hash.h (2115B)
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 */