1 #ifndef _ASM_HASH_H 2 #define _ASM_HASH_H 3 4 /* 5 * Fortunately, most people who want to run Linux on Microblaze enable 6 * both multiplier and barrel shifter, but omitting them is technically 7 * a supported configuration. 8 * 9 * With just a barrel shifter, we can implement an efficient constant 10 * multiply using shifts and adds. GCC can find a 9-step solution, but 11 * this 6-step solution was found by Yevgen Voronenko's implementation 12 * of the Hcub algorithm at http://spiral.ece.cmu.edu/mcm/gen.html. 13 * 14 * That software is really not designed for a single multiplier this large, 15 * but if you run it enough times with different seeds, it'll find several 16 * 6-shift, 6-add sequences for computing x * 0x61C88647. They are all 17 * c = (x << 19) + x; 18 * a = (x << 9) + c; 19 * b = (x << 23) + a; 20 * return (a<<11) + (b<<6) + (c<<3) - b; 21 * with variations on the order of the final add. 22 * 23 * Without even a shifter, it's hopless; any hash function will suck. 24 */ 25 26 #if CONFIG_XILINX_MICROBLAZE0_USE_HW_MUL == 0 27 28 #define HAVE_ARCH__HASH_32 1 29 30 /* Multiply by GOLDEN_RATIO_32 = 0x61C88647 */ 31 static inline u32 __attribute_const__ __hash_32(u32 a) 32 { 33 #if CONFIG_XILINX_MICROBLAZE0_USE_BARREL 34 unsigned int b, c; 35 36 /* Phase 1: Compute three intermediate values */ 37 b = a << 23; 38 c = (a << 19) + a; 39 a = (a << 9) + c; 40 b += a; 41 42 /* Phase 2: Compute (a << 11) + (b << 6) + (c << 3) - b */ 43 a <<= 5; 44 a += b; /* (a << 5) + b */ 45 a <<= 3; 46 a += c; /* (a << 8) + (b << 3) + c */ 47 a <<= 3; 48 return a - b; /* (a << 11) + (b << 6) + (c << 3) - b */ 49 #else 50 /* 51 * "This is really going to hurt." 52 * 53 * Without a barrel shifter, left shifts are implemented as 54 * repeated additions, and the best we can do is an optimal 55 * addition-subtraction chain. This one is not known to be 56 * optimal, but at 37 steps, it's decent for a 31-bit multiplier. 57 * 58 * Question: given its size (37*4 = 148 bytes per instance), 59 * and slowness, is this worth having inline? 60 */ 61 unsigned int b, c, d; 62 63 b = a << 4; /* 4 */ 64 c = b << 1; /* 1 5 */ 65 b += a; /* 1 6 */ 66 c += b; /* 1 7 */ 67 c <<= 3; /* 3 10 */ 68 c -= a; /* 1 11 */ 69 d = c << 7; /* 7 18 */ 70 d += b; /* 1 19 */ 71 d <<= 8; /* 8 27 */ 72 d += a; /* 1 28 */ 73 d <<= 1; /* 1 29 */ 74 d += b; /* 1 30 */ 75 d <<= 6; /* 6 36 */ 76 return d + c; /* 1 37 total instructions*/ 77 #endif 78 } 79 80 #endif /* !CONFIG_XILINX_MICROBLAZE0_USE_HW_MUL */ 81 #endif /* _ASM_HASH_H */ 82