/* 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 = 0 x61C88647 . * * The best way to do that appears to be to multiply by 0 x8647 with * shifts and adds , and use mulu . w to multiply the high half by 0 x61C8 . * * 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)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*0 x61c8) << 16 ) + a + b;#endif /* _ASM_HASH_H */
Messung V0.5 in Prozent C=92 H=100 G=95
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(vorverarbeitet am 2026-09-28)
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