arch/alpha/lib/strlen.S

   1 /*
   2  * strlen.S (c) 1995 David Mosberger (davidm@cs.arizona.edu)
   3  *
   4  * Finds length of a 0-terminated string.  Optimized for the
   5  * Alpha architecture:
   6  *
   7  *      - memory accessed as aligned quadwords only
   8  *      - uses bcmpge to compare 8 bytes in parallel
   9  *      - does binary search to find 0 byte in last
  10  *        quadword (HAKMEM needed 12 instructions to
  11  *        do this instead of the 9 instructions that
  12  *        binary search needs).
  13  */
  14
  15         .set noreorder
  16         .set noat
  17
  18         .align 3
  19
  20         .globl  strlen
  21         .ent    strlen
  22
  23 strlen:
  24         ldq_u   $1, 0($16)      # load first quadword ($16  may be misaligned)
  25         lda     $2, -1($31)
  26         insqh   $2, $16, $2
  27         andnot  $16, 7, $0
  28         or      $2, $1, $1
  29         cmpbge  $31, $1, $2     # $2  <- bitmask: bit i == 1 <==> i-th byte == 0
  30         bne     $2, found
  31
  32 loop:   ldq     $1, 8($0)
  33         addq    $0, 8, $0       # addr += 8
  34         nop                     # helps dual issue last two insns
  35         cmpbge  $31, $1, $2
  36         beq     $2, loop
  37
  38 found:  blbs    $2, done        # make aligned case fast
  39         negq    $2, $3
  40         and     $2, $3, $2
  41
  42         and     $2, 0x0f, $1
  43         addq    $0, 4, $3
  44         cmoveq  $1, $3, $0
  45
  46         and     $2, 0x33, $1
  47         addq    $0, 2, $3
  48         cmoveq  $1, $3, $0
  49
  50         and     $2, 0x55, $1
  51         addq    $0, 1, $3
  52         cmoveq  $1, $3, $0
  53
  54 done:   subq    $0, $16, $0
  55         ret     $31, ($26)
  56
  57         .end    strlen