/* Compare two memory blocks for differences in the first COUNT bytes. Copyright (C) 1995-2016 Free Software Foundation, Inc. This file is part of the GNU C Library. The GNU C Library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version. The GNU C Library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with the GNU C Library; if not, see <http://www.gnu.org/licenses/>. */ #include <sysdep.h> #include "asm-syntax.h" #define PARMS 4+4 /* space for 1 saved reg */ #define BLK1 PARMS #define BLK2 BLK1+4 #define LEN BLK2+4 .text ENTRY (memcmp) pushl %esi /* Save callee-safe registers. */ cfi_adjust_cfa_offset (4) movl %edi, %edx /* Note that %edx is not used and can so be used to save %edi. It's faster. */ cfi_register (edi, edx) movl BLK1(%esp), %esi cfi_rel_offset (esi, 0) movl BLK2(%esp), %edi movl LEN(%esp), %ecx cld /* Set direction of comparison. */ xorl %eax, %eax /* Default result. */ repe /* Compare at most %ecx bytes. */ cmpsb jz L(1) /* If even last byte was equal we return 0. */ /* The memory blocks are not equal. So result of the last subtraction is present in the carry flag. It is set when the byte in block #2 is bigger. In this case we have to return -1 (=0xffffffff), else 1. */ sbbl %eax, %eax /* This is tricky. %eax == 0 and carry is set or not depending on last subtraction. */ /* At this point %eax == 0, if the byte of block #1 was bigger, and 0xffffffff if the last byte of block #2 was bigger. The latter case is already correct but the former needs a little adjustment. Note that the following operation does not change 0xffffffff. */ orb $1, %al /* Change 0 to 1. */ L(1): popl %esi /* Restore registers. */ cfi_adjust_cfa_offset (-4) cfi_restore (esi) movl %edx, %edi cfi_restore (edi) ret END (memcmp) #undef bcmp weak_alias (memcmp, bcmp) libc_hidden_builtin_def (memcmp)