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 */
41static 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 */
61