| George Spelvin | 7b13277 | 2016-05-25 11:06:09 -0400 | [diff] [blame] | 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 */ |