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 /* SPDX-License-Identifier: GPL-2.0 */ #ifndef _ASM_HASH_H #define _ASM_HASH_H /* * Fortunately, most people who want to run Linux on Microblaze enable * both multiplier and barrel shifter, but omitting them is technically * a supported configuration. * * With just a barrel shifter, we can implement an efficient constant * multiply using shifts and adds. GCC can find a 9-step solution, but * this 6-step solution was found by Yevgen Voronenko's implementation * of the Hcub algorithm at http://spiral.ece.cmu.edu/mcm/gen.html. * * That software is really not designed for a single multiplier this large, * but if you run it enough times with different seeds, it'll find several * 6-shift, 6-add sequences for computing x * 0x61C88647. They are all * c = (x << 19) + x; * a = (x << 9) + c; * b = (x << 23) + a; * return (a<<11) + (b<<6) + (c<<3) - b; * with variations on the order of the final add. * * Without even a shifter, it's hopless; any hash function will suck. */ #if CONFIG_XILINX_MICROBLAZE0_USE_HW_MUL == 0 #define HAVE_ARCH__HASH_32 1 /* Multiply by GOLDEN_RATIO_32 = 0x61C88647 */ static inline u32 __attribute_const__ __hash_32(u32 a) { #if CONFIG_XILINX_MICROBLAZE0_USE_BARREL unsigned int b, c; /* Phase 1: Compute three intermediate values */ b = a << 23; c = (a << 19) + a; a = (a << 9) + c; b += a; /* Phase 2: Compute (a << 11) + (b << 6) + (c << 3) - b */ a <<= 5; a += b; /* (a << 5) + b */ a <<= 3; a += c; /* (a << 8) + (b << 3) + c */ a <<= 3; return a - b; /* (a << 11) + (b << 6) + (c << 3) - b */ #else /* * "This is really going to hurt." * * Without a barrel shifter, left shifts are implemented as * repeated additions, and the best we can do is an optimal * addition-subtraction chain. This one is not known to be * optimal, but at 37 steps, it's decent for a 31-bit multiplier. * * Question: given its size (37*4 = 148 bytes per instance), * and slowness, is this worth having inline? */ unsigned int b, c, d; b = a << 4; /* 4 */ c = b << 1; /* 1 5 */ b += a; /* 1 6 */ c += b; /* 1 7 */ c <<= 3; /* 3 10 */ c -= a; /* 1 11 */ d = c << 7; /* 7 18 */ d += b; /* 1 19 */ d <<= 8; /* 8 27 */ d += a; /* 1 28 */ d <<= 1; /* 1 29 */ d += b; /* 1 30 */ d <<= 6; /* 6 36 */ return d + c; /* 1 37 total instructions*/ #endif } #endif /* !CONFIG_XILINX_MICROBLAZE0_USE_HW_MUL */ #endif /* _ASM_HASH_H */