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