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 */