libkern/ppc/memcmp.s

   1 /*
   2  * Copyright (c) 2000-2001 Apple Computer, Inc. All rights reserved.
   3  *
   4  * @APPLE_LICENSE_HEADER_START@
   5  *
   6  * The contents of this file constitute Original Code as defined in and
   7  * are subject to the Apple Public Source License Version 1.1 (the
   8  * "License").  You may not use this file except in compliance with the
   9  * License.  Please obtain a copy of the License at
  10  * http://www.apple.com/publicsource and read it before using this file.
  11  *
  12  * This Original Code and all software distributed under the License are
  13  * distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  14  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  15  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
  16  * FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT.  Please see the
  17  * License for the specific language governing rights and limitations
  18  * under the License.
  19  *
  20  * @APPLE_LICENSE_HEADER_END@
  21  */
  22 ;
  23 #include <ppc/asm.h>
  24 #include <ppc/proc_reg.h>
  25 ;
  26 ; int   memcmp(const void *LHS, const void *RHS, size_t len);
  27 ;
  28 ; Memcmp returns the difference between the first two different bytes,
  29 ; or 0 if the two strings are equal.  Because we compare a word at a
  30 ; time, this requires a little additional processing once we find a
  31 ; difference.
  32 ;       r3 - LHS
  33 ;       r4 - RHS
  34 ;       r5 - len
  35
  36         .align  5
  37         .globl  EXT(memcmp)
  38 LEXT(memcmp)
  39
  40         cmpwi   cr1,r5,6                ; six is the most common length
  41         mr      r6,r3                   ; we want to use r3 for compare result
  42         mr.     r3,r5                   ; test length for 0
  43         bgt     cr1,Llong               ; handle long strings
  44         blt     cr1,Lshort              ; and short strings
  45
  46         ; six char strings are special cased because they are the most common
  47 Lsix:
  48         lwz     r8,0(r6)                ; first 4 bytes of LHS
  49         lwz     r7,0(r4)                ; and RHS
  50         xor.    r3,r8,r7                ; compare first 4
  51         bne     Ldifferent              ; first 4 differed
  52         lhz     r8,4(r6)                ; last 2 of LHS
  53         lhz     r7,4(r4)                ; last 2 of RHS
  54         xor.    r3,r8,r7                ; compare last 2
  55         beqlr                           ; done if equal
  56
  57         ; strings differ, so we must compute difference between first two
  58         ; differing bytes.
  59         ;       r8 = LHS bytes
  60         ;       r7 = RHS bytes
  61         ;       r3 = r8 xor r7 (r3!=0)
  62 Ldifferent:
  63         cntlzw  r9,r3                   ; count leading 0s in xor
  64         rlwinm  r10,r9,0,0,28           ; mask off low 3 bits, so r10 = 0, 8, 16, or 24
  65         subfic  r6,r10,24               ; r6 := (24 - r10)
  66         srw     r4,r8,r6                ; r4 = LHS differing byte
  67         srw     r5,r7,r6                ; r5 = RHS differing byte
  68         sub     r3,r4,r5                ; r3 = difference
  69         blr
  70
  71         ; handle long strings
  72 Llong:
  73         srwi    r0,r5,2                 ; r0 = word length
  74         mtctr   r0                      ; set up for loop
  75 Llongloop:
  76         lwz     r8,0(r6)                ; next 4 bytes from LHS
  77         addi    r6,r6,4
  78         lwz     r7,0(r4)                ; next 4 from RHS
  79         addi    r4,r4,4
  80         xor.    r3,r8,r7                ; compare next 4 bytes
  81         bdnzt+  eq,Llongloop            ; loop if ctr!=0 and cr0_eq
  82         bne     Ldifferent              ; these 4 bytes not equal
  83
  84         andi.   r5,r5,3                 ; more to go?
  85
  86         ; compare short strings (0-5 bytes long)
  87         ;       r5 = length (0-5)
  88         ;       cr0= set on length
  89         ;       r3 = if r5=0, then r3=0
  90 Lshort:
  91         beqlr                           ; 0-length strings are defined to be equal (r3=0)
  92         mtctr   r5
  93 Lshortloop:
  94         lbz     r8,0(r6)                ; get next byte from LHS
  95         addi    r6,r6,1
  96         lbz     r7,0(r4)                ; and next byte from RHS
  97         addi    r4,r4,1
  98         sub.    r3,r8,r7                ; compare
  99         bdnzt+  eq,Lshortloop           ; lloop if ctr!=0 and cr0_eq
 100         blr                             ; done, r3 set correctly by the subtract