libkern/ppc/memcmp.s

   1 /*
   2  * Copyright (c) 2000-2001 Apple Computer, Inc. All rights reserved.
   3  *
   4  * @APPLE_LICENSE_HEADER_START@
   5  *
   6  * Copyright (c) 1999-2003 Apple Computer, Inc.  All Rights Reserved.
   7  *
   8  * This file contains Original Code and/or Modifications of Original Code
   9  * as defined in and that are subject to the Apple Public Source License
  10  * Version 2.0 (the 'License'). You may not use this file except in
  11  * compliance with the License. Please obtain a copy of the License at
  12  * http://www.opensource.apple.com/apsl/ and read it before using this
  13  * file.
  14  *
  15  * The Original Code and all software distributed under the License are
  16  * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  17  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  18  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
  19  * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
  20  * Please see the License for the specific language governing rights and
  21  * limitations under the License.
  22  *
  23  * @APPLE_LICENSE_HEADER_END@
  24  */
  25 ;
  26 #include <ppc/asm.h>
  27 #include <ppc/proc_reg.h>
  28 ;
  29 ; int   memcmp(const void *LHS, const void *RHS, size_t len);
  30 ;
  31 ; Memcmp returns the difference between the first two different bytes,
  32 ; or 0 if the two strings are equal.  Because we compare a word at a
  33 ; time, this requires a little additional processing once we find a
  34 ; difference.
  35 ;       r3 - LHS
  36 ;       r4 - RHS
  37 ;       r5 - len
  38
  39         .align  5
  40         .globl  EXT(memcmp)
  41 LEXT(memcmp)
  42
  43         cmpwi   cr1,r5,6                ; six is the most common length
  44         mr      r6,r3                   ; we want to use r3 for compare result
  45         mr.     r3,r5                   ; test length for 0
  46         bgt     cr1,Llong               ; handle long strings
  47         blt     cr1,Lshort              ; and short strings
  48
  49         ; six char strings are special cased because they are the most common
  50 Lsix:
  51         lwz     r8,0(r6)                ; first 4 bytes of LHS
  52         lwz     r7,0(r4)                ; and RHS
  53         xor.    r3,r8,r7                ; compare first 4
  54         bne     Ldifferent              ; first 4 differed
  55         lhz     r8,4(r6)                ; last 2 of LHS
  56         lhz     r7,4(r4)                ; last 2 of RHS
  57         xor.    r3,r8,r7                ; compare last 2
  58         beqlr                           ; done if equal
  59
  60         ; strings differ, so we must compute difference between first two
  61         ; differing bytes.
  62         ;       r8 = LHS bytes
  63         ;       r7 = RHS bytes
  64         ;       r3 = r8 xor r7 (r3!=0)
  65 Ldifferent:
  66         cntlzw  r9,r3                   ; count leading 0s in xor
  67         rlwinm  r10,r9,0,0,28           ; mask off low 3 bits, so r10 = 0, 8, 16, or 24
  68         subfic  r6,r10,24               ; r6 := (24 - r10)
  69         srw     r4,r8,r6                ; r4 = LHS differing byte
  70         srw     r5,r7,r6                ; r5 = RHS differing byte
  71         sub     r3,r4,r5                ; r3 = difference
  72         blr
  73
  74         ; handle long strings
  75 Llong:
  76         srwi    r0,r5,2                 ; r0 = word length
  77         mtctr   r0                      ; set up for loop
  78 Llongloop:
  79         lwz     r8,0(r6)                ; next 4 bytes from LHS
  80         addi    r6,r6,4
  81         lwz     r7,0(r4)                ; next 4 from RHS
  82         addi    r4,r4,4
  83         xor.    r3,r8,r7                ; compare next 4 bytes
  84         bdnzt+  eq,Llongloop            ; loop if ctr!=0 and cr0_eq
  85         bne     Ldifferent              ; these 4 bytes not equal
  86
  87         andi.   r5,r5,3                 ; more to go?
  88
  89         ; compare short strings (0-5 bytes long)
  90         ;       r5 = length (0-5)
  91         ;       cr0= set on length
  92         ;       r3 = if r5=0, then r3=0
  93 Lshort:
  94         beqlr                           ; 0-length strings are defined to be equal (r3=0)
  95         mtctr   r5
  96 Lshortloop:
  97         lbz     r8,0(r6)                ; get next byte from LHS
  98         addi    r6,r6,1
  99         lbz     r7,0(r4)                ; and next byte from RHS
 100         addi    r4,r4,1
 101         sub.    r3,r8,r7                ; compare
 102         bdnzt+  eq,Lshortloop           ; lloop if ctr!=0 and cr0_eq
 103         blr                             ; done, r3 set correctly by the subtract