arch/m68k/include/asm/hash.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #ifndef _ASM_HASH_H
   3 #define _ASM_HASH_H
   4
   5 /*
   6  * If CONFIG_M68000=y (original mc68000/010), this file is #included
   7  * to work around the lack of a MULU.L instruction.
   8  */
   9
  10 #define HAVE_ARCH__HASH_32 1
  11 /*
  12  * While it would be legal to substitute a different hash operation
  13  * entirely, let's keep it simple and just use an optimized multiply
  14  * by GOLDEN_RATIO_32 = 0x61C88647.
  15  *
  16  * The best way to do that appears to be to multiply by 0x8647 with
  17  * shifts and adds, and use mulu.w to multiply the high half by 0x61C8.
  18  *
  19  * Because the 68000 has multi-cycle shifts, this addition chain is
  20  * chosen to minimise the shift distances.
  21  *
  22  * Despite every attempt to spoon-feed it simple operations, GCC
  23  * 6.1.1 doggedly insists on doing annoying things like converting
  24  * "lsl.l #2,<reg>" (12 cycles) to two adds (8+8 cycles).
  25  *
  26  * It also likes to notice two shifts in a row, like "a = x << 2" and
  27  * "a <<= 7", and convert that to "a = x << 9".  But shifts longer
  28  * than 8 bits are extra-slow on m68k, so that's a lose.
  29  *
  30  * Since the 68000 is a very simple in-order processor with no
  31  * instruction scheduling effects on execution time, we can safely
  32  * take it out of GCC's hands and write one big asm() block.
  33  *
  34  * Without calling overhead, this operation is 30 bytes (14 instructions
  35  * plus one immediate constant) and 166 cycles.
  36  *
  37  * (Because %2 is fetched twice, it can't be postincrement, and thus it
  38  * can't be a fully general "g" or "m".  Register is preferred, but
  39  * offsettable memory or immediate will work.)
  40  */
  41 static inline u32 __attribute_const__ __hash_32(u32 x)
  42 {
  43         u32 a, b;
  44
  45         asm(   "move.l %2,%0"   /* a = x * 0x0001 */
  46         "\n     lsl.l #2,%0"    /* a = x * 0x0004 */
  47         "\n     move.l %0,%1"
  48         "\n     lsl.l #7,%0"    /* a = x * 0x0200 */
  49         "\n     add.l %2,%0"    /* a = x * 0x0201 */
  50         "\n     add.l %0,%1"    /* b = x * 0x0205 */
  51         "\n     add.l %0,%0"    /* a = x * 0x0402 */
  52         "\n     add.l %0,%1"    /* b = x * 0x0607 */
  53         "\n     lsl.l #5,%0"    /* a = x * 0x8040 */
  54         : "=&d,d" (a), "=&r,r" (b)
  55         : "r,roi?" (x));        /* a+b = x*0x8647 */
  56
  57         return ((u16)(x*0x61c8) << 16) + a + b;
  58 }
  59
  60 #endif  /* _ASM_HASH_H */