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| 1 | /* SPDX-License-Identifier: GPL-2.0 */ |
|---|---|
| 2 | #ifndef _ASM_HASH_H |
| 3 | #define _ASM_HASH_H |
| 4 | |
| 5 | /* |
| 6 | * If CONFIG_M68000=y (original mc68000/010), this file is #included |
| 7 | * to work around the lack of a MULU.L instruction. |
| 8 | */ |
| 9 | |
| 10 | #define HAVE_ARCH__HASH_32 1 |
| 11 | /* |
| 12 | * While it would be legal to substitute a different hash operation |
| 13 | * entirely, let's keep it simple and just use an optimized multiply |
| 14 | * by GOLDEN_RATIO_32 = 0x61C88647. |
| 15 | * |
| 16 | * The best way to do that appears to be to multiply by 0x8647 with |
| 17 | * shifts and adds, and use mulu.w to multiply the high half by 0x61C8. |
| 18 | * |
| 19 | * Because the 68000 has multi-cycle shifts, this addition chain is |
| 20 | * chosen to minimise the shift distances. |
| 21 | * |
| 22 | * Despite every attempt to spoon-feed it simple operations, GCC |
| 23 | * 6.1.1 doggedly insists on doing annoying things like converting |
| 24 | * "lsl.l #2,<reg>" (12 cycles) to two adds (8+8 cycles). |
| 25 | * |
| 26 | * It also likes to notice two shifts in a row, like "a = x << 2" and |
| 27 | * "a <<= 7", and convert that to "a = x << 9". But shifts longer |
| 28 | * than 8 bits are extra-slow on m68k, so that's a lose. |
| 29 | * |
| 30 | * Since the 68000 is a very simple in-order processor with no |
| 31 | * instruction scheduling effects on execution time, we can safely |
| 32 | * take it out of GCC's hands and write one big asm() block. |
| 33 | * |
| 34 | * Without calling overhead, this operation is 30 bytes (14 instructions |
| 35 | * plus one immediate constant) and 166 cycles. |
| 36 | * |
| 37 | * (Because %2 is fetched twice, it can't be postincrement, and thus it |
| 38 | * can't be a fully general "g" or "m". Register is preferred, but |
| 39 | * offsettable memory or immediate will work.) |
| 40 | */ |
| 41 | static inline u32 __attribute_const__ __hash_32(u32 x) |
| 42 | { |
| 43 | u32 a, b; |
| 44 | |
| 45 | asm( "move.l %2,%0" /* a = x * 0x0001 */ |
| 46 | "\n lsl.l #2,%0" /* a = x * 0x0004 */ |
| 47 | "\n move.l %0,%1" |
| 48 | "\n lsl.l #7,%0" /* a = x * 0x0200 */ |
| 49 | "\n add.l %2,%0" /* a = x * 0x0201 */ |
| 50 | "\n add.l %0,%1" /* b = x * 0x0205 */ |
| 51 | "\n add.l %0,%0" /* a = x * 0x0402 */ |
| 52 | "\n add.l %0,%1" /* b = x * 0x0607 */ |
| 53 | "\n lsl.l #5,%0" /* a = x * 0x8040 */ |
| 54 | : "=&d,d" (a), "=&r,r" (b) |
| 55 | : "r,roi?" (x)); /* a+b = x*0x8647 */ |
| 56 | |
| 57 | return ((u16)(x*0x61c8) << 16) + a + b; |
| 58 | } |
| 59 | |
| 60 | #endif /* _ASM_HASH_H */ |
| 61 |
Warning: This file is not a C or C++ file. It does not have highlighting.