+/*
+ * Copyright (c) 2009 Apple Inc. All rights reserved.
+ *
+ * @APPLE_LICENSE_HEADER_START@
+ *
+ * This file contains Original Code and/or Modifications of Original Code
+ * as defined in and that are subject to the Apple Public Source License
+ * Version 2.0 (the 'License'). You may not use this file except in
+ * compliance with the License. Please obtain a copy of the License at
+ * http://www.opensource.apple.com/apsl/ and read it before using this
+ * file.
+ *
+ * The Original Code and all software distributed under the License are
+ * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
+ * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
+ * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
+ * Please see the License for the specific language governing rights and
+ * limitations under the License.
+ *
+ * @APPLE_LICENSE_HEADER_END@
+ */
+
+// ARM Assembly implementation of memcmp( ) from <string.h>
+// Uses Thumb2 if it is available, otherwise generates ARM code.
+//
+// -- Stephen Canon, August 2009
+//
+// The basic idea is to use word compares instead of byte compares as long as
+// at least four bytes remain to be compared. However, because memcmp( )
+// compares the buffers as though they were big-endian unsigned integers, we
+// need to byte-reverse each word before comparing them.
+//
+// If the buffers are not word aligned, or they are shorter than four bytes,
+// we just use a simple byte comparison loop instead.
+//
+// int bcmp(void *src1, void *src2, size_t length);
+// int memcmp(void *src1, void *src2, size_t length);
+
+#include <arm/arch.h>
+
+ .text
+ .syntax unified
+#if defined __thumb2__
+ .code 16
+ .thumb_func _bcmp
+ .thumb_func _memcmp
+#else
+ .code 32
+#endif
+ .globl _bcmp
+ .globl _memcmp
+ .align 3
+_bcmp:
+_memcmp:
+
+#ifdef _ARM_ARCH_6
+ subs ip, r2, #4 // if length < 4
+ bmi L_useByteCompares // jump to the byte comparison loop
+
+ orr r3, r0, r1 // if the buffers are
+ tst r3, #3 // not word aligned
+ bne L_useByteCompares // jump to the byte comparison loop
+
+.align 3
+L_wordCompare: // Here we know that both buffers are word
+ ldr r2, [r0], #4 // aligned, and (length - 4) > 0, so at least
+ ldr r3, [r1], #4 // four bytes remain to be compared. We load
+ subs ip, #4 // a word from each buffer, and byte reverse
+ bmi L_lastWord // the loaded words. We also decrement the
+ rev r2, r2 // length by four and jump out of this loop if
+ rev r3, r3 // the result is negative. Then we compare the
+ cmp r2, r3 // reversed words, and continue the loop only
+ beq L_wordCompare // if they are equal.
+L_wordsUnequal:
+ ite hi // If the words compared unequal, return +/- 1
+ movhi r0, #1 // according to the result of the comparison.
+ movls r0, #-1 //
+ bx lr //
+L_lastWord:
+ rev r2, r2 // If we just loaded the last complete words
+ rev r3, r3 // from the buffers, byte-reverse them and
+ cmp r2, r3 // compare. If they are unequal, jump to the
+ bne L_wordsUnequal // return path.
+ add r2, ip, #4 // Otherwise, fall into the cleanup code.
+#endif // _ARM_ARCH_6
+
+L_useByteCompares:
+ tst r2, r2 // If the length is exactly zero
+ beq L_returnZero // avoid doing any loads and return zero.
+ mov r3, r0
+.align 3
+L_byteCompareLoop:
+ ldrb r0, [r3], #1 // Load a byte from each buffer, and decrement
+ ldrb ip, [r1], #1 // the length by one. If the decremented
+ subs r2, #1 // length is zero, exit the loop. Otherwise
+ beq L_lastByte // subtract the loaded bytes; if their
+ subs r0, ip // difference is zero, continue the comparison
+ beq L_byteCompareLoop // loop. Otherwise, return their difference.
+ bx lr
+L_returnZero:
+ mov r0, ip
+L_lastByte:
+ sub r0, ip // Return the difference of the final bytes
+ bx lr
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