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

     /*
      * Copyright (c) 2000-2001 Apple Computer, Inc. All rights reserved.
      *
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     * circumvent, violate, or enable the circumvention or violation of, any
     * terms of an Apple operating system software license agreement.
     * 
     * Please obtain a copy of the License at
     * http://www.opensource.apple.com/apsl/ and read it before using this file.
     * 
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     */
    ;
    ;
    ; Strlen, optimized for PPC.  The routine we use is 2-3x faster
    ; then the simple loop which checks each byte for zero.
    ; For 0- and 1-byte strings, the simple routine is faster, but
    ; only by a few cycles.  The algorithm used was adapted from the
    ; Mac OS 9 stdCLib strcopy routine, which was originally
    ; written by Gary Davidian.  It relies on the following rather
    ; inobvious but very efficient test: 
    ; 
    ;       y =  dataWord + 0xFEFEFEFF
    ;       z = ~dataWord & 0x80808080
    ;       if ( y & z ) = 0 then all bytes in dataWord are non-zero
    ;
    ; The test maps any non-zero byte to zeros and any zero byte to 0x80,
    ; with one exception: 0x01 bytes preceeding the first zero are also
    ; mapped to 0x80.
    ;
    #include <ppc/asm.h>
    #include <ppc/proc_reg.h>
    ;
    ; int   strlen(ptr)
    ;
    ;
    
            .align  5
            .globl  EXT(strlen)
    LEXT(strlen)
    
            andi.   r4,r3,0x03              ; test alignment first
            mr      r9,r3                   ; store the original address for later use....
            bne     LalignSource            ; align the source addr if not already aligned
    Llentry:
            lis     r5,hi16(0xFEFEFEFF)
            lis     r6,hi16(0x80808080)
            subi    r3,r3,0x04              ; pre-decrement r3 for the lwzu
            ori     r5,r5,lo16(0xFEFEFEFF)  ; r5=0xFEFEFEFF
            ori     r6,r6,lo16(0x80808080)  ; r6=0x80808080
    
    LLoop:
            lwzu    r8,4(r3)                ; get the first 4 bytes and increment address
            add     r4,r5,r8                ; r4= data + 0xFEFEFEFF
            andc    r7,r6,r8                ; r7= ~data & 0x80808080
            and.    r4,r4,r7                ; r4= r4 & r7
            beq     LLoop                   ; if r4 is zero, then all bytes are non-zero
    
    ; Now we know one of the bytes in r8 is zero,
    ; we just have to figure out which one. 
    ; We have mapped 0 bytes to 0x80, and nonzero bytes to 0x00,
    ; with one exception:
    ; 0x01 bytes preceeding the first zero are also mapped to 0x80.
    ; So we have to mask out the 0x80s caused by 0x01s before
    ; counting leading zeroes to get the bytes in last word.
    
            rlwinm  r5,r8,7,0,31            ; move 0x01 bits to 0x80 position
            subf    r3,r9,r3                ; start to compute string length
            andc    r4,r4,r5                ; turn off false hits from 0x0100 worst case
            cntlzw  r7,r4                   ; now we can count leading 0s
            srwi    r7,r7,3                 ; convert 0,8,16,24 to 0,1,2,3
            add     r3,r3,r7                ; add in nonzero bytes in last word
            blr
    
    ; We must align the source address for two reasons: to avoid spurious page
    ; faults, and for speed.  
    ;       r4 = low 2 bits of address (1,2, or 3)
    ;       r3 = address
    ;       r9 = original address (still same as r3)
    
    LalignSource:
            lbz     r5,0(r3)                ; get the first byte...
            subic.  r4,r4,2                 ; test for 1, 2 or 3 bytes
            addi    r3,r3,1                 ; increment address
           addi    r6,r9,1                 ; now r6==r3
           cmpwi   cr1,r5,0                ; zero?
           beq     cr1,Lreturn             ; if its zero return zero
           bgt     Llentry                 ; address is aligned now if low bits were 3
   
           lbz     r5,0(r3)                ; get the next byte...
           addi    r3,r3,1                 ; increment address
           cmpwi   cr1,r5,0                ; zero?
           beq     cr1,Lreturn             ; if its zero return one
           beq     Llentry                 ; addr is aligned now if low bits were 2
   
           lbz     r5,0(r3)                ; get the next byte...
           addi    r3,r3,1                 ; increment address
           cmpwi   cr1,r5,0                ; zero?
           bne     cr1,Llentry             ; not zero, continue check (now aligned)
   Lreturn:
           sub     r3,r3,r6                ; get string length (0, 1, or 2)
           blr
   

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