1 /* SPDX-License-Identifier: GPL-2.0 */ 2 #ifndef _ASM_HASH_H 3 #define _ASM_HASH_H 4 5 #include <linux/wordpart.h> 6 7 /* 8 * If CONFIG_M68000=y (original mc68000/010), this file is #included 9 * to work around the lack of a MULU.L instruction. 10 */ 11 12 #define HAVE_ARCH__HASH_32 1 13 /* 14 * While it would be legal to substitute a different hash operation 15 * entirely, let's keep it simple and just use an optimized multiply 16 * by GOLDEN_RATIO_32 = 0x61C88647. 17 * 18 * The best way to do that appears to be to multiply by 0x8647 with 19 * shifts and adds, and use mulu.w to multiply the high half by 0x61C8. 20 * 21 * Because the 68000 has multi-cycle shifts, this addition chain is 22 * chosen to minimise the shift distances. 23 * 24 * Despite every attempt to spoon-feed it simple operations, GCC 25 * 6.1.1 doggedly insists on doing annoying things like converting 26 * "lsl.l #2,<reg>" (12 cycles) to two adds (8+8 cycles). 27 * 28 * It also likes to notice two shifts in a row, like "a = x << 2" and 29 * "a <<= 7", and convert that to "a = x << 9". But shifts longer 30 * than 8 bits are extra-slow on m68k, so that's a lose. 31 * 32 * Since the 68000 is a very simple in-order processor with no 33 * instruction scheduling effects on execution time, we can safely 34 * take it out of GCC's hands and write one big asm() block. 35 * 36 * Without calling overhead, this operation is 30 bytes (14 instructions 37 * plus one immediate constant) and 166 cycles. 38 * 39 * (Because %2 is fetched twice, it can't be postincrement, and thus it 40 * can't be a fully general "g" or "m". Register is preferred, but 41 * offsettable memory or immediate will work.) 42 */ 43 static inline u32 __attribute_const__ __hash_32(u32 x) 44 { 45 u32 a, b; 46 47 asm( "move.l %2,%0" /* a = x * 0x0001 */ 48 "\n lsl.l #2,%0" /* a = x * 0x0004 */ 49 "\n move.l %0,%1" 50 "\n lsl.l #7,%0" /* a = x * 0x0200 */ 51 "\n add.l %2,%0" /* a = x * 0x0201 */ 52 "\n add.l %0,%1" /* b = x * 0x0205 */ 53 "\n add.l %0,%0" /* a = x * 0x0402 */ 54 "\n add.l %0,%1" /* b = x * 0x0607 */ 55 "\n lsl.l #5,%0" /* a = x * 0x8040 */ 56 : "=&d,d" (a), "=&r,r" (b) 57 : "r,roi?" (x)); /* a+b = x*0x8647 */ 58 59 return (lower_16_bits(x * 0x61c8) << 16) + a + b; 60 } 61 62 #endif /* _ASM_HASH_H */ 63