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