libkern/ppc/strlen.s

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  30 ;
  31 ;
  32 ; Strlen, optimized for PPC.  The routine we use is 2-3x faster
  33 ; then the simple loop which checks each byte for zero.
  34 ; For 0- and 1-byte strings, the simple routine is faster, but
  35 ; only by a few cycles.  The algorithm used was adapted from the
  36 ; Mac OS 9 stdCLib strcopy routine, which was originally
  37 ; written by Gary Davidian.  It relies on the following rather
  38 ; inobvious but very efficient test:
  39 ;
  40 ;       y =  dataWord + 0xFEFEFEFF
  41 ;       z = ~dataWord & 0x80808080
  42 ;       if ( y & z ) = 0 then all bytes in dataWord are non-zero
  43 ;
  44 ; The test maps any non-zero byte to zeros and any zero byte to 0x80,
  45 ; with one exception: 0x01 bytes preceeding the first zero are also
  46 ; mapped to 0x80.
  47 ;
  48 #include <ppc/asm.h>
  49 #include <ppc/proc_reg.h>
  50 ;
  51 ; int   strlen(ptr)
  52 ;
  53 ;
  54
  55         .align  5
  56         .globl  EXT(strlen)
  57 LEXT(strlen)
  58
  59         andi.   r4,r3,0x03              ; test alignment first
  60         mr      r9,r3                   ; store the original address for later use....
  61         bne     LalignSource            ; align the source addr if not already aligned
  62 Llentry:
  63         lis     r5,hi16(0xFEFEFEFF)
  64         lis     r6,hi16(0x80808080)
  65         subi    r3,r3,0x04              ; pre-decrement r3 for the lwzu
  66         ori     r5,r5,lo16(0xFEFEFEFF)  ; r5=0xFEFEFEFF
  67         ori     r6,r6,lo16(0x80808080)  ; r6=0x80808080
  68
  69 LLoop:
  70         lwzu    r8,4(r3)                ; get the first 4 bytes and increment address
  71         add     r4,r5,r8                ; r4= data + 0xFEFEFEFF
  72         andc    r7,r6,r8                ; r7= ~data & 0x80808080
  73         and.    r4,r4,r7                ; r4= r4 & r7
  74         beq     LLoop                   ; if r4 is zero, then all bytes are non-zero
  75
  76 ; Now we know one of the bytes in r8 is zero,
  77 ; we just have to figure out which one.
  78 ; We have mapped 0 bytes to 0x80, and nonzero bytes to 0x00,
  79 ; with one exception:
  80 ; 0x01 bytes preceeding the first zero are also mapped to 0x80.
  81 ; So we have to mask out the 0x80s caused by 0x01s before
  82 ; counting leading zeroes to get the bytes in last word.
  83
  84         rlwinm  r5,r8,7,0,31            ; move 0x01 bits to 0x80 position
  85         subf    r3,r9,r3                ; start to compute string length
  86         andc    r4,r4,r5                ; turn off false hits from 0x0100 worst case
  87         cntlzw  r7,r4                   ; now we can count leading 0s
  88         srwi    r7,r7,3                 ; convert 0,8,16,24 to 0,1,2,3
  89         add     r3,r3,r7                ; add in nonzero bytes in last word
  90         blr
  91
  92 ; We must align the source address for two reasons: to avoid spurious page
  93 ; faults, and for speed.
  94 ;       r4 = low 2 bits of address (1,2, or 3)
  95 ;       r3 = address
  96 ;       r9 = original address (still same as r3)
  97
  98 LalignSource:
  99         lbz     r5,0(r3)                ; get the first byte...
 100         subic.  r4,r4,2                 ; test for 1, 2 or 3 bytes
 101         addi    r3,r3,1                 ; increment address
 102         addi    r6,r9,1                 ; now r6==r3
 103         cmpwi   cr1,r5,0                ; zero?
 104         beq     cr1,Lreturn             ; if its zero return zero
 105         bgt     Llentry                 ; address is aligned now if low bits were 3
 106
 107         lbz     r5,0(r3)                ; get the next byte...
 108         addi    r3,r3,1                 ; increment address
 109         cmpwi   cr1,r5,0                ; zero?
 110         beq     cr1,Lreturn             ; if its zero return one
 111         beq     Llentry                 ; addr is aligned now if low bits were 2
 112
 113         lbz     r5,0(r3)                ; get the next byte...
 114         addi    r3,r3,1                 ; increment address
 115         cmpwi   cr1,r5,0                ; zero?
 116         bne     cr1,Llentry             ; not zero, continue check (now aligned)
 117 Lreturn:
 118         sub     r3,r3,r6                ; get string length (0, 1, or 2)
 119         blr
 120