1 /* SPDX-License-Identifier: GPL-2.0 */ 2 #ifndef _ASM_HASH_H 3 #define _ASM_HASH_H 4 5 /* 6 * Fortunately, most people who want to run Linux on Microblaze enable 7 * both multiplier and barrel shifter, but omitting them is technically 8 * a supported configuration. 9 * 10 * With just a barrel shifter, we can implement an efficient constant 11 * multiply using shifts and adds. GCC can find a 9-step solution, but 12 * this 6-step solution was found by Yevgen Voronenko's implementation 13 * of the Hcub algorithm at http://spiral.ece.cmu.edu/mcm/gen.html. 14 * 15 * That software is really not designed for a single multiplier this large, 16 * but if you run it enough times with different seeds, it'll find several 17 * 6-shift, 6-add sequences for computing x * 0x61C88647. They are all 18 * c = (x << 19) + x; 19 * a = (x << 9) + c; 20 * b = (x << 23) + a; 21 * return (a<<11) + (b<<6) + (c<<3) - b; 22 * with variations on the order of the final add. 23 * 24 * Without even a shifter, it's hopless; any hash function will suck. 25 */ 26 27 #if CONFIG_XILINX_MICROBLAZE0_USE_HW_MUL == 0 28 29 #define HAVE_ARCH__HASH_32 1 30 31 /* Multiply by GOLDEN_RATIO_32 = 0x61C88647 */ 32 static inline u32 __attribute_const__ __hash_32(u32 a) 33 { 34 #if CONFIG_XILINX_MICROBLAZE0_USE_BARREL 35 unsigned int b, c; 36 37 /* Phase 1: Compute three intermediate values */ 38 b = a << 23; 39 c = (a << 19) + a; 40 a = (a << 9) + c; 41 b += a; 42 43 /* Phase 2: Compute (a << 11) + (b << 6) + (c << 3) - b */ 44 a <<= 5; 45 a += b; /* (a << 5) + b */ 46 a <<= 3; 47 a += c; /* (a << 8) + (b << 3) + c */ 48 a <<= 3; 49 return a - b; /* (a << 11) + (b << 6) + (c << 3) - b */ 50 #else 51 /* 52 * "This is really going to hurt." 53 * 54 * Without a barrel shifter, left shifts are implemented as 55 * repeated additions, and the best we can do is an optimal 56 * addition-subtraction chain. This one is not known to be 57 * optimal, but at 37 steps, it's decent for a 31-bit multiplier. 58 * 59 * Question: given its size (37*4 = 148 bytes per instance), 60 * and slowness, is this worth having inline? 61 */ 62 unsigned int b, c, d; 63 64 b = a << 4; /* 4 */ 65 c = b << 1; /* 1 5 */ 66 b += a; /* 1 6 */ 67 c += b; /* 1 7 */ 68 c <<= 3; /* 3 10 */ 69 c -= a; /* 1 11 */ 70 d = c << 7; /* 7 18 */ 71 d += b; /* 1 19 */ 72 d <<= 8; /* 8 27 */ 73 d += a; /* 1 28 */ 74 d <<= 1; /* 1 29 */ 75 d += b; /* 1 30 */ 76 d <<= 6; /* 6 36 */ 77 return d + c; /* 1 37 total instructions*/ 78 #endif 79 } 80 81 #endif /* !CONFIG_XILINX_MICROBLAZE0_USE_HW_MUL */ 82 #endif /* _ASM_HASH_H */ 83