memcmp.s

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    1 /*
    2  * Copyright (c) 2000-2001 Apple Computer, Inc. All rights reserved.
    3  *
    4  * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
    5  * 
    6  * This file contains Original Code and/or Modifications of Original Code
    7  * as defined in and that are subject to the Apple Public Source License
    8  * Version 2.0 (the 'License'). You may not use this file except in
    9  * compliance with the License. The rights granted to you under the License
   10  * may not be used to create, or enable the creation or redistribution of,
   11  * unlawful or unlicensed copies of an Apple operating system, or to
   12  * circumvent, violate, or enable the circumvention or violation of, any
   13  * terms of an Apple operating system software license agreement.
   14  * 
   15  * Please obtain a copy of the License at
   16  * http://www.opensource.apple.com/apsl/ and read it before using this file.
   17  * 
   18  * The Original Code and all software distributed under the License are
   19  * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
   20  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
   21  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
   22  * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
   23  * Please see the License for the specific language governing rights and
   24  * limitations under the License.
   25  * 
   26  * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
   27  */
   28 ;
   29 #include <ppc/asm.h>
   30 #include <ppc/proc_reg.h>
   31 ;
   32 ; int   memcmp(const void *LHS, const void *RHS, size_t len);
   33 ;
   34 ; Memcmp returns the difference between the first two different bytes, 
   35 ; or 0 if the two strings are equal.  Because we compare a word at a
   36 ; time, this requires a little additional processing once we find a
   37 ; difference.
   38 ;       r3 - LHS
   39 ;       r4 - RHS
   40 ;       r5 - len
   41 
   42         .align  5
   43         .globl  EXT(memcmp)
   44 LEXT(memcmp)
   45 
   46         cmpwi   cr1,r5,6                ; six is the most common length
   47         mr      r6,r3                   ; we want to use r3 for compare result
   48         mr.     r3,r5                   ; test length for 0
   49         bgt     cr1,Llong               ; handle long strings
   50         blt     cr1,Lshort              ; and short strings
   51 
   52         ; six char strings are special cased because they are the most common
   53 Lsix:
   54         lwz     r8,0(r6)                ; first 4 bytes of LHS
   55         lwz     r7,0(r4)                ; and RHS
   56         xor.    r3,r8,r7                ; compare first 4
   57         bne     Ldifferent              ; first 4 differed
   58         lhz     r8,4(r6)                ; last 2 of LHS
   59         lhz     r7,4(r4)                ; last 2 of RHS
   60         xor.    r3,r8,r7                ; compare last 2
   61         beqlr                           ; done if equal
   62 
   63         ; strings differ, so we must compute difference between first two
   64         ; differing bytes.
   65         ;       r8 = LHS bytes
   66         ;       r7 = RHS bytes
   67         ;       r3 = r8 xor r7 (r3!=0)
   68 Ldifferent:
   69         cntlzw  r9,r3                   ; count leading 0s in xor
   70         rlwinm  r10,r9,0,0,28           ; mask off low 3 bits, so r10 = 0, 8, 16, or 24
   71         subfic  r6,r10,24               ; r6 := (24 - r10)
   72         srw     r4,r8,r6                ; r4 = LHS differing byte
   73         srw     r5,r7,r6                ; r5 = RHS differing byte
   74         sub     r3,r4,r5                ; r3 = difference
   75         blr
   76 
   77         ; handle long strings
   78 Llong:
   79         srwi    r0,r5,2                 ; r0 = word length
   80         mtctr   r0                      ; set up for loop
   81 Llongloop:
   82         lwz     r8,0(r6)                ; next 4 bytes from LHS
   83         addi    r6,r6,4
   84         lwz     r7,0(r4)                ; next 4 from RHS
   85         addi    r4,r4,4
   86         xor.    r3,r8,r7                ; compare next 4 bytes
   87         bdnzt+  eq,Llongloop            ; loop if ctr!=0 and cr0_eq
   88         bne     Ldifferent              ; these 4 bytes not equal
   89         
   90         andi.   r5,r5,3                 ; more to go?
   91 
   92         ; compare short strings (0-5 bytes long)
   93         ;       r5 = length (0-5)
   94         ;       cr0= set on length
   95         ;       r3 = if r5=0, then r3=0
   96 Lshort:
   97         beqlr                           ; 0-length strings are defined to be equal (r3=0)
   98         mtctr   r5
   99 Lshortloop:
  100         lbz     r8,0(r6)                ; get next byte from LHS
  101         addi    r6,r6,1
  102         lbz     r7,0(r4)                ; and next byte from RHS
  103         addi    r4,r4,1
  104         sub.    r3,r8,r7                ; compare
  105         bdnzt+  eq,Lshortloop           ; lloop if ctr!=0 and cr0_eq
  106         blr                             ; done, r3 set correctly by the subtract

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FreeBSD/Linux Kernel Cross Reference sys/libkern/ppc/memcmp.s

FreeBSD/Linux Kernel Cross Reference
sys/libkern/ppc/memcmp.s