hash.h

/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _ASM_HASH_H
#define _ASM_HASH_H

/*
 * If CONFIG_M68000=y (original mc68000/010), this file is #included
 * to work around the lack of a MULU.L instruction.
 */

#define HAVE_ARCH__HASH_32 1
/*
 * While it would be legal to substitute a different hash operation
 * entirely, let's keep it simple and just use an optimized multiply
 * by GOLDEN_RATIO_32 = 0x61C88647.
 *
 * The best way to do that appears to be to multiply by 0x8647 with
 * shifts and adds, and use mulu.w to multiply the high half by 0x61C8.
 *
 * Because the 68000 has multi-cycle shifts, this addition chain is
 * chosen to minimise the shift distances.
 *
 * Despite every attempt to spoon-feed it simple operations, GCC
 * 6.1.1 doggedly insists on doing annoying things like converting
 * "lsl.l #2,<reg>" (12 cycles) to two adds (8+8 cycles).
 *
 * It also likes to notice two shifts in a row, like "a = x << 2" and
 * "a <<= 7", and convert that to "a = x << 9".  But shifts longer
 * than 8 bits are extra-slow on m68k, so that's a lose.
 *
 * Since the 68000 is a very simple in-order processor with no
 * instruction scheduling effects on execution time, we can safely
 * take it out of GCC's hands and write one big asm() block.
 *
 * Without calling overhead, this operation is 30 bytes (14 instructions
 * plus one immediate constant) and 166 cycles.
 *
 * (Because %2 is fetched twice, it can't be postincrement, and thus it
 * can't be a fully general "g" or "m".  Register is preferred, but
 * offsettable memory or immediate will work.)
 */
static inline u32 __attribute_const__ __hash_32(u32 x)
{
	u32 a, b;

	asm(   "move.l %2,%0"	/* a = x * 0x0001 */
	"\n	lsl.l #2,%0"	/* a = x * 0x0004 */
	"\n	move.l %0,%1"
	"\n	lsl.l #7,%0"	/* a = x * 0x0200 */
	"\n	add.l %2,%0"	/* a = x * 0x0201 */
	"\n	add.l %0,%1"	/* b = x * 0x0205 */
	"\n	add.l %0,%0"	/* a = x * 0x0402 */
	"\n	add.l %0,%1"	/* b = x * 0x0607 */
	"\n	lsl.l #5,%0"	/* a = x * 0x8040 */
	: "=&d,d" (a), "=&r,r" (b)
	: "r,roi?" (x));	/* a+b = x*0x8647 */

	return ((u16)(x*0x61c8) << 16) + a + b;
}

#endif	/* _ASM_HASH_H */