LXR linux/arch/m68k/include/asm/hash.h

   1#ifndef _ASM_HASH_H
   2#define _ASM_HASH_H
   3
   4/*
   5 * If CONFIG_M68000=y (original mc68000/010), this file is #included
   6 * to work around the lack of a MULU.L instruction.
   7 */
   8
   9#define HAVE_ARCH__HASH_32 1
  10/*
  11 * While it would be legal to substitute a different hash operation
  12 * entirely, let's keep it simple and just use an optimized multiply
  13 * by GOLDEN_RATIO_32 = 0x61C88647.
  14 *
  15 * The best way to do that appears to be to multiply by 0x8647 with
  16 * shifts and adds, and use mulu.w to multiply the high half by 0x61C8.
  17 *
  18 * Because the 68000 has multi-cycle shifts, this addition chain is
  19 * chosen to minimise the shift distances.
  20 *
  21 * Despite every attempt to spoon-feed it simple operations, GCC
  22 * 6.1.1 doggedly insists on doing annoying things like converting
  23 * "lsl.l #2,<reg>" (12 cycles) to two adds (8+8 cycles).
  24 *
  25 * It also likes to notice two shifts in a row, like "a = x << 2" and
  26 * "a <<= 7", and convert that to "a = x << 9".  But shifts longer
  27 * than 8 bits are extra-slow on m68k, so that's a lose.
  28 *
  29 * Since the 68000 is a very simple in-order processor with no
  30 * instruction scheduling effects on execution time, we can safely
  31 * take it out of GCC's hands and write one big asm() block.
  32 *
  33 * Without calling overhead, this operation is 30 bytes (14 instructions
  34 * plus one immediate constant) and 166 cycles.
  35 *
  36 * (Because %2 is fetched twice, it can't be postincrement, and thus it
  37 * can't be a fully general "g" or "m".  Register is preferred, but
  38 * offsettable memory or immediate will work.)
  39 */
  40static inline u32 __attribute_const__ __hash_32(u32 x)
  41{
  42        u32 a, b;
  43
  44        asm(   "move.l %2,%0"   /* a = x * 0x0001 */
  45        "\n     lsl.l #2,%0"    /* a = x * 0x0004 */
  46        "\n     move.l %0,%1"
  47        "\n     lsl.l #7,%0"    /* a = x * 0x0200 */
  48        "\n     add.l %2,%0"    /* a = x * 0x0201 */
  49        "\n     add.l %0,%1"    /* b = x * 0x0205 */
  50        "\n     add.l %0,%0"    /* a = x * 0x0402 */
  51        "\n     add.l %0,%1"    /* b = x * 0x0607 */
  52        "\n     lsl.l #5,%0"    /* a = x * 0x8040 */
  53        : "=&d,d" (a), "=&r,r" (b)
  54        : "r,roi?" (x));        /* a+b = x*0x8647 */
  55
  56        return ((u16)(x*0x61c8) << 16) + a + b;
  57}
  58
  59#endif  /* _ASM_HASH_H */
  60