3 | This file is part of the GNU C Library. |
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| 4 | |
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| 5 | The GNU C Library is free software; you can redistribute it and/or |
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| 6 | modify it under the terms of the GNU Lesser General Public |
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| 7 | License as published by the Free Software Foundation; either |
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| 8 | version 2.1 of the License, or (at your option) any later version. |
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| 9 | |
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| 10 | The GNU C Library is distributed in the hope that it will be useful, |
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| 11 | but WITHOUT ANY WARRANTY; without even the implied warranty of |
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| 12 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
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| 13 | Lesser General Public License for more details. |
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| 14 | |
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| 15 | You should have received a copy of the GNU Lesser General Public |
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| 16 | License along with the GNU C Library; if not, see |
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| 17 | <https://www.gnu.org/licenses/>. */ |
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| 18 | |
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| 19 | #include <sysdep.h> |
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| 20 | #include "asm-syntax.h" |
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| 21 | |
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| 22 | #define PARMS 4+4 /* space for 1 saved reg */ |
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| 23 | #define BLK1 PARMS |
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| 24 | #define BLK2 BLK1+4 |
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| 25 | #define LEN BLK2+4 |
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| 26 | |
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| 27 | .text |
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| 28 | ENTRY (memcmp) |
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| 29 | |
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| 30 | pushl %esi /* Save callee-safe registers. */ |
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| 31 | cfi_adjust_cfa_offset (4) |
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| 32 | movl %edi, %edx /* Note that %edx is not used and can |
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| 33 | so be used to save %edi. It's faster. */ |
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| 34 | cfi_register (edi, edx) |
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| 35 | |
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| 36 | movl BLK1(%esp), %esi |
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| 37 | cfi_rel_offset (esi, 0) |
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| 38 | movl BLK2(%esp), %edi |
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| 39 | movl LEN(%esp), %ecx |
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| 40 | |
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| 41 | cld /* Set direction of comparison. */ |
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| 42 | |
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| 43 | xorl %eax, %eax /* Default result. */ |
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| 44 | |
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| 45 | repe /* Compare at most %ecx bytes. */ |
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| 46 | cmpsb |
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| 47 | jz L(1) /* If even last byte was equal we return 0. */ |
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| 48 | |
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| 49 | /* The memory blocks are not equal. So result of the last |
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| 50 | subtraction is present in the carry flag. It is set when |
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| 51 | the byte in block #2 is bigger. In this case we have to |
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| 52 | return -1 (=0xffffffff), else 1. */ |
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| 53 | sbbl %eax, %eax /* This is tricky. %eax == 0 and carry is set |
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| 54 | or not depending on last subtraction. */ |
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| 55 | |
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| 56 | /* At this point %eax == 0, if the byte of block #1 was bigger, and |
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| 57 | 0xffffffff if the last byte of block #2 was bigger. The latter |
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| 58 | case is already correct but the former needs a little adjustment. |
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| 59 | Note that the following operation does not change 0xffffffff. */ |
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| 60 | orb $1, %al /* Change 0 to 1. */ |
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| 61 | |
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| 62 | L(1): popl %esi /* Restore registers. */ |
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| 63 | cfi_adjust_cfa_offset (-4) |
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| 64 | cfi_restore (esi) |
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| 65 | movl %edx, %edi |
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| 66 | cfi_restore (edi) |
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| 67 | |
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| 68 | ret |
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| 69 | END (memcmp) |
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| 70 | |
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| 71 | #undef bcmp |
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| 72 | weak_alias (memcmp, bcmp) |
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| 73 | #undef __memcmpeq |
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| 74 | strong_alias (memcmp, __memcmpeq) |
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| 75 | libc_hidden_builtin_def (memcmp) |
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| 76 | libc_hidden_def (__memcmpeq) |
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| 77 | |
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