mirror of https://gitee.com/openkylin/linux.git
61 lines
2.1 KiB
C
61 lines
2.1 KiB
C
/* 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 */
|