61 lines
2.1 KiB
C
61 lines
2.1 KiB
C
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/* 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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* If CONFIG_M68000=y (original mc68000/010), this file is #included
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* to work around the lack of a MULU.L instruction.
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*/
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#define HAVE_ARCH__HASH_32 1
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/*
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* While it would be legal to substitute a different hash operation
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* entirely, let's keep it simple and just use an optimized multiply
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* by GOLDEN_RATIO_32 = 0x61C88647.
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*
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* The best way to do that appears to be to multiply by 0x8647 with
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* shifts and adds, and use mulu.w to multiply the high half by 0x61C8.
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*
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* Because the 68000 has multi-cycle shifts, this addition chain is
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* chosen to minimise the shift distances.
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*
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* Despite every attempt to spoon-feed it simple operations, GCC
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* 6.1.1 doggedly insists on doing annoying things like converting
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* "lsl.l #2,<reg>" (12 cycles) to two adds (8+8 cycles).
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*
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* It also likes to notice two shifts in a row, like "a = x << 2" and
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* "a <<= 7", and convert that to "a = x << 9". But shifts longer
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* than 8 bits are extra-slow on m68k, so that's a lose.
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*
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* Since the 68000 is a very simple in-order processor with no
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* instruction scheduling effects on execution time, we can safely
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* take it out of GCC's hands and write one big asm() block.
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*
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* Without calling overhead, this operation is 30 bytes (14 instructions
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* plus one immediate constant) and 166 cycles.
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*
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* (Because %2 is fetched twice, it can't be postincrement, and thus it
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* can't be a fully general "g" or "m". Register is preferred, but
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* offsettable memory or immediate will work.)
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*/
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static inline u32 __attribute_const__ __hash_32(u32 x)
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{
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u32 a, b;
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asm( "move.l %2,%0" /* a = x * 0x0001 */
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"\n lsl.l #2,%0" /* a = x * 0x0004 */
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"\n move.l %0,%1"
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"\n lsl.l #7,%0" /* a = x * 0x0200 */
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"\n add.l %2,%0" /* a = x * 0x0201 */
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"\n add.l %0,%1" /* b = x * 0x0205 */
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"\n add.l %0,%0" /* a = x * 0x0402 */
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"\n add.l %0,%1" /* b = x * 0x0607 */
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"\n lsl.l #5,%0" /* a = x * 0x8040 */
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: "=&d,d" (a), "=&r,r" (b)
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: "r,roi?" (x)); /* a+b = x*0x8647 */
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return ((u16)(x*0x61c8) << 16) + a + b;
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}
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#endif /* _ASM_HASH_H */
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