This provides a multiply by constant GOLDEN_RATIO_32 = 0x61C88647 for the original mc68000, which lacks a 32x32-bit multiply instruction. Yes, the amount of optimization effort put in is excessive. :-) Addition chains found by Yevgen Voronenko's Hcub algorithm at http://spiral.ece.cmu.edu/mcm/gen.html Signed-off-by: George Spelvin <linux@xxxxxxxxxxxxxxxxxxx> Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> Cc: Greg Ungerer <gerg@xxxxxxxxxxxxxx> Cc: linux-m68k@xxxxxxxxxxxxxxxxxxxx --- arch/m68k/Kconfig.cpu | 1 + arch/m68k/include/asm/archhash.h | 58 ++++++++++++++++++++++++++++++++++++++++ 2 files changed, 59 insertions(+) create mode 100644 arch/m68k/include/asm/archhash.h diff --git a/arch/m68k/Kconfig.cpu b/arch/m68k/Kconfig.cpu index 0dfcf128..bf3de464 100644 --- a/arch/m68k/Kconfig.cpu +++ b/arch/m68k/Kconfig.cpu @@ -40,6 +40,7 @@ config M68000 select CPU_HAS_NO_MULDIV64 select CPU_HAS_NO_UNALIGNED select GENERIC_CSUM + select HAVE_ARCH_HASH help The Freescale (was Motorola) 68000 CPU is the first generation of the well known M68K family of processors. The CPU core as well as diff --git a/arch/m68k/include/asm/archhash.h b/arch/m68k/include/asm/archhash.h new file mode 100644 index 00000000..2532cf92 --- /dev/null +++ b/arch/m68k/include/asm/archhash.h @@ -0,0 +1,58 @@ +#ifndef _ASM_ARCHHASH_H +#define _ASM_ARCHHASH_H + +/* + * The only 68k processors that lack MULU.L and so need this workaround + * are the original 68000 and 68010. + */ +#if defined(CONFIG_M68000) || defined(CONFIG_M68010) + +#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 GCC 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. + */ +static inline u32 __attribute_const__ __hash_32(u32 x) +{ + u32 a, b; + + asm( "move.l %2,%0" /* 0x0001 */ + "\n lsl.l #2,%0" /* 0x0004 */ + "\n move.l %0,%1" + "\n lsl.l #7,%0" /* 0x0200 */ + "\n add.l %2,%0" /* 0x0201 */ + "\n add.l %0,%1" /* 0x0205 */ + "\n add.l %0,%0" /* 0x0402 */ + "\n add.l %0,%1" /* 0x0607 */ + "\n lsl.l #5,%0" /* 0x8040 */ + /* 0x8647 */ + : "=&d" (a), "=&r" (b) + : "g" (x)); + + return ((u16)(x*0x61c8) << 16) + a + b; +} +#endif /* HAVE_ARCH__HASH_32 */ + +#endif /* _ASM_ARCHHASH_H */ -- 2.8.1 -- To unsubscribe from this list: send the line "unsubscribe linux-m68k" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
