/* 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 */