LXR linux/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
  23strlen:
  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
  32loop:   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
  38found:  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
  54done:   subq    $0, $16, $0
  55        ret     $31, ($26)
  56
  57        .end    strlen
  58